[Lowfer] TH and HS

[John Andrews]

Bill,

> This is where I'm confused. Wouldn't increasing the step-up ratio actually
> reduce the Q since the R seen by the loop would be lower? But this is a
> series circuit...... Hmmm. I'll have to work this out on paper.

The resistive component of the series tuned circuit is the the sum of the
radiation resistance (vanishingly small), the various loss resistances (in
the 1 ohm category, as you mentioned), and the resistance reflected into the
circuit via the coupling transformer. My 1:2 transformer feeds a 50 ohm
preamp with a 20 dB return loss at the input. Therefore the transformer
pretty much sees 50 ohms, and transforms it down to 12.5 ohms in series with
the loop. So you'd figure the Q from dividing the loop reactance by about
13.5 ohms, to use the numbers above.

> Looks like one of the biggest objectives to successful RX looping is to
come
> up with a simple remote-tuned 1000pf(+) variable capacitor. I don't trust
> those tuning diodes in my high-noise environment. They seem to sensitive
to
> out of band noise and modulate the desired signal. This new loop without
> diode tuning or pre-amp is the first loop I've used that is almost dead
> quiet when I tune the receiver off to the side of the loop resonant point.
> (And the Q is no higher than the others) I suppose one of my big
compression
> mica timmers could be motor-driven with a compact gear reduction unit.

As I've noted here before, I use 8 small relays that add capacitors in
parallel. The nominal values are 50 pF, 100 pF, 200 pF, 400 pF, 800 pF, 1600
pF, 3200 pF and 6400 pF. That would theoretically provide 50 pF steps, were
it not for my sloppiness in construction and in picking the caps. But I can
tune from below 50 kHz to almost 500 kHz in this manner. I just have a box
with a bunch of miniature toggle switches to select the combinations. I've
got a fun project planned to program a microcontroller to read the R75
frequency via the receiver's serial port, and tune the antenna
automatically.

JA