[600MRG] Rain amplitude modulation

[Ben Gelb]

We've been getting some rain in the SF Bay Area finally, which has
presented a fun opportunity to observe fluctuations in ground losses during
rain events. Below are some plots from last night of the R component of my
antenna impedance (captured while WSPRing) as well as some precipitation
data from a local weather station.

I retuned the antenna a few weeks back to present right around 50 ohms to
the TX. Last night it ranged from about 35 ohms to 48 ohms (or really 140
to 192 ohms before the transformer). Rr is estimated at about 0.2 ohms, so
that variation is really all variation in ground loss (and I guess a pretty
large corresponding change in antenna efficiency).

Maybe a really clever WSPR listener could estimate the rate of rainfall at
my QTH based on changes in signal strength... hi.

Hopefully the plot images come through... or everyone is going to wonder
what I'm talking about!

My measured feedpoint resistance (stepped down from loading coil by 4:1
ratio).




Rainfall data from the Weather Underground station in my neighborhood
(wraps at 12 midnight, so you have to use your imagination to fix the line,
but there's a big step from the downpour around midnight):


73,
Ben N1VF
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