Antenna expansion

I know when summer is here when I have to keep on tweaking the tuning of my MFJ magnetic loop antenna throughout the day. Having an antenna farm in the attic (or loft as it is more commonly called over here) protects the antennas from the depredations of the elements but it does subject them to extremes of temperature during the summer months. As the loft warms up during the morning the metal of the magnetic loop expands. Because the magnetic loop is a very sharply tuned antenna this has an effect on the SWR. I don’t know what the temperature in the loft reaches on a sunny day but I wouldn’t want to go up there.

I’m not talking about a small change. I may tune the antenna to achieve a 1.2:1 SWR first thing in the morning and by lunchtime it can have increased to 2:1 or more. This wouldn’t be so noticeable if I was moving around the bands retuning as I go. But I use the magnetic loop for my 30m APRS station which stays on 10.1473MHz all day and every day. (It does a jolly good job there, by the way.) If I don’t pop into the shack now and again and give the antenna a quick tune I could be subjecting the transceiver to a higher SWR than is good for it.

The magnetic loop is the only antenna I have that will cover 30m, so I don’t have any alternative for my APRS system. This need for retuning affects all the bands I can use the loop on, not just 30m.

I suppose my multiband dipole also expands and contracts with temperature, but because the tuning is broader the effect on the SWR is less noticeable. Outdoor antennas have their tuning affected by rain or ice, of course, so I’m not alone in having to put up with weather effects on my antennas.

Julian Moss, G4ILO, is a regular contributor to and writes from Cumbria, England. Contact him at [email protected].

One Response to “Antenna expansion”

  • Stephen G0PQB:

    I obtained a digital thermometer with a probe on a short length of wire and that tells me the attic temperature and also gives the upstairs ambient room temperature but not a bad piece of kit to have as it will let you know if you getting near frosty conditions in the attic which could affect your plumbing in winter time. I obtained the thermometer from Clas Ohlson who are local to me in Watford but I don’t think they have a branch in your part of England. If you can’t get a suitable one locally I can always get one and post it to you Julian.

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