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Welcome to my #20 page.
This radio is a dual spider coil radio. The antenna input coil
is wound with 70 turns of 40/44 litz for an inductance of 350 µH. The
inductance of this coil was selected due to the antenna I was using. My antenna
is a 75 foot piece of wire about 10 feet over the ground. If I use my
80 meter dipole (or one half of it), I found that the inductance had to
be smaller. You may have to experiment with this for your own use. The
spider form AOL cd)
is attached to a dowel that can be pulled out or pushed in from the
front panel. This varies the coupling between the antenna and detector
coils.
The detector coil is a biggie. The coil is made from about
58 feet of 660/46 litz wire. The form
is made from a 1/16 inch styrene sheet. The piece is 8 inches wide and
8½ inches tall. The inside diameter is 3 inches and this form
has 11 slits. After winding, adjust the positions of the wires so they
aren't touching an adjacent wire. This will keep the coil capacitance to a minimum.
The inductance is 240 µH.
For most efficient operation, the coil is not tapped. With the
use of the detector circuit shown plus an audio matching transformer, the
high impedance is dealt with properly. There is a small variable capacitor that
couples from the top of the coil to the diode. This adjusts the impedance
and the signal level. It is possible to come up with the best sensitivity
while maintaining selectivity. A 27 mh rf choke
is used as a dc ground return path. A 330 pf capacitor removes the rf from the
circuit and the audio is sent to a transformer.
The RC network shown on the diagram maintains a high dc
resistance in the detector circuit for minimum audio distortion. Some
use a 500k pot to adjust for the least distortion. I found that around
here, that setting just didn't change much, so I put in a fixed resistor
equal to the transformer impedance.
The variable capacitors should be of high quality. Ceramic
insulators on the stator will give you a high Q circuit. Be sure to
look for a rotor wiper on the capacitor too. These capacitors are few
and far between but if you can find them, that just adds to the
performance of your radio. I don't think that my Remler capacitors
are that wonderful but I like how they look. A vernier dial mounted
on the antenna and detector capacitor will help you not miss stations.
The small 25 pf coupling capacitor should be mounted back from the front
panel with an insulated coupling so that hand capacitance doesn't upset
the settings.
About the name "Arachniodyne". My friend Fred Wise in MD
came up with that! Great name Fred!
This radio is kind of expensive to build, so keep some
money stashed away for this project.
Best wishes from -- Dave N2DS
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