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This is the Crystal Radio to End all Crystal Radios.
Hello there. Can't you just feel the tension rising? With this new radio I shift
gears again. Large litz wire, high quality variable capacitors, schottky diodes.
It won't get much better than this. Last night I am proud to announce that I heard
KMOX in St. Louis on this radio. That is my furthest DX ever. I am sure that the
ionosphere was hot and the wind was blowing my way, but a heard station is a heard
station. Enough bragging and on with the show.
This radio is built in modular form.
Mike Peebles
gave me the idea when he had his modular crystal set published in the
Crystal Set Society
newsletter. The first part built is the
detector. This consists of the main tuned circuit with my
660/46 Überlitz Coil. This is of my
own design. I am a big fan of litz spider coils,
and this is a big litz spider
coil. I believe I hit crystal set heaven with this coil. The detector unit also
has a selectivity / sensitivity control using a differential capacitor. Besides
all of this there is, of course, the detector diode itself. I made a board with
a three position link switch and places for three diodes to be connected. This
allows me to easily compare 3 diodes in the same set. After the diodes is an
audio output transformer and RC network. The 27 mh choke
provides a DC return path for the diode circuit.
The variable capacitor is an old one made by National Radio. I added a
6:1 vernier drive between the capacitor and the knob.
I can tune this set accurately without missing stations. I made a dial scale
from white styrene. The numbers are made by using a Brother P-Touch label
maker.
The selectivity and sensitivity are handled by a differential
capacitor and a piston trimmer. This system was first used in the
Hobbydyne II circuit by Jim Frederick. The piston trimmer is for tracking adjustment. You adjust
the piston trimmer until the main tuning needs no touch up after turning the
differential capacitor. This is a breakthrough
in weak signal reception on a crystal set.
The audio transformer matches the very high (100 k ohm +) detector
circuit with the much lower impedance headphones. I am using a 100k to 1.5k ohm
transformer. If you can find a high quality transformer, go for it.
My antenna tuning unit is the other section of this dx radio.
The coil is made from 660/46 litz.
The hub diameter of the coil is 3 inches. The coil form made from 1/16 inch
styrene sheet is 6-1/2 inches wide and 7-1/2 inches tall. There are 33 turns
on it for an inductance of 150 µH. The variable capacitor has 600 pf
per section. Smaller ones can be used, however the low end of the band might
not tune. You can get away with a 500 pf per section capacitor.
Earlier I had used a 150 micro henry coil made from 40/44 litz
wire. This worked quite well. One night very late, I scanned the dial and
with this set up I heard 40 stations. Not bad for a September night.
The two units are placed next to each other. The coil distance
for good overall operation are about 15 inches apart. Careful adjustment of
the antenna tuner and detector tuning are required. The Q of these types
of coils have been measured at over 1000! This ain't your grand pappy's
crystal set.
If you build one of these, let me know how it plays for you.
Happy crystal set dx. Dave, N2DS
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