Re: LF: Mains Harmonics ??

[DL4YHF@aol.com]

Hello group,

I can confirm what Alan said about the mains harmonics. The amplitude ratio between fundamental and 2750th harmonic is not easy to determine (we don't know if the generator's waveform and the efficiency of the QRM-TX-antenna at different frequencies). 

The "wanderers" are indeed caused by the AM modulation on the 25...50 kHz switching mode supplies, including these:
- modern compact fluorescent lights which use a terribly small electrolyte cap so the XX kHz current through the tube has a strong 100 Hz modulation. Old (large) fluorescent tubes sometimes produce strong broadband QRM when the discharge is unstable.
- hi-tec ovens where the heat is generated by eddie currents ("induction"). Though I don't know the fundamental "TX" frequency I suspect them to be strong QRM sources.


There are weaker lines spaced 100 Hz on the waterfall, caused by symmetric load to both half waves (like in a brigde rectivier, or a conventional light dimmer with diac and triac).

A stable ladder of harmonics separated 50 Hz is even weaker. I found a not-so-obvious source for this: It is a coffee machine with a single rectifier diode to reduce the "standby power" which heats the plate under the can (solution: drink all the coffee before listening to DX signals and turn the machine off ;-)


Regards,
  Wolf  DL4YHF