Topband: water saturated ground effect

[Herb Schoenbohm]

On my Cushcraft MA160V, the resonant SWR point has lowered from almost 
5:1 to 1:1 without the Amidon unUn in line. All 26 of the buried radials 
are attached. The main difference is that the normally dry ground is 
saturated from 6 inches of rain in the past two weeks. Can the water 
saturated soil really make this much of a difference? The last time this 
antenna showed this good of an SWR, was when I had no radials attached, 
in dry dirt, as expected- Of course attaching radials only began to show 
the true impedance mismatch, thus the need for the Amidon UnUn to match 
the coax to the antenna- Bottom line, the antenna is acting like I 
disconnected the radials, which are very much intact- My guess is that 
as the soil drys out, my SWR at sresonance will once again rise, and i 
will need the UnUn again for matching reasons- does this sound right>?
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> 73  Paul  N0AH
Paul,

What you experienced could be a misleading indication as ground 
conductivity with moisture depends a lot on the actual type of soil. 
with a vertical that is less than a quarter wave the direct feed 
impedance should be very low.  What could be happening is that in order 
to get to a 1:1 you probably have a significant loss resistance which 
brings in the 1:1 reading.  This is similar to what placing a non 
inductive resistor across your feedline.  IMHO it is better to measure 
with a bridge the actual base resistance with a ground system of known 
efficiency on 160.  If you have 26 radials all 1/4 wave long on 160 the 
amount of moisture (fresh water moisture) in the ground you should not 
get that kind of variation and not for an antenna that has a  low feed 
point impedance.  On a dry salt lake or high salinity soil the results 
with mixing water would be more noticeable but I don't think this is 
what you have there.

My point here is that one of the reasons the FCC requires AM stations to 
have a ground system as part of their application is so there is a 
constant radiation with a certain power level that will maintain the 
radiation pattern and microvolts per meter at a given distance.  A good 
adequate radial system is supposed to prevent the very phenomenon you 
described.

Herb Schoenbohm, KV4FZ
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