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SWR and external condition

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by OK1TK, Mar 21, 2021.

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  1. OK1TK

    OK1TK Ham Member QRZ Page

    Hello guys, can external and weather condition affect SWR ? I assume yes, but could someone explain how ?

    I have EFHW wire antenna ca 24m long. It is placed in not very good condition, just ca 5 -7m above the ground one side is "shielded" by metal-concrete building (one end) and also by a slope on the second end. It is located in the city center. I use Xieugu G90 and its internal tuner. I have CM choke on the feed line and I use our central heating as ground (2nd floor appartment building).

    The antenna was hanged mid of DEC 2020 and I was surpised I had really good SWR on 20m band 1.1-1.2. Within ca mid of JAN till I would say mid of FEB the SWR was ca 1.5 and within MAR it was at about 2.0. But since yesterday the SWR back on 1.0 on 20m and 1,1 on e.g. 15m.

    There were no changes done by my antenna system. Is this external condition what affected my SWR ? I would to understand it just a bit.

    Thanks for any replies.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The internal tuner in the G90 should handle those variations fine if you use it.

    But one thing that changes antenna resonance is water (rain) when raindrops cling to the wire and don't fall off, and ice is much worse since it almost always sticks to the wire until it melts. Either of those things will make the antenna electrically longer even though the physical length doesn't change.

    Could be something as simple as that. Could be something else, of course.
  3. N7EKU

    N7EKU Ham Member QRZ Page


    I think rain will also impact ground conductivity a lot, and since your antenna is so close to the ground it would impact the tuning.

  4. KD4EB

    KD4EB XML Subscriber QRZ Page

    We had a soaking rain all last night and my SWR got noticeably worse when I checked it this morning, so I needed to lengthen my counterpoise from what it was yesterday. I also suspect that temperature changes cause the wire to expand and contract, and the weight of the wire itself will stretch the wire over time.

    About 20 years ago we had cable TV interruptions which were attributed to sunshine and temperature changes affecting electrical contacts in the box on the street.

    I would guess your antenna system is perfectly normal and just requires periodic inspection and adjustment.

  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    In reality, rain impacts the "radiator" more than the earth ground below it, almost always.

    I've run soil conductivity tests at two very different locations: The Jersey shore, where I used to live 34 years ago (!) and here in L.A. where I live now.

    In both cases, made many soil conductivity measurements before and after very soaking rains: Not much difference at all. Soil conductivity is based mostly on the elements naturally in the soil, and rain doesn't change them.

    But rain changes the characteristics of the radiators, though. In every case I've seen (even just using a water hose with a lot of pressure and a spray nozzle), I can lower the resonant frequency of almost any kind of antenna just by getting it wet.
  6. N7EKU

    N7EKU Ham Member QRZ Page

  7. W4HWD

    W4HWD Premium Subscriber QRZ Page

    Weather can and does affect SWR, more on some antennas then others.

    Take my multi-band doublets. Both have 300 ohm ladderline matching sections. When it rains, SWR changes. When the rain stops, SWR goes back to normal. Why? Because when ladderline gets wet, the electrical length of the line changes.

    No big deal, it's not a massive change, the tuner does it's thing, and when the antennas dry out, the tuner goes back to it's programmed "dry" settings.

    In my case, the ground going between wet and dry has nothing to do with it. In fact, my ground-mounted 160m inverted-L sees little to no change in SWR between wet and dry ground; and it's radials are in the ground. Maybe that's an indication that I have enough radials; I don't know, but that, and it's performance on the air, would suggest I do.
    Last edited: Mar 22, 2021

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