I started pondering ways to make useful measurements of 70-75 kHz commercial
signals coming from across the pond. My receiver has a useless S-meter, so
we'll skip that approach. Here's one possibility with a surprising result:

The following was done between 0130 and 0200 tonight. My loop preamp is not
at the antenna, but rather in the shack. I put a variable 50 ohm attenuator
between the output of the preamp and my receiver. Tuning to 75.0 kHz, I
heard what I assume to be HBG. I started ARGO, with the 3 sec/dot screen
(basically because I want instant results). I found that it took 65 dB of
attenuation to remove the HBG line from the screen. Dropping down to 73.30
kHz, it took 60 dB of attenuation to kill the rather broad line from the FSK
signal. Considering that the Rugby station's power is spread over 100 Hz
(though concentrated at 73.2 and 73.3 from the look of it), that seems
reasonable. In any case, that's a lot of attenuation.

Of course, there's a lot to be gained by narrowing the BW, so I ran ARGO at
60 sec/dot, and had to crank in another 13 dB to kill HBG. There's a
considerable time lag in making such measurements, so fading sets some
limits on accuracy. But I did end up barely being able to see the signal
at -77 dB. Interesting, eh?

One possible objection to the method is that the absolute value of the
attenuation is probably in error due to the output impedance of the preamp
and the input impedance of the receiver not being 50 ohms. While that's
true, at least the changes in attenuation needed to kill the signal under
various band conditions would still be valid. This will enable me to track
things on a day-to-day basis.

Thoughts and criticisms would be appreciated.

John Andrews, W1TAG