[Lf] 73 kHz Signal Measurements]

[Andre Kesteloot]

John Andrews wrote:

> 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