Re: MW hum - higher frequencies only - 200Hz
OK - I have done some more tests, walking all around the area with my PL330, trying both the ferrite bar and telescopic antennas. Here are my findings...
1) The noise is worse with the rod antenna. This suggests magnetic coupling and further re-enforces the notion that this is mains modultation.
2) Although 'very' present in the garden, away from the properties, the noise quickly diminishes as I move further away from the residential area. There is no noise when standing directly under the 3 phase, 11Kv transformer.
3) I agree with Simon in that the although only on the higher part of the band, the noise is only actually a problem with stronger signals. 1251Khz for Smooth which is a good signal, my pantry tx when tuned high in the band etc. When pulling in Libra AM from much further away, it is clean.
4) Perhaps most interesting of all - the noise is everywhere in the home with 1 exception... My fuse box is on the landing along with the meter. It is just as noisy all around the fuse box, BUT, move the radio above the meter so that it is across the incoming mains tails, and the noise is completely gone.
So I have to agree with others here, that it is indeed mains modulation hum interacting with strong RF signals, and the ring mains are clearly forming some kind of radiating loop. The 200Hz must just be a harmonic. I have tried adding plug(s) with 100nF capacitors in them, but it makes no difference. Nor does isolating the main breaker.
No matter - I have moved the pantry TX to a lower frequency and the rest of the band is clear. Just interesting!
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