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Here I take a new step in my crystal set building with
this interesting radio. This crystal set uses two of my favorite
circuits. It also uses a basket weave type coil. More on this later.
I would like to bring to your attention the circuit
diagram below. There are two circuits that I have favored. First
is the series-parallel tuning with one coil, such as in my set
#12 and others. The selectivity is controlled
by the series tuning capacitor. The circuit resonance is controlled
by the parallel capacitor. By careful adjustment of the two capacitors,
a good balance between selectivity and sensitivity can be achieved.
One bad feature of a single coil radio is the inadvertent
pickup of shortwave stations. This is called "ghosting". A double
coil receiver filters out this undesirable reception. By changing the
switch position, another coil is placed in the circuit.
This circuit acts like my
#6 radio (in the
series tuned switch position). The first
capacitor becomes part of a series tuned circuit with the antenna
coil. The rf is inductively coupled to the detector coil. My design
allows for variable physical separation or coupling of the two coils. It is
sensitivity vs. selectivity. Moving the coils closer does increase
the volume, but the selectivity is impaired. There is no free lunch,
as they say.
The basket weave coil that I mentioned earlier is called
a rook coil.
There is information there on how to obtain or build a coil
form to wind these coils. It is very easy to make coils this way.
However the coils are a little delicate and require some gluing.
They do look pretty cool. The antenna coil is
wound with 40 strand of 38 gauge litz and the detector coil is wound
with 3/22/38 gauge litz. I chose the smaller litz for the antenna
coil so there would be fewer windings, thus a smaller coil. The antenna
coil has an inductance of 240 microhenries. The coil form diameter is
slightly larger than 4 inches and has 43 turns of wire. The detector
coil has 40 turns with a diode tap at 25 turns This coil measures
around 210 uh.
Of special note is the little track I built to slide
the antenna coil. I used the same method to hold the coils as in
my #16 set. The wood blocks are 2 inches long
and 1/2 inch square. The wooden dowel rods are 10 inches long and
3/16 inch in diameter. The diameter of the rod is just right for
holding the coil. The antenna coil can be slid several inches to
adjust the coupling of the two coils. If you are in the big city
you might want to have the coils further apart, but if you are in
the sticks, you might want to slide them closer. The only downside
that I see to this arrangement is that a lot of adjusting could
wear the litz coils. Perhaps some plastic tubing (soda straws)
could be inserted between the dowel and the windings to eliminate
this possible problem.
For switching the circuit, I used my familiar brass link
technique. They just look neat. The front panel material is garolite.
I use wide masking tape over the face to lay out where the holes should
be drilled. This works very well. I use a forstner bit to make the half
inch holes for the variable capacitors.
This radio tunes quite well. It looks like this one
covers the whole band, but around here there is nothing below 660 khz,
so I am not certain. The basket weave coils have low internal capacitance,
so the tuning range is wider than with cylinder coils. The 38 gauge
litz is somewhat large for this frequency, but the wire does work
very well when wound on a rook coil form.
Go ahead and build this radio. I don't think you will
be sorry that you did. Best wishes and good DX. Dave - N2DS
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