LF: Big vs small RX antennas

[MarkusVester@aol.com]

Hi John and LF group,

> Just curious -- since the signal to noise ratio in my receiving installation
> appears to be limited by external (man-made and atmospheric) noise, rather
> than noise in my preamp/receiver, how would a 100 foot tower improve that
> situation?
>
> John Andrews, W1TAG


A big vertical does help a lot against local noise-sources (neighbours' TVs and SMPSs), but with anything originating further away than a couple of 100m, it makes absolutely no difference.

However the directivity provided by a magnetic receive loop can be valuable. If all of the noise was coming in isotropically from the horizon, the figure-eight pattern would theoretically have 3dB better SNR than a vertical, and a cardioid combination would gain 4.8dB. In practice, of course one can often null out a source of QRM or a thunderstorm front and have far greater improvement.

The only problem I am having with small loops is that they seem to be much more prone to local pickup than the E-field antenna - at least in my suburban area which has underground mains wiring. The 86cm-diameter pair in the garden is often swamped by notorious 100Hz-modulated carriers which at the same time I can hardly see on the marconi.

73 de
Markus, DF6NM