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It finally had to come to this. A 660/46 litz wire crystal set.
The litz wars have started. This is kind of a "litzkrieg". I imagine that
we will need a Large Litz Non-Proliferation Treaty. Perhaps
Owen Pool could monitor compliance?
This set is based mostly on the recent work that
Jim Frederick
did on his high performance crystal set. His is a variation of this earlier
Australian
design.
The standout feature of Jim's set is the
use of a differential variable capacitor to adjust how the diode is tapped
in the circuit. A differential capacitor is a dual gang variable capacitor.
The difference is as the capacitor is rotated, one side increases in capacitance
while the other side decreases. These were used in the old days to make low
drift oscillator circuits. Here, this capacitor is used as an rf voltage divider.
As the capacitance between the high side of the coil and diode increase, the
volume will tend to increase. But the selectivity decreases. If you need
to tune sharp, make the coupling looser. The beauty of using a differential
capacitor is that the detector tuning shifts very little as that circuit sees
almost no net change in capacitance. Note how I mounted the capacitor. It stands on
an insulator back from the front panel. The shaft coupling and the shaft
extension are also non-metallic. If you don't take these steps, you will
land in "hand capacitance hell".
The RF choke is to provide a DC path for the detector to work.
It is a 25 millihenry choke. Smaller ones were tried but those are more
for the shortwave frequencies. Anything above a 10 mh choke will work fine.
The output is made to work with an audio
matching transformer. I use a 100,000 ohm to 1500 / 6000 ohm transformer.
The transformer (and that 100k resistor and .01 capacitor) are
not on the main board. I have one output circuit now for all my sets. The
transformer drives my semi-homemade sound powered headphones. A crystal earphone
may be used instead if a 100k - 220k ohm resistor is placed across the output
connections.
There are two coils in this radio. The main detector coil is
wound using a
rook coil form.
I used a 4 inch form. The coil is wound using the "One up, Two down"
(1U2D) scheme. This
coil has 47 turns (240 uh). It takes just about 50 feet according to my
little spreadsheet program.
Other less expensive
litz wire may be used or even magnet wire. The coil efficiency and selectivity
(Q)will be less. It depends on how crazy you want to get.
The antenna coil has 64 turns of 100/46 litz.
You can also use 40/44 litz too. The inductance
measures to 300 microhenries. The ferrite rod I used is 6 inches long and
3/8 inches in diameter. This is a rod I had in my collection for a long time.
I am not sure of the material of this one but it was originally an AM antenna coil.
Here is a recommended place
to buy ferrite rods. Buy the Ferrite-61 material.
The ferrite coil is mounted on a slide arrangement for variable coupling
of the two coils. The closer the coils are together, the louder the signal,
but you lose selectivity. You just can't have it both ways!
The variable capacitors are also important. I used ones that
have a ceramic insulator, rather than the phenolic insulator. The circuit
Q also depends on having good variable capacitors. I used styrene instead
of garolite as I am not sure of the rf properties of the garolite.
This radio has a vernier dial on the main tuning capacitor so I could
tune without missing stations. A half a turn tuning doesn't cut it with
the big litz. It might be also handy to put a vernier on the input capacitor
too. In lieu of the vernier dials, a small band spread capacitor could be
placed in parallel with the main ones.
How does it perform? It does tune sharp and I can hear
lots of stations. The band is at its worst during the summer months.
This radio is the loudest set I have ever had. There are no blowtorch
stations near by, so can hear all the locals during the day really well.
At night I can hear stations all over the Northeast US.
I can't wait for winter conditions (not the winter weather).
Best wishes and good DX. Dave - N2DS
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