[TenTec] SWR and Rain

[Dr. Gerald N. Johnson, electrical engineer]

Sherrill WATKINS wrote:
> 
> Frank:  Interesting thought.  However, I once had a home made
> open wire transmission line and no such occurrence ever happened,
> even with ice and snow.  If conduction was occurring on the spacers
> because of water, etc. then the swr would go down, not up.  As you
> know, water absorbs RF, which heats it.  This will cause the
> line to act like a dummy load and the swr will go down, not up.  This
> is a mistake that most amateurs assume, i.e. because
> their antenna system has a low swr that everything is ok when just
> the opposite may be happening.
> - 73-s- Corn - k4own.
> 
Not necessarily true! Losses at the feed line insulators will LOWER the
impedance of the load through the transmission line but may either RAISE
or LOWER the SWR as seen at the feed point according to the distance
from the feed point, the magnitude of the normal load impedance, and the
initial swr in the line.


Water only absorbs RF at certain frequencies, near resonance of the
water molecule. One is about 650 MHz. It just happens to work out at 440
MHz that 9913 SWR when filed with water is low, it could change the SWR
to high because the dielectric constant of the water (at lower
frequencies especially) would lower the characteristic Z of the coax,
while not change the characteristic Z of the SWR meter. The line still
might be matched, just not the transmitter.

73, Jerry, K0CQ

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