The loss changes are frequency dependent. Maximum loss from water in
coax is at about 650 MHz, but severe at 440 MHz. At lower frequencies
the mismatch may obscure the added losses or be obscured by the added
losses. In any case the SWR need not rise nor fall because the SWR
depends on the load Z, the transmission line Z, AND the transmission
line losses. If the water changes the Z of the line and the losses the
input Z of the line will be a complex value depending on the length of
the dry line and the length of the wet line, how wet, and the load. All
that can be said, is that sometimes a wet line will have a different
SWR, sometimes it won't. More often the mismatch will affect signal
transmission loss.
The SWR meter is calibrated to 50 ohms, when the transmission line Z
changes due to water the SWR should rise, unless the line was tuned and
then all bets are off about the direction of change.
Coax holds water in more of the electric field space than open wire line
so water has more of an effect. Loss in a length of coax won't change
the input power unless it detunes the transmitter too. The input power
should be determined completely by the transmitter (providing the tuner
is tuned for matched load to the transmitter), not the loss in the coax.
Ice often totally detunes large VHF yagis. A little did that to my M2
5WL this week. Couldn't hear the packet node 65 miles away on 145.01 MHz
until the ice melted. The dielectric constant of ice is fairly large and
frequency dependent because of the polar molecule.
73, Jerry, K0CQ
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