[Lowfer] New Loop NG

[Peter Barick]

John Andrews writes:

>>> w1tag@charter.net 04/15/02 06:58PM >>>
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.
*PB: Yes, ultimately and I had planned on using one, but initially I
was looking for resonance.

2. Describe the balun you used. Also describe the preamp that you have
available.
*PB: The balun is 1:1, 2 X25 t. on a 75 core, from the LWCA site. The
preamp is Lyle's High-Gain single ended with provision for loop or low-Z
inputs. I had previously used the amp indoors w/ a parallel tuned box
loop and was fimilar w/ how it tuned and the general performance of a
loop like that.

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.
*PB: I can try that. Presently the 2X25 turns balun is side-by-side
wound. Can you reco a winding scheme for the 1:4 (pri : sec) balun?

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.
*PB: The rx is Drake R-8 w/ 50 and 500 ohm inputs. I used the 50 ohm w/
coax.

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.
*PB: Right. When adding the link I shorted the balun winding.

Thoughts, anyone?

John Andrews, W1TAG

*PB: Thanks for your helpful input, Peter