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Swr and rain?


Master of puppets
Jul 9, 2009
If antenna is in a free space wet/dry SWR change indicate shitty antenna system.
If it just just over the roof wet/dry can change SWR.
For example:
I have 1/4 GP with radials laying on wooden roof. Wet/snow change SWR from 1:1 to 1:1.8


W9WDX Amateur Radio Club Member K5KNM
Feb 11, 2007
Yes, Water adds capacitance and more capacitance will change the tuning.

Antenna tuners do just that, they add capacitance and/or inductance dependent on the band and what's needed. Usually higher bands like CB only need some capacitance.

So yes, when the antenna dries, it should revert to as it was before.


Sr. Member
Oct 18, 2011
does it change when it rains? Then back to normal when dry..
Yes that's perfectly possible. Because RF flows over the surface of a conductor and not through it anything that affects that surface and alters the velocity factor of the conductor will alter how electrically long the radiator appears to RF. You will most likely find that the SWR went up on higher channels and down on lower ones.

BJ radionut

Supporting Member and 6m addict
May 9, 2008
35 miles East of Indianapolis
Very common issue. Roofing material is made from asphalt/petroleum products (oil). Then add rainwater which can contain many impurities gathered while falling. You can then have "reflective" surface near field of the antenna. The antenna can also absorb moisture thru joints/feedline etc. This is possible or can/will change the impedance of the overall system.
A ground plane antenna (with radials) at least a 1/4 (quarter)wave above this kind of surface, may help minimize this annoyance.
Metal roofing surfaces can produce similar annoying issues, it's just Mother Nature doing her thing.
I have a beam for 6 meters...tuned perfect when dry.
(Center frequency 50.150)
Rains, the center frequency drops to about 50.125. +/-
(Antenna appears LONGER)
Not really issue since the antenna is very Broad banded, it just drops the resonance down slightly. Most times I pay little or no attention to this, Ice is a whole different issue.
Unless the VSWR gets above 2:1...
where normal is about 1.3-1.5 :1...
I really don't care...My tube amps don't care and running barefoot I have radios with internal antenna tuners to "clean" it up.
All the Best
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