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After 20 comes 21. It is too bad I am all out of those
Remler capacitors. Ok, on to 21. This crystal radio features two
coils. One tunes the top half of the broadcast band and the other
coil tunes the bottom part. This was done to increase efficiency and
control the bandwidth a little bit better. I also wanted to see if
having separate coils was better than tapping a single coil. Right
now the jury is out on this matter. I write these pages soon after
I build them and wait until the evening to test them.
I won't spend much time on the construction details
as these are well described in my other sets. I did something that
might be of note with this set and that involves the rounded edges
on the front panel. The contour of that edge matches the knob size.
I traced the knob edge along the corner as a cutting guide. I cut
the edge with a hacksaw at a 45 degree angle and then cut two more
times at 30 and 60 degrees. I then file and finally sand the
edges.
Before I cut that edge, I drew a 45 degree line from each corner
down through the center part of the garolite panel. Then I placed
the tuning shaft of the capacitors on these lines, 4 inches up from
the bottom of the panel. This gives this radio a better visual
flow. By the way, before I do any measuring and cutting on the
panel, I cover it with masking tape and mark my hole positions
on the tape. I am not sure if I had mentioned that before.
The coils have a tap in the middle for the diode
connection. I found that a 50% tap was a good compromise between
volume and tuning sharpness (selectivity). I checked this out
with my #20 set. This applies only with
these type of coils and circuit.
The low frequency coil has a total of 42 turns with
a measured inductance of 220 microhenries. The high frequency
coil has a total of 30 turns with a total inductance of 140
microhenries. If you use a different size of wire, the number
of windings will be different. With the 3/22/38 litz wire that
I use, the holes on the low frequency coil are at 1-1/16 inch
intervals (2-1/8 inches end to end). The high frequency coil
has holes placed at 3/4 inch intervals (1-1/2 inch total distance).
I use a Styrene 4 inch sewer pipe coupler as a coil form. The
outside diameter is 4-1/2 inches. This pipe is available from
Genova Products
(# S40140) or from Home Depot. Don't buy PVC couplers, only
styrene.
The basic circuit is like my #17 and other sets.
This is my favorite crystal set circuit. The only problem with this
circuit is because of the single coil construction, there can
be some shortwave leakage. A trap tuned to 6 mhz should take care
of this problem.
I labeled this set in german, just has I had with
my #14 set. It looks kind of cool. I use
a Brother P-Touch labeler. Besides that my
ancestors are krauts,
errrr, I mean german.
I write these pages so that you might get an idea or two
for building your own crystal set.
The main thing is that you have fun doing it. I have not
had this much fun with radio in 20 years, so I am happy to
share all this joy with you.
Best wishes from -- Dave N2DS
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