With many local noise sources to contend with and too many nearby AM
signals w/ splatter and digital sideband hash, I've concluded nulling
is as important as gain.
Nulling is a relatively effective approach to dealing with noise and interference, provided that you only have to contend with a single source of the interference. If so, it's a practical matter of using a second antenna, adjusting the phase and amplitude, and then combining the two so as to cancel the undesired signal.
In the general sense, the problem of cancelling interference is far more demanding and I have yet to see anything that is truly effective. An article that was published in Wireless World many years ago described a system in which the cleaner sideband of an AM signal was selected and the noisier sideband cancelled. I think it was Sony that implemented this in an all-band receiver and it was termed as "adaptive sideband". It is somewhat effective provided that there is a single interfering station and the sidebands of that signal do not overlap the carrier frequency of the desired signal which would place interference on the otherwise "cleaner" sideband of the desired signal.
I spent a considerable amount of time trying to devise a system that would recognize a double sideband (DSB) signal that was symmetrical about a carrier and eliminate anything that was not symmetrical. It's not as simple as you might think. For instance, when you demodulate a DSB signal using a Costas loop, the quadrature arm contains the asymmetrical signals, however those that are above the carrier are subtracted from those below the carrier while in the inphase arm the two are summed together, which leaves you with no opportunity to cancel completely. Doing a quadrature phase shift and then combining allows you to cancel the interference on one side or the other, but not both simultaneously.
Chris
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\ _______,--------------' Chris Trask / N7ZWY
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