After great deliberation and a search of the Internet, some new and
possibly interesting potential ( no pun intended ) theories on Rain-
Induced static have come to light:
1) Typical raindrops are resonant to frequencies near the 9th harmonic
of the FM broadcast band. The RF energy contained in these raindrops
mixes with local signals present in every ham shack to produce short
duration pops at whatever frequency you are tuned to. The strength of
the "static"--which is actually a signal, is additive, that is, larger
surface area antennas, and stronger rain produce louder "pops" because
each raindrop contains a "packet" of RF energy.
2) The feedpoint impedance of a large HF yagi is dramatically and
rapidly changed by the presence of water or snow. The "static" is due
to instantaneous, very large SWR mismatch, and in fact is a desensing
of the receiver caused, in effect, by having a poor antenna connected. In
some
cases, the high SWR condition acts like a diode detector for instantaneous
wide bandwidth noise. The "loop" nature of quad antennas is a natural
preventer of this effect.
3) Under certain conditions, the volume and intensity of the rain produces
a "comb filter" effect that despreads the strong spread spectrum signal
present in the Washington, DC and northern California regions. Like (1)
above, the despread signal causes instantaneous receiver overload at
the intended receive frequency.
4) Aliens are doing it, but only at Contest stations.
Happy April 1st...I mean May 1st.
73, and I hope the rain stops so I can work on my antennas.
Brian/ND3F
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