24 June 02 Hard copy and hard lessons: When I first got
the Internet, I was copying stuff right and left. Turns out that
a good idea. I was updating my links page today, and several nice
seem to have taken a powder. Keep in mind that postings to the
are often ephemeral; webmasters die, just give it up, or change their
and/or their address, and the good dope they had posted often goes away
well. If you see something out there you like, go ahead and run
off, 'cuz it might not be there next time you go looking for it.
a hard copy, and it's yours. You can always copy it again if it
changed. Just loading it onto your dumputer isn't the best option
In the last 6 months I have changed out computers, and then had a
drive go south on the new one. Dell got me a new drive in less
a week, but it was a slick drive, and everything had to be reloaded.
Mike Tuggle sent me a thumbnail shortwave listening log with some 20 stations on it from his Hawaii location, and also sent me some resource sites for help in chasing down the shortwave broadcasters. Since we are exploring new territory here, I have consolidated my shortwave links in a separate section of my links page . We will add a shortwave segment to the next contest, so get to work. You multilinguals out there should have an advantage over those of us with Georgia Cracker as a first language.
22 June 02 Been kinda slow in the laundry room lately. Seems like school being out doesn't necessarily give me the time to get out there and clean things up for the next round of construction and testing. Something about having small kids, relatives to visit, etc. Took a simple shortwave crystal set to my nemesis location, the Shenandoah Valley of Virginia, strung about 50 feet of wire out and searched around a bit. Had enough success to pursue the idea of shortwave sets as the new paradigm for kid radios. Here's my take on the issue: most kids today are only vaguely aware of the AM broadcast band, and the station formats are usually geared to an adult audience. Yeah, with even an oatbox crystal set, you can usually hear a station or two, but the initial excitement quickly wears off as the listener realizes that the stations received are, to them, BORING. Also, in many locations, such as the aforementioned Valley, getting more than one or two stations requires a high level of effort. When you get on the HF bands, however, a whole world, literally, opens up. With even a simple set and basic antenna, you can hear HCJB in Equador, Radio Canada International, and a host of other stations. This has been my experience on the east coast of the US, and I would like to hear from others in different parts of the world to see what they are getting.
6 Apr 02 The 4th annual listening contest is over. Being late in the year, conditions were not the same as we had last year, but there were still some respectable scores. Send your entries in so we can tally up the results. Some new and some changed web links out there; will get around to updating my links page as soon as the yard is cleaned up and I have a round tuit to spare. Meanwhile, we are seriously thinking about adding a shortwave category to the next listening contest; pretty simple rules for this one; just want to see what technical improvements we can come up with and how to sort out the stations. Get building.
18 Feb 02 Had a couple of nice eyeballs with other Rockheads
in the last month. Gil Stacy came down from Savannah to check out
some of the 5 gang variable capacitors from Fair Radio - very
nice caps with a drive that takes about 90 turns to tune lock to lock -
have to get them with the drive from the catalog. Met Kevin Wise,
AI4X, at the Georgia Science Teachers Association Conference and helped
him put on a crystal radio workshop for teachers. Fortunately, we
had an outside door to the room, and were able to string up a couple of
makeshift antennas outside so the sets
would get a signal. We need more people like Kevin getting kids
on the ground floor on basic electricity and electronics. When
schools talk technology these days, they just want to teach kids how to
use applications on a computer; nice goal, but somebody has to teach
them how the computer works too; crystal radios are a good
place to start IMHO. Thought I'd borrow some solder from one of
our technology teachers awhile back, and learned that he doesn't even
have a soldering iron! Go figure.
A Science Fair crystal radio kit clone made by Maxitronix, model MX-901 came on the market a few months ago, and is being sold by several vendors. The earplug has a very high failure rate, based on my sample of the 10 kits I bought for my students. I'll stick with the SK 103. Kevin uses the SK 103 for his workshop, and can get them built by a group of pure novices, tested, and work in some instruction in under an hour.
Yahoo! is moving their clubs to their groups format. Crystal Radio Think Tank just transitioned, with more to follow.
Has anyone found a good supply of germanium diodes for a good price in small quantities? I really miss those little 10 packs from RS.
Ben Tongue has posted some new stuff to his site; a thinking man's crystal set, featuring single and double tuning, and all manner of things you never thought of. Check it out.
5 Jan 02 Okay, time for another listening
contest. Rules linked from the main page. Everyone seems to
have been busy this fall/winter, so we have been a little late getting
off the mark, but you have over two months to get ready. Next
year, look for some different categories. I don't plan much new
for my own rig this year, but it looks like some people out there have
been playing some serious
catch up ball, and we should get some interesting entries.
The FCC, after being stuck in stasis for over a year, has finally started updating their AM station search page, so now I can update my log book and get some time in before the contest finding out what's out there. Have some 660 strand litz wire, and ought to wind some new coils, but have to clear the laundry table off again before I can do anything.
20 November 01 I spent a few frustrating
to upgrade the shortwave xtal set with an antenna tuner, and found that
basic Tuggle setup just didn't cut it; something about having an
that is longer than a quarter wavelength on the higher
frequencies. Then a timely post on the Crystal Set Radio Club by
Steve Holden (#8430) suggested that the two capacitors need to be
independent instead of ganged, since on some bands you are tuning the
antenna as a Marconi and on others as a Hertzian. This tends to
complicate things, so I played around with the connections to the
little poly film variable I was using and the coil and think I came up
with a single knob tuner, or that is what my grid dip oscillator and
set is telling me. I ended up with a coil with half as many turns
as the main tank coil, and put it in parallel with the 160
pF section the capacitor, and put the ground lead to the 60 pF
section. Still have some more testing to do, but it seems to be
working well, and I have the two coils now separated by more than a
coil diameter, and have to tune more carefully. By the way, I had
a few of my kids build the simple version, and it might be a winner.
Restoration Hardware seems to have flogged off all of its rocket radios, and Radio Shack is out of the 10 packs of germanium diodes. Derf Electronics seems to have access to a plentiful supply of all manner of germanium diodes, but only deal in wholesale lots starting at about $100.00. Dan's small parts also has a number of germaniums for sale, and also has some other good deals on transistors, FETs, and other parts.
14 November 01 Spent a pleasant morning a
few weeks ago at a ham swap meet. I set up a table with some xtal
set literature, handed out some flyers with some propaganda and some
web addresses and sold a few parts and as well as some rocket radios
some Technokit crystal sets. Had a handout to go with the rockets
with modification suggestions. Over half the people who came to
table had made crystal radios as kids, so we shared a few nostalgic
Trying to come up with some projects for my students, I started playing around with an oatbox set, and decided that it is really a neat kid radio for such a simple thing. I was able to separate 4 locals, and when I took the ground connection off, found myself solidly in the 49 meter shortwave band. Then I decided to see if I could make a kid radio for the shortwave band, and it worked pretty well too. A lot of times, with kid radios, you get a couple or so local stations, and that's about it. Then you have to start paying close attention to a lot of things. With a drop dead simple shortwave set, I'm hearing 4 or 5 separate international shortwave bands and a grunch of stations. What was really neat was hearing a program on the 49 meter band, then tuning up and hearing it on the 41 meter band as well.
15 September 01 Finally got tired of
trying to catch dx with poor conditions and a bum
antenna, so strung up a hundred or so feet of wire the other day.
Much better. Still not much in the way of good dx yet, but at
least I can't blame it
on the wire. Have been playing around with some simple sets and
trying things out, such as the audion circuit. Decided that the
crystal earplug is still hard to beat for the price. Also found
that the audion type headphone amp with the darlington pair beats the
single transistor version all hollow. The old standby crystal
convertible amp still does a
fine job with the cheap 2k headphones. Also put the crystal
convertible on the air in the regen mode and had some fun with
that. I used a
store bought regen kit from MFJ to see what's happening on the short
and there's plenty out there. Fooled around with a xtal set on
wave a bit too, and got some stations, but my setup was just a few
thrown together, and selectivity was very poor. I think it's
time I built a serious short wave xtal set before I start gearing up
the next listening contest.
This is the season for the ham fests to start cranking up in my area, and there are three next month within a couple of hours' drive. I plan on setting up a crystal radio table in the flea market of a couple of them and see what kind of interest I can gin up. Also plan to take a multimeter and see if there are some nice intermediate impedance (say, 600 ohm) headphones out there for a good price. Go to the ARRL site below to check out the ham fests in your area.
20 August 01 Got a nice plug this week on
the ARRL web site. Thanks
guys, and we'll continue to help you poison the minds of our nation's
youth - hi. I notice that a lot of the Rockheads out there are
also hams, and know that a lot of hams started out with crystal
radios. With that being the case, let me make a suggestion or
two. Go to your local hamfest and set up a crystal radio table in
the tailgate flea market section. Show off a couple of your rigs
and have some
of the literature that is available out there for all to see.
you can talk Becca into providing you some complimentary copies of the
newsletter to hand out. Have a handout available with web
that you find useful, and maybe a set of instructions for a couple of
xtal sets. Heck, buy a bunch of the inexpensive sets available
Transtronics and flog them off at near cost, as well as some crystal
and germanium diodes. I guarantee you'll get some nibbles and
than a few nostalgic conversations. If you don't want to be the
crystal radio guy in your zip code, you'll have to get some more on
and this is one way to do it. Most old timers may remember fondly
first crystal radio, but I doubt that the majority have any idea what's
going on the hobby lately.
2 August 01 Restoration Hardware is flogging off its stock of rocket radios for a nice price, so I went and bought a few. Nice looking little radio, but it has a couple of problems (which is probably why they are discontinuing it). It uses a ferrite core inductor in parallel with a 200 pF capacitor, and is tuned by sliding the core in and out of the coil, giving a 2.8:1 frequency tuning range. It tunes from about 480 to 1300 kHz. By replacing the capacitor with one of 150 pF, the tuning range is about 560-1570 kHz, allowing me to tune my two loudest locals at the top of the band. The germanium diode was a little on the insensitive side, so replacing it gave me a louder signal. It has a single wire with an alligator clip for an antenna, and you connect the clip to something metal, such as an antenna, to get a signal. If you want to add an antenna/ground to it, you can put a 50 pF capacitor on the ground side of the tank circuit, and connect a ground wire to it. The diode taps to the top of the tank circuit, and the crystal earplug has a 100 kohm resistor across it. Things to try include adding a tap about halfway on the coil for the detector, and using a 47 kohm resistor across the earplug. It is still in the "toy" category, but with a couple of quick mods is okay to hand a 10 year old boy to try out.
20 July 01
Hard lessons: lightning works! Vaporized about 50 feet of antenna between two attachment points the other day, not to mention other collateral damage to TV, cable, phones, hot water heater, and garage door opener. Guess my plans to swing another antenna up just moved up on the list of things to do.
Okay, so maybe Mike Lee's homebrew building contest set wasn't really pathetic, and, after all, his worked much better than mine, and was a lot prettier. There may be another contest in 2002, and I'm already conceptualizing mine, so watch this space.
Doing my periodic web search for new sites, I found several out there recently, and you can dig them up on my links page. If you make up a new site of your own, let me know and I'll get it posted.
18 June 01 Not a lot going on in the
laundry room lately, but now that school is out, I have a bunch of
things to try, including swinging an antenna oriented at right angles
to the current
one to see what happens with reception. I notice the FCC site is
getting more out of date every passing day for getting AM station
Lee Freshwater's Broadcast Band DX logbook seems to be doing the best
job in keeping up to date.
Looks like Radio Shack is going to quit carrying the nice little 10 packs of 1N34A type germanium diodes; both of my stores are out, and I'm hearing the same thing across the country. Also raised the prices about 10 to 20 percent on many of their small parts; 'course that is to be expected from time to time. There still seem to be plenty of diodes out there, however, so don't get into a panic. If you do run across a RS store with the germanium diodes still in stock, however, go ahead and clean 'em out, particularly if you intend to work with kids.
Saw a number of postings lately on what is termed a "rook" coil". Wound like a basket coil, but on a solid form with slits cut into it longitudinally for going in and out instead uf using posts. Haven't seen anything to make me want to go out and build one yet. Only technical comment I have seen is that it seems to obey Wheeler's formula for inductance, as one would expect. I'll wait for a more detailed technical evaluation before I rush right out.
Charlie McGinnis recently sent me a sample of Germanite, which contains copper, iron, and germanium. Trying it out on a rock stand, it works fairly well as a detector, but is not as "user friendly" as either galena or iron pyrite. I was able to find a spot on it as sensitive as my good galena, but it took more poking around, and didn't hold the point as well.
4 May 01 The XSS had to reformat the
Rap'nTap forum, but it works well at the new address. Doesn't do
hyperlinks, but every dog has its fleas. In the XSS bookstore you
can now buy a two-gang 365 pF variable. Okay, no excuses now,
time to build a serious dx set with a Tuggle front end. I'll show
you how soon.
I came in second in my building contest category, while Mike Lee ran a pathetic next to last - too bad for me that there were only two entries - hi. Visit the building contest site if you haven't already and see what a little ingenuity can do.
Had a few of my students build the basic MK 484 rig recently. Took only a little prodding and instruction to get them going, and they were able to get them built with minimal assistance from me. I took a pretty straightforward approach, giving them a layout plan, a completed set (make it look like this one), and step by step written instructions. Any of you Elmers out there that want a set of my instructions for your own kid groups, I now have them posted here.
7 Apr 01 The results of the listening
contest have been tallied and the winners announced. Building
contest results should be out imminently. A fun time was had by
Took a trip to Jacksonville, FL today for a ham tailgate swapfest. Picked up a nice little Micronta (Radio Shack) digital multimeter for $10, and it does a nice job - now I can return the one I have been borrowing for the last year back to my school lab. Also picked up a couple of cheap used headphones for a buck each. One was 600 ohms dc resistance (modified to be 300) and the other was about 300. Putting them on my big rig with a matching transformer, they did a middlin' job. Both worked very nicely with my one transistor amp, and are both lightweight and comfortable. For local use, without the amp, they should be okay. You Elmers out there take an ohmmeter with you to those swap meets and pick up any working set of phones that cost next to nothing. If they aren't up to snuff for xtal set use as is, use one of my little amps, either the convertible version or the audion type, depending on the impedance of your phones, and you won't have to mess with those crystal earplugs. Okay, so the plugs are plenty sensitive, but they tend to have a short half-life on occasion, don't fit into little ears very well, and, let's face it, just aren't as sexy as a real set of cans. Next swap meet I'm bringing back a box of them.
16 Mar 01 We're still gathering in the
votes, but Mike Tuggle is still showing us how it's done. Rumor
has it he has recently heard Radio Vision Christiana down in Grand
Turks/Caicos on his Lyonodyne 17 model crystal set - from Hawaii!!
We have in the contest also a couple of 2000 plus mile contacts over land, one of them running only about 500 watts. So much for these "toys" that only hear a couple of stations.
The XSS has announced it will soon start carrying the dual gang 365 pF variables; the heart of the Tuggle tuner. No more switching tuning setups and dealing with taps for full band coverage with your antenna tuner. Guess I'll post a set of plans for a scaled up version of the project radio for those of you wanting to get in on the hot dx action.
Perhaps, as the winter dx season slowly winds down, it might be a good time to get up a rig to check out the tropical bands (that's shortwave, mates). They keep coming into my BC band receiver, so maybe I ought to go up there and chase after them a bit. Anyone have any tried and true plans? At first blush, it ought to be easy to make the transition - we'll see.
07 Mar 01 While the building contest
scoring the entries (you really ought to visit that site; it will be up
a long while), the listening contest results are still coming in, and
the results impressive. Had some more activity with the hobby
this year, with some better than expected results, as people start to
what you can hear with a rather modest setup, and single dial tuning.
Yes Virginia, the XSS does have the crystal earplugs in stock; just haven't had it listed in the web bookstore.
02 Mar 01 The building contest and the listening contest are over for another year. Lots of neat sets that were submitted for the building contest, and the judges will have to do some wrangling over these. Listening contest entries are starting to come in, and there are some nice scores and reception reports being posted. Results in a couple of weeks, I hope. I got a personal best in the open class and about the same as last year in the hobby class. It will be awhile before I can go back to using two earpieces - hi. A couple of things are apparent, however; the sets and the operating skills are still improving. This year, for the first time, several people have mentioned the superiority of reception of their xtal set over the little superhets we use for locking on the frequency, particularly under noisy band conditions. On more than a couple of occasions I had to bring my superhet near the crystal set's antenna tuning coil to catch what I had already heard on the xtal set. Now if that station in (Columbia??), had just given an ID in english...Happiness is when you hear a new station for only 15 seconds, during which you clearly hear the call sign and location. Misery is listening to Radio Disney for 15 minutes or more on several nights, with nary an ID, and you know that there are 3 such stations on that frequency, and that all are in range. Whyizzit that only the clear channel stations ID every 5 minutes? Seems like the lower the power, the less they ID.
04 Feb 01 I have decided to go with big
litz wire coils for the next listening contest. 420/46 or
something like that, and not cheap. Wound a couple on a 4 inch
diameter basket. First thing I noticed was that the optimum coil
spacing between antenna and detector coils went from about 4 inches to
a foot. We'll see how they pan out in the contest. Of
course, the pros from Dover have already gone up to 660 strand
litz. Claim it improves performance
at the top of the band, where the stations all seem to crowd
together. Just love these simple radios!
Just received a copy of K.G. Kott's Book "Theory, Design, and Operation of Crystal Receivers" from the XSS . It's a delightful book, with a bunch of good information, particularly on antennas, coils, coupling, and a mess of other stuff. It should be on your bookshelf.
14 Jan 01 Nothing much changes over time. Read some letters to the editor of Time Magazine in the August 7, 1944 issue; looks like a typical sampling from Rap 'n Tap:
In reference to the "Foxhole Radio" in Time (July 17). I am the orientation man in our Co. While giving a lecture I spoke of the foxhole radio and drew a diagram for the rest of the Co. to see. There were quite a few sceptical ones ; so I promised that one would be made to see if the idea was practicable. One was made. It didn't work....
My honor and your magazine are at stake. Does the radio work or not, and why? I will keep to my guns until you reply. Please hurry! There is no time to lose.
(Pvt.) Thomas Rees
Camp Butner, N.C.
....To date we have tried several gauges of coil wire- the article gave no particular gauge- with no success. Should we dig a foxhole?
I've tried Lieut. Rupert's razor-blade radio. It works! But it takes tops in patience, skill & headphone sensitivity, plus excellent ears....
Selectivity may be gained by adding a variable condenser for tuning, connecting it across the coil if the coil's natural frequency is too high, or in series with the ground wire if the natural frequency is too low...
Anybody out there have the 17 July issue? The same issue had a Bell Telephone ad showing a sailor wearing a sound powered chest set. See it on Darryl Boyd's page.
7 Jan 01 Electrical grounds: since I
tout from time to time the use of the screw on the cover plate of
electrical outlets for a ground, I decided I needed to put one into my
test it against my cold water pipe ground. At first blush, it
to work about the same as the water pipe ground I have. I did
about a 10 ohm dc resistance between the two, for whatever that is
and haven't done a real head-to-head of one vs the other yet, but will
my supply of round tuits gets replenished.
Darryl Boyd just did us a nice service and put up a super page on Sound Powered phones and much, much more. Take a look.
The listening contest is just around the corner, so get your antennas up and start finding out what your usual stations are. Incidentally, this is really a low key event, and anybody, and I mean anybody is more than welcome to join in. It gives you an opportunity to see what you can dig out with your crystal set and then see what others can do in the same time frame. You never know what you can do with one of these little sets until you really give it an honest try.
Stole some mossy zinc from the chemistry teachers at school the other day and tried them in my rock stand; thought I was getting something, but it was too tenuous to make a call. However, while I was poking around with a stiff catwhisker, I did get a nice signal off the corrosion on the alligator clip I was using for a mineral holder. Ain't this hobby neat?
23 Dec 00 In case you hadn't noticed, the
winter dx season is in full swing. I have found once again that
some new stations may be found in the two hours just before sunset and
just after sunrise. Got to get them quick, however, since they
don't tend to
stick around too long. These are the ones I probably spend the
time chasing down. Always a good idea to have the digital
superhet handy to go back quickly to the freq on the hour to try
to get the station ID.
Picked up a piece of Bornite, aka peacock ore, at the rock display of a novelty shop the other day and found it to be useful as a detector. Any of you out there who have been reluctant to try a rock stand for whatever reason can drop me a line and I'll send you a cheap but effective knock down version I made up; seems I have a supply of the necessary ingredients, modest perhaps, but well in excess of my immediate needs. I'll even throw in a couple of bits of rock for you to try with it. You'll need a piece of scrap wood to mount it on; take you about five minutes to get it up and running.
A few fellows are getting together to mount a propagation study to which you can contribute, I think. Watch this space for details.
4 Dec 00 Rockhead confessions: As I have mentioned before, my first xtal set was a Cub Scout model, which had a slider and a galena crystal detector mounted on a cardboard chassis; also had a single earpiece Cannon headphone. It got my interest up a bit, even though I had only modest success with it. Not much later, I discovered diodes, such as the 1N60 and 1N21, and never went back to the rocks. It was hard enough finding a station without also having to find a sweet spot on the crystal, and diodes eliminated one of those problems instantly. Just this last week, however, a comment from Hal Pollner got my interest back up. I actually had two rockstands already; one is a commercial job, and it is poorly designed - lots of slop, backlash, and a very stiff cat whisker. The other, which I won in the last dx contest, is one of Larry's Custom Detectors , and it is at the very opposite end of the spectrum; a true work of craftsmanship. Anyway, after trying Larry's out, and finding that it worked very nicely, I set about to make one of my own. I had a couple of rocks from a craft store display, one of galena and the other of iron pyrite, so I broke some chips off each. My stand is pretty crude looking, but works fine (but not as well as Larry's). It uses an alligator clip to hold the rock, and a length of 3/32 " brass tubing with a wooden ball hot glued on one end, and run through a large fahnstock clip, with a piece of fine nichrome wire wound in a coil to slide on the rod, with the end looping around to touch the rock. Doggoned if the darn thing didn't work. It was neat to use it to listen to a bunch of dx stations at night such as the Grand Old Opry from Nashville using a truly retro detection method. Okay, now I am ready to build a serious homebrew rig for the building contest.
26 Nov 00 Solid State fun: Spent
playing around with simple transistor circuits over the holidays.
up with an enhanced MK 484 set, which still tickles me with its
performance, a beefed up audion amplifier, and some investigations into
the rf preamp I've had posted for a couple of years but never got past
the breadboard stage.
My current "best buy" in headphones is a set of HS 16 low impedance magnetic phones with a Mouser TM 117 matching transformer. The phones, built for military use by a variety of manufacturers (mine are Murdocks) in the '50s are currently readily available, as are the transformers. No, they don't match up to the sound powered phones and matching transformer setup I use in competition, but they're darn close, easier to find, and will cost you about 1/3 as much. And, you can tap across the whole tank coil in your double tuned rig, get higher signal levels, and lose the out of band intruders. Scott's Crystal Radios has 'em. Ben Tongue's Web Page has a matching configuration using 4 of the TM 117s (go ahead, they're only 2 bucks each) which he says will give you even better performance (and no doubt will).
22 Nov 00 Limitations and lamentations: Every radio ever built has its drawbacks. These range from low sensitivity, poor audio quality, cumbersome, cross modulation, birdies, noise, and need an antenna and headphones to just plain stink. Crystal radios and simple powered radios are no exceptions. Battery voltage can greatly affect regenerative receivers and the MK 484 as well; you can "fix" that, but there are costs involved, usually with more smarts, parts and complexity. Ever take your latest wonder somewhere to show off, and find it lacking? I have. Sometimes a set will work just fine with your antenna, ground, phones, and ambient noise level in the shack, but be a bust elsewhere or under different conditions. I bought a couple of very cheap kits a couple of years ago - they stunk to high heaven. Now I have to stop short of calling the seller a crook, 'cuz they probably worked a little better for him in his location - maybe he was just tickled the design worked at all - it did have a couple of neat construction features, but an oatbox set works better in my opinion. What's the message here? Well, just be skeptical out there. If you make a set and like it, wring it out before you "take it on the road". I come up with little radios for my students to build, and have to remind myself to test them under conditions less than my best before I put the design to bed. Heck, anyone with a 1N60, a crystal earplug, and 100 feet of wire can have a working radio in about 10 minutes, and be tickled that it works, but you have to accept a few limitations.
18 Nov 00 Dead Artists: Dick Smith is following the lead of RS and discontinuing its basic xtal set; don't know if a new model in the wings. I think I got the last 6, on sale. I'll build one, and probably put the rest up for contest prizes. Uses a flat ferrite coil of 695 uH and only 160 pF of the variable - the other 60 pF section is unused. I'm thinking a Tuggle mod maybe. I just checked out the latest Goldmine on-line catalog; the place where they usually have the little polyfilm variable capacitors is vacant! What's going on here? Well, at least at last count Mouser had over 6000 of the xtal earplugs in stock. The polyfilm variables are still listed at Dan's, Ocean State Electronics, kitsrus.com and Electronix Express.
I finally got an acceptable prototype of the MK 484 radio suitable for student construction, and we built a few at school recently. With the coil prewound and the capacitor prepped with leads, took about 1 1/2 hours for the kids to assemble. Only had two construction glitches, readily fixed (one cold solder joint and one lead to the wrong pad), and they all worked fine. I'll post the details for any of you interested Elmers out there. I've also come up with a couple of easy mods to it which make it into a respectable dx set; takes only 4 more parts (or even 3 if you prefer).
29 Oct 00 Been so busy with school lately that I missed the action. Anyway, there is a new xtal set forum out there, as well as more activity on one of the old ones -go to the links page and browse.
My latest kid radio was 80 turns of #24 plastic coated telephone wire wound on 3 inches of a 3.25 inch diameter pasteboard core (for 400 uH), with one tap at 24 turns for the detector and xtal earplug. Used an antenna loading coil of about 17 turns spaced 1/2 inch away. Pretty conventional design, with a 220 polyfilm variable doing the tuning honors. I mounted it on a cardboard "chassis" with masking tape (coil), and double sided tape (capacitor). Found some small fahnestock clips to use for antenna ground connection and mounted them with brass paper brads. It worked pretty well, but, as expected, I started losing it below about 680 kHz or so with it hooked up to the 50 foot "student" antenna, and started getting bleed from the high end locals at the low end of the tuning range. This was not unexpected, since the antenna is too darn short for the low end of the band without some help. We built 5 of them in a couple of hours, had them tested and out the door. I took mine home and hooked in a little 200 uH ferrite core AM tuning coil from the Goldmine in series with the antenna and loading coil, and got the bottom of the band. In building the prototype, I put some more taps on the tuning coil, and tried connecting antenna and ground to it instead of using the loading coil; as expected, the loading coil is a better way to go. With some more turns on the loading coil and some taps, it makes a decent enough antenna tuner, better if you make it so you can vary the separation between the coils; you Elmers out there need to keep these lessons in mind when mentoring. Without an antenna and ground, a crystal set remains in the "toy" category; it can probably pick up something with the antenna connection connected to "something metal". With short antennas, failure to provide some means of loading the antenna to near-resonance will limit the radio a lot.
2 Oct 00 Magic bullets: after scouring the local Radio Shacks for different samples of the current lots of 1N34A type diodes in two states, we have yet to locate anything closer than 3 dB to the 12101 3RT diode (innocently bought in a Radio Shack 1N34A type germanium diode pack). A couple came close: Mike found one in Hawaii with two brown bands on one end, marked 008. I found one marked 1N45 23 049. The markings, by the way, are on the glass portion of the diode, so you have to use a magnifying glass to spot them. Both of these tested better than average, so you might look for them, and compare them to your "best".
Kid Radios: Last night I made up an oatbox set, using #24 plastic coated telephone wire (about #22 with the coating on, for 31 turns per inch).. Wound it on a 20 ounce plastic water bottle, getting about 75 turns using 55 feet of wire, with taps every 5 turns (think every 8 would have been enough). With a crystal earphone, a "leaky" diode (no 47k resistor needed), and a couple of alligator clips, I started playing with it. With my "student" antenna and an electrical socket switch plate cover screw for a ground, I picked up my two top of the band locals (1440 &1490), the all-sports station at 790, and could hear in there the other local at 1100, as well as a 50 kw station 70 miles down the road on 690. Replacing the antenna with about 15 feet of wire draped around the door, I still picked up the two stations at the top of the band clearly. Removing the ground connection and putting the antenna back on, I picked up two or 3 shortwave stations. An interesting twist is that the oatbox liked my 50 foot student antenna better than it did the 120 foot long wire. Since you tune this baby by changing taps, I found that changing taps for the detector was important too - one alone didn't do it. Now if I just had a double slider coil...One can clearly make a case for the Oatbox as a first crystal set - if it doesn't work in your location, nothing will.
17 Sep 00 Magic bullets: Been going back and forth with Mike Tuggle (aka the Big Kahuna) on our best diodes. I sent him some, which he thought mediocre (actually, so did I), and he sent me one of his best in return, a 1N297. So now I pull out my whole inventory, and start testing them head to head for weak signal sensitivity. My clear winner was out of a Radio Shack 1N34A "type" pack, and I had 5 of them (well, 7 if you count the other two I found installed later). I also had about 3 which I graded "second best", and then some I found "good". Mike's was in the "good" lot. So, back to the mailbox and send Mike one of each grading. His findings confirmed mine. The best ones were about 7 dB more sensitive than the "good", and really made an audible difference. Before you go running off to RS, a final note: all of the best ones were marked 12101 3RT . So take a magnifying glass with you to check them out. I apparently picked up a pack a little over a year ago, and that was the house number of the month. As a side note, I got some diodes for school today from my local RS, and they were actually marked 1N34A. This summer, I was seeing 1N27s in the store (good or worse category). Oh yeah, the 1N60 diodes were in the worse category, but are fine for kid radios. Actually, for picking up the locals, just about anything works. Some diodes shine with strong signals, but I was looking for weak signal (dx) performance.
Kid radios: Getting back to basics again, I pulled out my RS and Tectronics kits again today and hooked them up to my student antenna (see 26 July 00 notes). I found an antenna loading coil an absolute neccessity to reach the bottom of the band, below about 680, where my 5 kw stations about 70 to 80 miles away reside. It took the whole coil of 150 turns of #24 around a toilet paper core to get the 5 kw station 80 miles away at 550 kHz. I tried it connected to the set two ways: The first was to connect it to the "long antenna" tap on the set, and grounded the set. The other way was to just connect both antenna and ground to the loading coil, and inductively couple it to the set, either in parallel with or in line with the set's coil. Both worked, and for the latter, selectivity was variable as well. Yes, you can do some pretty good work with a single tuned set, but you won't find one to your liking in the marketplace; you'll have to build one yourself. For kid kits, make the loading coil and add it after the initial "shine" wears off.
3 Sep 00 Things have been slow in the laundry room since the beginning of another school year. About a month or so ago I picked up a pair of Utah magnetic phones, 150 ohm, and a pair of Murdock HS 16 phones, 125 ohm, also magnetic. The latter were made in '59, and there appears to be a modest supply of them available. Anyway, both were very sensitive, and are good dx chasers when used with a matching transformer. The HS 16 was clearly the better of the two.
26 July 00 Diodes: I was playing around with a little kit radio today, the one from Techtronics, just to check it's performance with a short antenna, and I noticed that not only did the crystal earplug not have a 47k resistor in parallel with it, but didn't need it, and that the diode was kind of funny looking. I reached into my junk box, the part with carefully labeled and sorted parts, and pulled out some 1N60 diodes I had gotten from Dan's recently. Yep, they were the same, and I had already tested them and found them lacking in sensitivity against my standards, including the ones I use for high level detection. Okay, time to experiment. I tried a variety of diodes in place of the 1N60, and guess what? They all pretty much stunk... until I put in the 47k resistor, and then they acted very nicely. Even most of the 1N34A type diodes benefited from the resistor, but the '60 didn't really care one way or the other. I suspect it has a fair amount of back leakage so the crystal earplug has a path to discharge between peaks.
Racheting back: one nice thing about my project radio is that I always have a tank circuit and an antenna tuner and lots of taps when I am trying something out, such as how my latest "student" radio will play in Peoria. I finally realized that I had better have a "student" antenna up as well, just to ensure I wasn't overestimating the capabilities of the sets, so I took my alternate antenna, which had outlived its need, and cut it back to 50 feet of wire out the window - 18 feet high(roofline) and the rest over to a tree horizontally. I even got it pointing north, where most of my big signals come from. It still tunes normally with the antenna tuner, but it doesn't grab the daytime stations quite like the older one, a little more than 100 feet long, did. It suffers at night too, as was expected. However, it will still pick up bona fide dx. Next effort will be to add a parallel line or two to the horizontal part in flattop fashion and see how much difference that makes.
23 July 00 The more I try the less I know. I have been playing around a bit with simple circuits in the last two weeks, and had some interesting revelations on the effect of connecting the antenna to the top of the tank coil. Let's assume, for the sake of discussion, that a grounded tank circuit consisting of a parallel coil and capacitor, is a quarter wavelength long. In that case, adding some 9 or 10 feet of wire to the top of the circuit would make it resonant, at the top of the BC band, about 100 kHz lower; WRONG. The effect is much more profound than that; it makes it resonant by about 3 times that expected. Adding as little as 3 feet of wire to the top of the tank can lower the resonant frequency by about 100 kHz, or enough to tune you well off a station you had tuned. That little capacitor you see with some set designs between the antenna and the top of the tank circuit is a necessity to keep from detuning the circuit. If you are like me, you probably assumed it just made the antenna "work better". If you actually know something about this business, you will talk about cancelling the inherent capacitance of a short antenna. Many communications receivers have an antenna tuning control, which is often simply a small variable capacitor to cancel antenna capacitance, to keep from affecting the tuning of the antenna circuit of a superheterodyne. With crystal sets, the problem is more serious. In the least respectful manner, one can get away with a very low Q front end with a superheterodyne ( and many do), since the serious work is done by the local oscillator, the mixer, and the IF filtering. With crystal sets, you have to pay attention to the details.
11 July 00 Some thoughts on magic bullets and free lunches: I have spent a lot of time messing around with simple radios this year, the latest being the Audion Receiver. All work to one degree or another, and can be lots of fun as long as you understand their limitations and obey the laws of nature. For example, more than one person has built a loop receiver of the "cigar box" variety, and suffered great frustration when it didn't work, or performed poorly. Any number of xtal sets have been designed and advertised, and faithfully built by the unsuspecting newbie, who doesn't know (perhaps the designer didn't either), that it can turn out to be really neat or a complete bust depending on expectations and how it is used. Let's review some basic first order axioms: (1) Sensitive headphones hear more. (2)A good antenna and ground are essential for optimum performance. (3)Impedance matching and hi Q do make a difference; a big difference.(4) Failure to observe (3) can negate any benefits of (1) and (2). Lets take a look at an example of this. Where do you connect the antenna to a simple set with a coil and a variable capacitor for the tank circuit? With a very short antenna, on the order of 10 to 20 feet, you are using it more as a simple signal collector, and should best connect it to the end of the coil opposite the ground end, but will still need to interpose some capacitance between it and the set, on the order of a pF per foot of antenna. As the antenna gets longer, you collect more signal, but the antenna becomes a real part of the resonant circuit, and affects tuning even more drastically. You now have to start connecting the antenna to a tap on the coil - the longer the antenna, the closer the tap is to the ground end of the coil. This gives you better impedance matching, and you are now working with what is, in effect, two tuned circuits, sharing the same coil. You will reach a limit here, no matter how much you try, and should start looking at separate tuning for the antenna. As an intermediate step, you can try a coupling coil, and your selectivity will increase. There is a cost in complexity and parts as you get more performance. Fortunately, it ain't that bad as soon as you learn some basics and dare to experiment.
7 July 00 Got a msg from Tom Rodriguez that Mouser no longer carried the 140/60 pF polyfilm variable capacitor I have finally come to know and love. Said they no longer had a supplier - this happened a couple of years ago with the dual ganged 260/260 pF polyfilm variable they and others used to carry. I checked the online catalog, and, sure enough, it no longer comes up. Also checked Circuit Specialists site, and they don't have it either. Electronix Express, Dan's, and the Goldmine still list it. The same variable is used by Transtronics. Radio Shack uses a single polyfilm capacitor of about 265 pF. What's on the horizon for availability? I don't know, but the news from Mouser bodes ill for those who build sets with kids. Unless the bottom drops out of the low end transistor radio market, and all the little portable radios start using tuning diodes ( it could happen), I am confident that something will be out there. On the other hand, we could become an even more elite niche hobby, using only vintage or homebrew capacitors.
27 June 00 Loopers revisited: After playing around with the MK484 radio a bit, and noting how it liked to have a ground for better performance, I went back and reviewed what I had done with simple radios (see my 17 Feb 00 comments). With one of the simple radios, I got something with just a single connection to a ground. So, today I did a little more with the loop idea. Hooking my good sound powered phones and detector to a tank circuit with a 5 inch diameter coil of #24 wire, about 265 uH, I found that even without a ground I could hear a few local stations, albeit weakly. Then I connected a ground the the base of the tank circuit and the signals improved considerably, and I was able to hear a 50 kw station about 70 miles away (daytime). Then I replaced the sp phones with a mouser xtal earplug, and still could hear something, but lost some loudness (actually, a lot). Okay, time to play around. I hooked up my little headphone amp and the stations came in much better. Tried it with smaller coils, and the signals went down, down with the coil diameter. Forget it without the amp on the smaller coils too. A ferrite core coil was useless. Guess the next thing to try is a larger coil made of some big wire. The message is clear, however; a looper will work, particularly if it is grounded. The headphone amp, just like I have on my crystal convertible makes it work even better, even though it isn't a crystal set anymore. There ain't no free lunch, and the laws of physics still apply. There is nothing magic about a loop crystal set, but in some cases you can get one working for you. I still don't recommend it for a first time project, however, unless you live uncomfortably near several really strong locals, or just want to listen to the peanut whistle just up the road.
22 June 00 I finally got around to
MK 484 IC radio. This was a breadboard model, unlike the neat
little package Phil Miller Tate sent me, and which you can see on his
Yahoo! photo site. Still, once I got started, it was a snap to
build, worked as advertised, and now I can fiddle with it to prototype
a "student " model. See it HERE ..
See the pix on Phil's photo page now - go to my links.
Had some fun chasing DX for a very informal Summer Solstice DX Party yesterday. There is still some nice DX to be had, but not as much of it, and the band is a bit noisier. Mike Tuggle still managed to get from Hawaii to the west coast, barely.
17 June 00 Getting the most out of a single-tuned rig; some thoughts and experiments. I had much better success in the hobby class of the latest dx contest than I thought I would have. My set was pretty basic, sort of; I used a main tuning coil 4 inches in diameter using close wound #20 plastic coated hookup wire. Inductance was about 250 uH. In parallel with it was a XSS 365 pF capacitor. The detectors, connected to the top of the tank coil, were a parallel pair of Hewlett Packard 5082-2835 Schottky diodes at night, and a single 1N34A-type diode from Radio Shack in the daytime. These led to a matching transformer a'la Ben Tongue, then to a second 4k:600 ohm transformer for my sound powered phones. The antenna was about 160 feet of wire in the air, between 50 and 30 feet off the ground. I used this in combo with a "Tuggle" antenna tuner for the Open class. For the Hobby Class, I couldn't tune the antenna ( per the rules), so did the next best thing; I used a loading coil of about 35 turns of #24 hookup wire on a 5" form, inductively coupled to the tank coil. The number of turns for the loading coil was arrived at experimentally, and got me down to the bottom of the band - it seemed to work acceptably at the higher frequencies as well. The reason I mention this is that conventional design wisdom for antenna transfer/ loading coils used in simple sets is that the antenna coil should have about 1/6 as many turns as the tank coil; following this philosophy, my coil would have had about 10 turns. I have used tapped antenna coils before without a capacitor, and they worked pretty well; a variable capacitor added to make it an antenna tuner gives you sharper tuning, but a tapped coil works pretty well by itself. However, my thinking now is that perhaps the number of turns for an untapped loading coil can be better selected than just using x number of turns. I plan on working with this some more.
29 May 00 High impedance headphones and other myths: From my earliest experiences with radios, I heard that the gold standard of headphones was 2000 ohm magnetic phones, and higher was even better - this is dc resistance by the way, so it is simple to check. Getting back into the xtal set hobby after a long hiatus, using ceramic earplugs since that was all I had readily available, I yearned for a set of the 2k phones so I could get some "real" dx. When I finally purchased a set, new, I even held off using them for a few weeks until I had a set I felt "worthy" of them. You guessed it; they really stunk. I had an old telephone (not so old really, it was just left unattended and unattached for too long to resist, so I liberated it), and the earpiece from it rivaled the ceramic plugs, but it only read 165 ohms of resistance. Then Larry Pizzella tells me how he used top quality walkman type earphones of 16 or so ohms of resistance, feeding them from a small transformer, and the hunt was on. My best set now is a pair of surplus sound powered phones, fed by a transformer (actually a couple) from the detector. I also have several nice 2k and 4k phones around. Here's my take on phones - dc resistance doesn't matter much; it is the sensitivity of the phones that does. So why the mystique of the 2k phones? Well, I suspect it is because of a couple of things. First is impedance matching between the resonant tank circuit of the set and the diode/phones. The second is about getting enough signal to the detector. The detector attached to a good set of 2k phones can be tapped high enough on the tank coil to give an adequate match and also provide enough signal level to overcome the detector signal threshold, about 300 millivolts for the commonly used galena and germanium detectors. The optimum tap point seems to be between 1/2 and 1/3 of the coil's length from the ground end of the tank circuit. You can tap all the way to the top of the tank coil, and many set designs do this, but while you get a even higher signal level to the diode, your selectivity AND sensitivity are really slammed due to an impedance mismatch, and you end up hearing even fewer stations since the big guys dominate the dial. As Ben Tongue and Al Klase have shown us, the thing to do is to get the most sensitive phones you can find, then use a transformer to match their impedance up to the top of the tank circuit. Yes, transformers involve a little more complexity and cost. However, decent ones with little loss can be had for anywhere from a couple of bucks and up (way up if you like). Go to Ben's pages for what you can use and how to use them. The balanced armature phones seem to be the most sensitive, and date back to the Baldwin headphones of the early part of the last century. Sound powered phones use this design, and they can be found in a number of similar applications, including some telephone handsets. Getting back to the 2k magnetic phones, I suspect that somewhere between 2k and 4k is the max practical limit for good sensitivity at a reasonable cost and which be successfully used without an expensive transformer. As far as the ceramic (crystal) earplugs go, they still work fine, the price is right, and they are good for kid (both small and large) projects - don't knock 'em. Oh yeah, my crappiest set of 2k magnetics work nicely with my one transistor amplifier.
16 May 00 So the y2k dx contest is over and the results announced. I retained the Georgia State Championship, and felt pretty good about it. However , Mike Tuggle swung in from Hawaii, knocking off west coast stations at 2500+ miles and the price of poker just went up. Just goes to show that location matters too, and Mike had a clear shot over the ocean. But on the BC bands! Unheard of. The details of his set will trickle out over the next few months, I am sure, but for now think big basket coils using big Litz. Another lesson hammered home is that the details do matter. A dB here, a dB there, and soon it adds up to more stations heard. In other words, don't sniff at marginal improvements.
Let me put in a plug for the Xtal Set Society . The web is great, and so are our on-line forums. I see something nearly every day that puts new ideas into my kit bag. However, digging through the archives for some of these nuggets is frustrating and time consuming. If you want to record it for posterity, or at least past the time when Al Gore uninvents the internet, how about sending something to Rebecca for the newsletter? A description and drawing of your favorite circuit is always welcome, and can be seen by the 700 subscribers that aren't wired in. Any hobby needs a common meeting place backed up by some hard copy to survive, such as a magazine or newsletter, and in this niche hobby the XSS is it. You can renew your subscription with your next submission. I am working on a couple of things to send in myself, so if you don't want to relive my crappy drawings, beat me to it. Twenty years from now, no one will remember your favorite URL, but the XSS literature will still be around.
More thoughts on Q: A popular myth in crystal sets is that selectivity is gained at the cost of sensitivity. Let me be one of the first to say it ain't so. There are several ways commonly used to improve selectivity in a set. One is to tap the detector down on the tuning coil. This gives you a better impedance match between the detector/phone circuit and the tank circuit, meaning better energy transfer. It also keeps the Q of the tank circuit preserved a bit as well. Lately some sharp minds have realized that transforming the detector circuit impedance up to that of the tank circuit not only keeps the efficiency of the tank circuit high while giving a good impedance match, but also bumps up the signal voltage to get it over the detector barrier. Yes, tapping down on the coil is a good idea, to a point, and tapping down even lower seems to improve selectivity, but at a cost in overall signal levels. In other words, there is an optimum tap point for impedance matching, if you have to do that, but tapping down even further isn't really doing much for you, particularly when you are trying to dig out the weak ones. If you have a tank circuit with an inherently high Q, you also have minimized resistance losses in the circuit, meaning more signal to detect. Using double tuning, that is, having an antenna tuner feeding the tuning circuit of the set, also improves both selectivity as well as sensitivity - the extra tuned circuit doesn't cost you. First, your antenna is now most efficient at the frequency of interest, therefore you have more signal to work with. Second, you are rejecting both man made and natural noise with the second tuned circuit, so your signal to noise ratio goes up. As further evidence of this, consider how wave traps on strong locals allow you to nudge up closer to them to hear weaker stations. When your set can separate stations in adjacent channels, you will start to appreciate Q. Even a simple coupling coil from an untuned antenna can help your selectivity; by controlling the coupling, you keep the Q of the tank circuit up, and have started the transition from a couple of stations heard to some real performance. Bottom line: good selectivity is essential to high sensitivity.
6 May 00 The winter dx season is over, so
broke out my superconvertible the other day to see it it still
worked. Using it as a regenny I picked up some daytime stations
that I had never
heard before. It is a tricky little set to operate, being a two
operation, but can really perform. I also checked out the simple
transistor audio amp that is featured on the basic convertible, just to
how it stacked up from my earlier enthusiasm - it too gave me pleasant
and will give a so-so set of headphones new life.
More simple radios. If you haven't tried it out yet, get one of the Radio Shack xtal sets and give it a try using my project page on it. Better still, try out some of your own ideas. Rick Moss sent me an email or two with some pix of his very modified set. Seems like every time I get back to basics I learn another trick or two.
Cutting out the intruders: In my neck of the woods, it seems, the hf broadcasters seem to sneak into your crystal set and make it tough sledding. Traps for these guys seem to work a little, but let your guard down just a little and they come roaring back. One technique that seems to work fairly well is to use a tuned antenna, but they still like to sneak in, suggesting that something else is at work here. No, they don't come in without at least an antenna or ground attached to the set, which rules out the need for a Faraday shield around the whole set. For the contest, I decided to try out Ben Tongue's scheme of impedance matching and using a high level detector tap, that is, tapping the detector to the top of the tank circuit instead of down the coil. An unexpected benefit of this arrangement was that I heard no intruders . My educated guess is that by tapping the detector down on the coil, needed when you don't have the matching transformer per Ben's scheme, you actually have two resonant circuits at work, the second being resonant above the BC band, and perhaps due in part to self resonance of the tank coil. Better made coils with less self capacitance, and a corresponding higher self resonant frequency, may help cut out the intruders as well. Incidentally, Al Klase has used the high impedance matching for the detector for some time now, and I was inspired to give it a try after he kicked my behind using it in last year's contest.
I see on the web that there are some more simple crystal sets available on the market, including one that looks like the old Radio Shack set. If you buy one of these little sets, how about giving me a short review of it? Every now and again something pops up that is better or worse, and it's nice to get the word out. I only link vendors I have tried and liked or haven't tried but have no reason not to like.
17 Feb 00 Simple Radios: I know I tend to rant about these on occasion, but there must be some reason or reasons they just won't go away. . You know the type I am thinking of; parallel tank circuit, usually with antenna and detector connected at one end, and ground at the other. No taps on the coil, and either the capacitor or the inductor is variable as in the "rocket" radio. Sometimes they are a "foxhole radio" or just a simple slider, without even the capacitor, but relying on coil capacitance to get resonance. I had several of these in the distant past, and thought they were pretty neat....then. I even posted a query to my panel of experts, and got back possible explanations for their being out there ranging from "technical incompetence", "corporate greed", and rip-off to "pinko commie plot". All of these have some merit, but then I finally figured out what I think is the real reason they keep showing up: they are not only simple but they work. I went and made one up tonight, hooking detector and a lead with an alligator clip to the top of the tank, and started touching metal, not even bothering to use a ground. Regular antenna - worked. Switch plate screw on wall - worked. Antenna on cordless phone (used to use the metal finger stop on dial telephone)-worked. Metal lamp-worked. Also tried using just a ground lead - heard something, but when I hooked ground to bottom of tank and put finger on antenna slot, could make something out. When I hooked up a decent antenna and ground, however, it still worked, but didn't tune as well until I went "back to the taps". By working, I mean I got two or three stations. This doesn't seem like much, but I don't live in the shadow of any powerhouse stations, and with the antenna alone got the three night time locals clearly. Also, when you can imagine the technical ability and mental state of a 10 year old boy, a very simple radio without batteries that usually gets something no matter what is a pretty neat thing to have. I remember attaching mine to fences, window screens, and anything metal about the house, mentally cataloging what worked, and what didn't. Simple radios have a whole bunch of limitations, and don't really impress the totally jaded techie, etc. etc., but they work. So, my possible recommendation for you out there when a 10 y.o. asks for a crystal radio, make up one of these, put an alligator clip on the end of a yard of antenna wire, and hand it over. You can set the hook when you start to get questions about how to make it work better. After all, it works, and that's the first hurdle.
15 Feb 00 Headphones: It seems to me that the limiting item for newcomers and would-be newcomers to crystal radio is finding good earphones. The supply of quality high impedance earphones is pretty much limited to antique sets, and the price is going up. I have a pair of new 2000 ohm magnetic phones from the XSS, but they are not very sensitive, and useless for going after dx in my neighborhood. High impedance crystal earplugs, which I get from Mouser, are very good, and the price is right, but these are fairly delicate, and a little knocking around can put them out of commission. Larry Pizzella uses high quality Sony walkman earbuds, feeding them with a matching transformer. Scott Balderston of Scott's Crystal Radios rounds up the older headphones, and evaluates their sensitivity before he puts them up for sale at reasonable prices. The Navy still uses sound powered phones for reliable communication on ships, and these work very well if you can find them. The are still being made, and come on the surplus market from time to time. Scott found a set for me, and they are the most sensitive phones on my wall. They are only about 160 ohms dc resistance (600 ohms impedance), but a matching transformer works very nicely for them. I tried a couple of 1000 ohm dynamic earplugs from Mouser, but, as expected, they were not even as good as the new magnetics from the XSS. A good set of cans will last and last, and are a sound investment, so get the best you can find. If you want to improve the results you get from the crystal earplugs, wear a set of sound attenuation earmuffs over them - you can get them from hardware stores and gun shops. The will reduce the background noise level immensely.
5 Feb 00 Radio on a chip: Sometime
while I was "out", Plessey/Ferranti came out with the ZN414 and the
ZN416 chips, which have three terminals, just like a transistor, but
are crammed with the components for a tuned RF receiver with agc.
All you add is a small handful of parts for biasing, etc, and a tank
circuit, battery, and phones, and you have a nice little
receiver. Just recently, I heard of the MK484 chip, which does
the same thing ( the ZNs are out of production, I understand, but still
available from time to time. I think Mouser sells an SK
replacement version of it , Mouser stock number 599-SK3246A, and
would like someone out there to tell me if it is - 7 July 00 someone
me it wasn't). The MK 484 is available in Australia and the
UK. Philip Miller Tate was kind enough to send me a working model
of a radio using
the 484, and is it ever hot. No external antenna needed, and it
pulls them in. Uses a tank circuit and 4 more parts; two
and two resistors. So, I went and ordered some of the chips, and
see how I fare in building one of these little sets myself. Ought
make a nice school project, particularly since it doesn't need an
or a whole lot of patience to operate (two items in short supply for
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