Those of you who have looked through
my pages might notice that I try to make many of my sets so they can be built
by someone with modest ability and access to parts. This is driven
in part because I try to make designs that my students can build successfully.
On the other hand, for my own use, I have spent a couple of
hundred dollars on mega-strand litz wire for high Q coils, have invested
similarly in sound powered phones for my competition sets, and, thanks to
the kindness of others, have some pretty decent high Q capacitors. I
also realize that many people just starting out in crystal radios are just
trying to make the first jump from the "toy" crystal sets to something with
better performance, and might not be ready to invest a lot of time and money
until they are convinced that crystal sets can do all the things we claim,
performance wise. For this reason I have chosen to try to design and construct
a set that will give real DX performance using readily available parts, and
which can be upgraded as the builder's inclination dictates.
design I have chosen is tried and true, using double tuning - a "Tuggle"
tuner for the antenna with inductive coupling between the antenna circuit
and the detector tuning circuit. The schematic is shown below:
I provide a tap on the detector coil suitable for using a standard
set of high impedance earphones, or crystal earplug, and also allow for tapping
the detector to the top of the detector tank circuit. Set layout and
construction should be fairly straightforward.
Parts used in the set are available from the Xtal Set Society
and your local Radio Shack:
From XSS: 365 pF air variable capacitor, dual gang Cat #365-2
365 pF air variable capacitor, single gang
Cat # 365
Germanium Diode Cat # DIO
Crystal earplug Cat # Ear
47k resistor Cat # RES
Fahnestock clips Cat # FC25 - this gives
you 25 clips, but you will only need about 5.
From Radio Shack:
2 (80 foot) coils of plastic coated
solid Hook-Up wire, 20 gauge Cat # 278-1216
Pack of alligator clips
Anything else I might need can be had from local stores, such
as Ace Hardware, the drug store, lumber store, etc. I needed some wood
to mount my parts on, some screws, such as for putting the "chassis" together
and mounting the fahnestock clips. Much of this stuff I already had
laying about, but it's easy to come by, and doesn't cost much.
Coils and Variable Capacitors:
I got both of my capacitors from the XSS, and, having
access to a capacitance meter, measured their ranges. The single gang
cap measured 20 - 400 pF. The two gangs on the double ganger measured
15 - 373 and 15 - 385. Incidentally, when I was inspecting the two
ganger, I heard a slight rubbing as I was turning the shaft. My initial
check with a resistance meter to check for a short between the plates indicated
that it was okay, but I still heard the rubbing. I then put the resistance
scale on the meter to 20 Megohms, and found that one side of the capacitor
gave a reading of several Megohms when nearly closed. I was able to
find and slightly bend the offending plate, one of the outside ones, and
the rubbing stopped. I had probably bent the plate accidentally while
playing with the capacitor. Since I have sufficient range on the capacitors
for tuning the broadcast band, I used a nominal max capacitance to calculate
the inductance needed in the coils and came up with 250 uH as the value to
shoot for. I had already decided to use the #20 hookup wire as it was
available in the store and my experience was that it makes a pretty decent
coil. With the plastic coating, it takes 15 turns per inch when closewound,
which just about gives you a one wire diameter spacing between the turns.
I then went to Wheeler's formula to calculate the dimensions for a
and found that a coil with a diameter of 4 inches and a length of 4 inches, using 60 turns of wire should do it.
Now for the coil forms; I bought a sheet of poster board
at the stationary store and wrapped it around an empty 40 ounce JifTm
butter jar, using liberal white glue and a bit of masking tape, to make two
ply coil forms. I made each form 5 inches long to give me some working room;
you can always trim off any excess later. Diameter worked out just
right for me with the jar to make the form on. I know that the experts
will tell you that this isn't the absolute best material, but it is easy
to get and work with, and works pretty well. I put a coat of polyurethane
varnish on the finished forms to keep them from absorbing moisture and to
stiffen them up a bit for winding. The antenna tuning coil was just
60 turns of wire closewound with each end tinned to connect to the capacitor.
On the detector coil, I added a tap at 20 turns from one end to serve
as a detector tap.
Again, relying on the kindness of strangers, I measured
the two coils and found them to be 241 uH; close enough to calculated.
Putting them on the test bench I came to the horrible realization that
this is probably the absolute worst time of year to test a crystal set for
performance - hi. However, I will continue to press on and see how
the setup works. Initial indications are that I might want to pull
a couple of turns off the coils as I still seem to have some capacity left at
the bottom of the band. I'll think about it. Again, relying on the kindness of strangers, I went to Brian Hawes'
site and downloaded his coil calculator, which uses Nagaoka's formula for
calculating coil inductance, and it came up with a calculated value of 241
uH for both coils - go figure.
Now that the coil and cap combination seems to be working,
it's time to concentrate on the base for the set. Looks like the optimum
coil separation is going to be about one coil diameter, so I will have to
give them a support about 15 inches long. Scrounging around in the
dust bin, I found a piece of 5/8 inch plywood for the base, and a 48 inch
dowel that is 1/4 inch in diameter; just fine for making rails to support
the coils. I also want to get the coils at least one coil diameter
away from metal, so will put the rails about 9 inches above the chassis,
using some 1/4 inch plywood side supports for the rails. After cutting
and drilling, I put a quick stain and finish on all the wood (left everything
out in the sun). Assembly starts tomorrow.
I mounted the caps on the base of the chassis using 6-32
screws, although I have also used double sided sticky tape for this in the
past and used it for a couple of years. Again relying on the kindness
of friends, I used a 6-32 tap to allow me to fasten a solder lug onto the
frame of the capacitors in a convenient location. The single gang cap
has tapped holes only on the bottom of the frame, and the double ganged cap
has tapped holes on the bottom and the front of the cap; not particularly
convenient for connecting to the capacitor, but usable in a pinch. For
making my connections, I had some flexible #18 stranded insulated wire, left
over from some other project. Electrical zip cord works pretty well for this
too. Since I can never leave a project alone, and might want to play
around with the connections a bit, I used alligator clips to make connections
to the coils, and also on the ground and antenna leads. For knobs,
I drilled out the holes on a pair of large wooden cabinet knobs and forced
them onto the capacitor shafts.
here is where a picture of the assembled set will go
antenna and ground, I gave the set a whirl. I got full band coverage,
but stations got rather close together at the top of the band, as expected.
I tried it with three different antennas, with nominal lengths of 50,
100, and 150 feet, and they all worked about as I thought they would. I
decided to take 4 turns off the antenna coil and one turn off the detector
coil, and got a little more room at the top of the band. The crystal
earplug worked much better when I put on a pair of hearing protection muffs
on to block background noise. Interestingly, I found that using the
tap on the detector coil didn't work nearly as well as I had expected, even
though that is supposed to provide a better impedance match to the resistor/earplug
combination. Instead, just connecting the detector to the top of the
tank circuit worked better, even when using a pair of 2000 ohm magnetic phones.
As expected, using an impedance matching transformer and some sound
powered phones worked even better, but that violates the "off the shelf"
precept of the set. The same results happen with coils and capacitors of
higher efficiency (Q); performance improves. Optimum performance was
achieved when the separation between the coils was about one coil diameter.
In my location, I don't have any power house stations,
but a peanut whistle station at 1490 kHz with a strong ground wave makes
it tough to get to the dx stations above that frequency. Every year
at contest time I have to work my buns off to sneak up there to snag stations.
Just for grins, I took a trick from Ben Tongue's Web Pages
and moved the tap on the detector coil to the center of the coil.
Then I connected one end of the capacitor, along with the diode, and
then put a short jumper between the two ends of the coil. By measurement,
this gave me an inductance of 34 uH, but all of a sudden, there were clearly
two stations at 1570 and 1600 that I usually have to struggle to get. The
configuration for the detector circuit is depicted below, as is a way to
reconfigure the antenna tuning, using only one section of the two gang capacitor,
which raises the resonant frequency of the antenna circuit a bit, but really
doesn't work as effectively as the straight "Tuggle".