Posted: 12/17/2015 10:45:41 AM EDT
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I have a 80/40/30/20 trap/fan dipole mounted @ 35' with a row of pine trees about 10' along side of it. Whenever we get a hard rain, the SWR on the upper bands will go up as much as 50% (i.e 2:1 -> 3:1). Oddly, the lower bands are only slightly affected. Shortly after the rain stops, the SWR slowly returns to normal. I think it's mostly the pine trees causing it.
Anyone else notice this with their antennas? |
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What type of feed line & how long? I've heard some mention that ladder line can get funky when wet.
Mine will change a little when raining, but not much. I'm using a 80-10 fan dipole & coax feed line. Nearest tree is about 25' away. Rain doesn't seem to bother my I-Max 2000 vertical antenna. SWR never changes on it, no matter the weather. |
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For a few months now since the last wind storm snapped the end Dacron support rope, the top section of my Inverted L has been laying on the pine branches. I see little if any SWR change wet vs. dry.
Water on ladder-line as mentioned above, or water in a trap or getting inside the coax is also possible. i |
My SWR changes when I get a hard rain. I believe it is do to the electrical ground depth. In the desert the electrical ground is lower than the ground level and changes as the earth is wet. This effects the height above electrical ground and make the antenna look like it is lower. The rain may also effect open ladder line and other connections. There is a reason they call it antenna theory and not antenna science. My inverted vee and vertical both have a change of SWR when it rains, I tuned them when it was dry. 
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It's fed with 75 Ohm RG-11U. I placed a 75 Ohm load resistor at the antenna end of the coax and the analyzer showed it essentially flat (~1.2:1) 80-10 Meters, wet or dry. Yeah, I got wet that day.
The ground beneath the antenna is high and well drained, but is still damp when the SWR falls back to normal levels. As far as i can tell, the antenna works the same wet or dry. |
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The 40/80 Meter pair are virtually unchanged.
On the 20/30 Meter pair, the inner elements (20 meter) resonance shifts downward 240 KHz while the outer (30 Meter) elements resonance is virtually unchanged. The antenna is a 40/80 trap dipole with a 20/30 trap dipole suspended 10" below it fan dipole style. If it was the center balun or the coax changing, all bands should be affected. If it was the 20 Meter traps, both the 20 and 30 Meter resonance should be affected as the trap also acts as a loading coil on 30 Meters. I agree that it's most likely the proximity of the pine trees. Anyway, wet or dry the antenna gets out just fine. All I have to do is tap the TUNE button on the radio to make it happy. |