Steve,

Do I take it that your loop lies in a horizontal plane?  Most of the frame
loops for LF receiving are in a vertical plane.  Thus there is a further
matter of polarisation to consider (and it is near field polarisation) for
mutual coupling considerations.

The absolute gain of your short vertical will be higher than your loop, so
whatever figure of mutual coupling is found to apply, on receive the loop
will be affected more by the tuning situation of the vertical than the other
way round.  With smaller frame loops for receiving the impact of the
vertical for reception on the loop is even more pronounced (due to the much
higher absolute gain of the vertical).

I find that on receive, unless the LF vertical transmitting antenna is
detuned, it receives local QRM from a fairly wide area and re-radiates it to
any LF receiving antenna on my property.

Your measurement is very interesting information.  Another application of it
is to confirm that survival of any pre-amplifiers on receiving antennas when
transmitting on another antenna.

73, Bob ZL2CA

Subject: LF: Coupling between two antennas

Hi All, from GW4ALG.

You may recall that, regarding my two LF antennas (a single-turn
balanced loop of 65 m perimeter; and my 12 m vertical _in very
close proximity_ to the loop), I wrote:

> . . .  when monitoring the TX SWR into the resonant vertical . . .,
> I found no change in SWR when:
> - tuning the loop antenna either side of resonance (with the
> coax-side of the tuner terminated in 50 ohms);
> - disconnecting the shack end of the balanced feeder from the
> tuner;
> - applying a short circuit to the shack end of the balanced
> feeder; or,
> - grounding the shorted balanced feeder.
> . . .   I'm very surprised that there appears to be no significant
> interaction between the two antennas.

Dick PA0SE commented:
> Even when the coupling between vertical and loop is so weak that
> the SWR of the vertical does not change when tuning the loop the
> coupling can still be strong enough for signals from the vertical
> getting  into the loop when receiving.

To try and quantify the degree of coupling between my two
antennas, I carried out the following procedure:
1) The loop antenna & tuner system was tuned to 135.9 kHz and
terminated in a 50 ohm carbon resistor.
2) My 400 watt transmitter was operated at 135.9 kHz into the
resonant 12 m vertical.
3) The voltage across the 50 ohm resistor was measured using a
Tektronix 465 oscilloscope.

The measured voltage was 8 volts peak-to-peak, corresponding to a
power of about 60 mW.

I then did the test again, this time feeding power into the loop,
and measuring the voltage developed across the 50 ohm resistor
(which was now used to terminate the coax feeder from the 12 m
vertical antenna).  Again, I measured 8 volts peak-to-peak across
the resistor.

I had expected greater coupling between my two antennas.  In
practice, there appears to be an isolation of something
approaching 40 dB between the balanced magnetic loop and the 12 m
vertical.

Regards to all,
Steve GW4ALG