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I've seen SWR spike up by about 0.3 because a car passed over 30 feet away when my VNA was scanning my antenna on my SUV.  I have also seen spikes in SWR because birds fly by the antenna when running such scans.  Changing SWR in a mobile environment is not anything new, and is always happening weather you realize it or not.  The antenna is reacting to the changing environment, which is constantly changing when your vehicle is moving anyway.  The small change you seem to be concerned about is normal, and if it wasn't happening I would be concerned about the antenna system anyway.  Such small changes in SWR don't have any noticeable difference anyway, actually you wouldn't notice even much larger changes, such as going from a perfect match to a 2:1, so your .2 or .3 SWR change is beyond negligible.  SWR has very little to do with how well an antenna works.


There are also other factors that can affect your receive, depending on environmental occurrences that affect wide areas at any given time, and that is before you add in the man made stuff that will also have an effect.   Ever hear of the solar cycle?  That alone can affect the amount of static you get based on what  is happening with said cycle.  Putting a CB at a certain setting, and expecting nothing to require a change to said setting ever is unrealistic, there is to much changing for that to happen.  There are times when some settings will be stable for a long time, possibly even a year or two (generally at the lower point of a solar cycle) but that is as good as it gets.


You are correct about one thing, the radios are not the cause of SWR found on an external SWR meter, but that is because the meter can only measure SWR on its antenna side.  A radio will never, in and of itself, cause a high SWR reading on an external SWR meter, and if a radio ever seems to do this then there is something else wrong with the antenna system.



The DB