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Author Topic: Temperature and SWR  (Read 1712 times)
M5AEO
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Posts: 336




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« on: April 23, 2010, 10:14:45 PM »

I have noticed that the SWR readings on my VHF/UHF antenna seem to vary with temperature.  This morning at 0400 the average reading across the two bands was about 1.5:1, whereas in the evening the reading is a little higher around 1.8:1.
Is it possible that ambient temperature has an effect on the antenna? Has anyone else noticed this phenomenon?

(The antenna is a vertical whip with groundplane located on the balcony about 55 feet agl).

Jonathan, M5AEO.



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IZ5PQT
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Posts: 89




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« Reply #1 on: April 24, 2010, 12:39:17 AM »

Hi Jonathan

you should be sure that it is not just an occasional fluctuation. If the same pattern occurs every day then I would suspect humidity rather than temperature. This occurs to my antenna on lower frequancies.

73s Giovanni
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K5DVW
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Posts: 2193




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« Reply #2 on: April 24, 2010, 06:38:08 PM »

Nope, never measured any VSWR variation with temperature, at least not over typical day to night temperature swing. I think you have some other problem such as water in the coax, that is changing vs heating in the sun (possibly).
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K0ZN
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Posts: 1777




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« Reply #3 on: April 24, 2010, 09:02:19 PM »

Hi.

It is very unlikely, that, with a properly made, properly matched antenna, with good coax and properly connected connectors and DRY connectors, the SWR would change with temperature. Here in the central U.S. Great Plains, we regularly see annual temperature swings of over 100 deg. F and daily swings can be 40+ F at times and over a lot of years of hamming, I have never seen a VHF antenna change SWR simply due to ambient temperature change.  Rain and ice can, and often do, change the SWR on a VHF antenna, but not simply temperature if all other aspects of the antenna and transmission line are correct.

Moisture from dew condensation *might* be an issue. Keep in mind that at VHF it is not impossible that it could be some object near the antenna that is changing its interaction with the antenna. Check to make sure your coax is DRY and the connectors do not have moisture in them and all mechanical connections on the antenna and system are good and tight.

73,  K0ZN
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W4VR
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Posts: 1214


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« Reply #4 on: April 25, 2010, 08:31:16 AM »

Sounds like possibly a bad connection in the transmission line or antenna itself.
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