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Introduction
I sure love the smell of cut garolite in my kitchen shop. Welcome to my #74 radio page.
This is a medium difficulty project only because of the complexities surrounding the wiring
of the contra coil and switch.
The rest is smooth sailing!
The Idea
The idea behind this set is for a fairly inexpensive and easy radio to build that will
work with a wide range of antennas. This is accomplished by having a dual coil inductance radio.
By using standard quality 410 pF variable capacitors and a medium strand count litz wire,
a very nice radio can be constructed. I hope this project gives you some good ideas for
making your own crystal set. Feel free to change the design to suit your tastes and budget.
Circuit Description
This radio operates as a single coil radio. There is a single capacitor in series with
the antenna to the top of the tank, and another variable capacitor across the tank and
then to ground. This has been my favorite radio tuning circuit for single coils and
my favorite ATU or antenna tuning unit on dual tuned circuit radios.
With the adjustment of the series and parallel capacitors you can tune your radio
so there is the best balance between sensitivity and selectivity. This is my no free lunch
crystal radio theory: It seems that if you go for maximum sensitivity, the selectivity suffers
and vice versa. By having the adjustable antenna coupling with the series capacitor
and the parallel capacitor to
bring the tank back into resonance, it is easy to adjust for the best receiving conditions.
The coil is the "contra style" with the best technical description by it's
inventor, Ben Tongue.
It is a good way to make an efficient dual inductance value coil. Not only there are no
loose coil ends ever, but in the high range you have double the litz strands working for you.
It is truly a free lunch scenario. The inductance change is 4:1. In this case from 240 to
60 µH. Coil taps to change the frequency range is so old school!
I decided that following Ben's example, that I would put a tap on the coil to provide
a means for a little extra selectivity (at the price of less sensitivity). So I tapped the
top coil (when operated in series for the low band) at about 2/3 the way up. This gives that
extra bit of control to this radio. The tap goes to the diode.
This frequency range selection and selectivity adjustment is handled by a 3 pole,
4 position ceramic wafer rotary switch. Two of the poles switch the windings in a series or parallel
configuration, while the other pole is used to switch the diode connection to the coil tap
or top of the tank. I could have used two switches to do this, but this gave me the maximum
benefit from the switch I had. My switch is a 4 pole, 5 position, but I used what I needed.
The first two postions are the low range, sharp or broad. The last two positions
are the high frequency range, sharp or broad.
After the diode, it is only a matter of follow through. The resistance - impedance
equalization, or Benny consists of a 180k ohm resistor with a .1uµF capacitor
in parallel. This goes to the top of the impedance matching autotransformer, that I call an
Überformer. This provides a
much better impedance match to the tank circuit than connecting
headphones directly to the diode output.
The Coil
Now some of you are saying, "What? Dave, the king of the spider coil is using a cylinder
coil?". Let me explain. This is the coil that I made to test the contra coil principles. It is the
first contra coil that I made. It has been sitting on top of my computer ever since I was
finished testing it. So why not honor it by putting it in a radio?
The coil form is made from ABS plastic and is available at your local, friendly
Home Depot store. If you can, avoid the PVC type. This coil form, or I mean sewer pipe
coupling had ABS written on the tag. The diameter of this form is 4-½ inches or about
11,5 cm.
There is a ¼ inch gap (6 mm) between the coils. I drilled 8 small holes
to feed the wires through. The picture below shows how the holes are cut. See, I saved
1000 words. :)
Each coil is 22 turns of 165/46 litz wire. Make sure that the coils are wound in opposite directions.
I started each coil in the center and wound the coils in the same direction around
the form. Look closely at how the coil winding starts at the center of the form.
Label one coil L1 and the other L2. The wires in the center are
the "start" wires and the outside wires are the coil "finish" wires. It is important to
know one from the other.
The coil tap for the diode is 15 turns out from the center on L2. If you make yours
the same way as I did, then wiring according to my pictorial diagram makes it easier.
Construction
I used a piece of 3/16 inch (4,5 mm) thick garolite as the base and a 1/8 inch thick (3 mm)
for the panel. I like using the thicker material for a base, especially for a larger radio.
It gives the radio good perspective (art lingo). The width of the radio is 12 inches (30 cm).
The height of the front panel is just under 6 inches. I sometimes make the measurements just
under 6 inches as the material is 24 inches long. Taking a little for the scroll saw blade,
I can get 4 pieces from the material that way. No one would care if the panel was 5-7/8 inches
instead of 6, now would they? The front panel is held to the base by three small metal angle
brackets. I used flat head screws with countersunk holes. All the screws on the base are
flat headed types with countersink holes
There is a 6:1 vernier in front of each variable capacitor. To support the capacitor
behind the vernier, I made a bracket from plastic. This piece is held down by two angle
brackets.
The coil is mounted with two 2 inch (50 mm) plastic stand offs.
I wired the switch to the coil while the pieces were outside the radio. It made
it easier to wire.
Operation
The frequency range switch splits the band at around 1000 kHz. There is overlap in the
bands. This can be used to your advantage, depending on if the signal is weak or you
need more selectivity. There are several ways to get to this magic balance point.
You might switch the frequency range and retune if you are near the middle of the
band. You can set the two capacitors to achieve the best balance and there is always
the broad - sharp selectivity positions on the main switch.
Conclusion
Once you figure how to wire the main switch, the rest of the radio is easy. The contra
coil lends itself to a fine working radio so long as the two variable capacitors are separate
and not ganged. I am working on a single dial version of this set. It will require some
additional circuitry to make it work right. Keep watching my web site for further developments.
Thanks for stopping by for a visit. 73 and good crystal DX. Dave - N2DS
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