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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