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I have never built a radio with more shape than this. This is not
some mickey mouse radio. This radio is a high performance set suitable as an entry in
the Crystal Radio Contest in the "Hobby Class" section. Since this is a single coil
set, a little more finesse is needed to pick out the weak stations buried among the
blowtorch senders. I used mammarial efforts in building this set!
I call this a high performance radio because several of my conditions
have been met. The coil is made up of the highest quality 660 strand, 46 gauge
litz wire. Also, this coil is wound on a homemade spider coil form. The distributed
capacitance is very low, so this helps the sharpness and the tuning range. The coil is
similar to my Überlitz ® coil
but wound to about 170 µH.
The variable capacitors are also high quality. There are ceramic studs
to insulate the stator which makes for a very low rf loss capacitor. In a high
tank impedance situation, having low loss components around the coil is very
important. Connected to the capacitors are vernier dials. If the set tunes sharper,
having slow tuning is a must. Stations can be peaked very easily. The number scales
also help to reset the dials to particular setting.
A schottky diode becomes important
in a high performance radio.
Since the high Q coil is not tapped, a very high impedance exists at the top of
the tank. The advantage of this selective circuit can be ruined by using a regular
detector circuit and diode. I used a low capacitance trimmer, schottky diode
and a 27mh choke. This combination performs very well.
Last but not least, since the impedances are very high, much higher
than a 2000 ohm set of headphones, a transformer should be used to get a closer
match to the detector and tank. Ideally, a 100,000 ohm or higher input, heavy iron
transformer, matched down to your headphones is what the doctor is ordering here.
If you can't find one of those, a Bogen T725 speaker
matching transformer will do
you. If you can find some good sound powered headphones, this will be the icing
on the cake! Examples of transformer coupling can be found on my
audio transformers page.
Here is how to use this type of crystal set. The selectivity (4-30 pf)
capacitor should be set to the mid range to begin with. If you would like higher
selectivity, reduce the value. If you want more volume, increase the value. If you
want, this could be a front panel adjustment, but make sure to set the capacitor
back a ways from the front panel and use an insulated shaft to connect to the knob.
Otherwise you will upset the tuning when you move your hand near the knob.
When tuning the band, I like to start at the top end. I set both
capacitors at the minimum value. Then I adjust the antenna capacitor and listen
for the stations. As the capacitance is increased, the received frequency will lower.
Keeping the antenna capacitor at a minimum value, decreases the likelyhood of
bad selectivity and short wave ghosts. You won't be able to tune the band with
the detector capacitor at the minimum setting. Next move this capacitor about 10%
higher in capacitance. Readjust the antenna capacitor to minimum and work down
the band again. When you are tuned to a station, rocking both capacitors for the
best volume with least interference will sharpen the tuning. Once you find the
optimum settings, write down the dial settings so you can later return to the
station. Changing the antenna will change the dial settings.
73 and good crystal DX. Dave - N2DS
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