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Let us assume that I have an adjustable antenna, between 160m - 6m. And let's also assume that using an antenna analyser I have estimated the SWR for that antenna for a specific frequency. If my equipment is the same, and I plan to work on the same frequency range at which I measured the SWR then why would I need to analyse the SWR of the antenna again? Or keep measuring the SWR periodically?

Can I assume that once I have established the SWR for an antenna then it should remain the same?

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If nothing about the physical situation changes, then the SWR will not change. But something might change in the long run, perhaps:

  • Damage to the feed line (water intrusion, sun damage to insulation, chewing by small animals, etc.)
  • Damage to the antenna (corroded junctions, parts broken off, wire fatigue).
  • Plants growing near or on the antenna.
  • Your neighbor erects a shed with metal (reflective) walls.
  • The wire stretches or the mount points move

All of these things and more might at some point affect the antenna, such that you need to readjust it or repair it. Some of these are instant, some could take 10-15 years.

I don't mean that you should routinely recheck your antenna with an analyzer. But, if you think it might not be performing like it used to, or if something changed in its environment, then it might be worth checking.

Also, higher-than-normal readings on a SWR meter used while transmitting (whether built into your transmitter or in a separate unit) can be the first sign that something might have gone wrong, and you should check your antenna system for damage (or something else wrong, such as just not being plugged in).

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  • $\begingroup$ Example: my club made a 160m full wave loop for field day one year using insulated copper wire. When we took the antenna down at the end of the weekend, we noticed the wire had sagged and was significantly longer. If we had been depending on the length for tuning, this could have been a problem. $\endgroup$
    – user10489
    Commented Jul 31, 2021 at 12:37
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    $\begingroup$ One of the things that will influence your SWR is also moisture, as I know Kevin knows. :). If you have significantly different weather conditions (for example, a rainy day or snow on the ground/trees/house), you will likely find your SWR changes a fair amount. I'm not suggestion that you need to do anything about this, but it's not a terrible idea to check it or use an inline meter so you are aware if it becomes so high that it could result in some damage. :) $\endgroup$
    – David Hoelzer ♦
    Commented Aug 21, 2021 at 21:09

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