[Lowfer] New Loop NG

[John Andrews]

Peter,

Some thoughts on your posting about the receive loop.

1. I think it may not be fair to expect the loop to produce signal strengths
comparable to the wire. The purpose of using the loop is to gain
directivity, and perhaps some freedom from local noise e-fields that are out
of proportion to their h-fields. A preamp is necessary with most loops and
receivers, regardless of the method of tuning or coupling to the loop.

2. Describe the balun you used. Also describe the preamp that you have
available.

3. In the series-tuned configuration, the input impedance of the receiver
(or preamp, if used) is reflected through the balun into the series circuit.
In my case, the input impedance of my preamp is an honest 40-50 ohms. Your
receiver may have a higher input impedance. If that is so, then it might
explain the broadness of the tuning. It might be very instructive to couple
to the receiver through a step-up transformer or balun, and see what
happens. That will also bring your signal level up going into the receiver.
Try 1:4 as a start.

4. To expand on the above, let's assume that you are correct in saying that
the loop resonates with 200 pF at 200 kHz. There might be another 100 pF in
distributed capacitance in the loop, so the actual tuning capacitance could
be 300 pF. That is a reactance of about 2650 ohms at that frequency. If you
introduce an honest 50 ohms in series with that tuned circuit, the Q becomes
53, and the 3 dB BW is 3.7 kHz, which should be noticeably sharp.  On the
other hand, if the receiver input is actually 200 ohms (not impossible), the
BW increases to 15 kHz, which is not so pronounced a peak.

5.  Your description of the behavior when the link-coupled method was used
is a little puzzling. In that case, the loop should just be tuned with the
cap, and the link fed to the receiver or preamp. The balun has no use. If an
unterminated balun is connected in series with the loop and the tuning cap,
things SHOULD fall apart.

Thoughts, anyone?

John Andrews, W1TAG