Topband: Shunt feed question/observation

[Michael Sapp]

Hello Topbanders: I'm using an MFJ-1793 80/40M vertical in a ground plane
config on 160m. It and the radials are up 13 feet, with four 1/4 wave
radials for 80M, two for 40M and 2 for 20m.  The 1793 uses an inductive
shunt match to ground at the base and the shunt inductor is 3.9 uh.  This
is the classic mobile whip shunt feed circuit and the equation predicts
this value for 3.5 MHz. Also this antenna is feed with 152 ft of mostly
buried rg-213 and I added an extra 26 feet of RG-213 to get the electrical
half wave transmission line length at 1.8 MHz  (492/f  * Vf). So the feed
point impedance should be replicated or very close in the shack. A vacuum
relay is used to switch in a 45 uh inductor in series with the monopole at
the base of the antenna.  After five days of continuous rain, I found a 1:1
SWR match at 1805 khz with the 3.9 uh shunt inductor active in the circuit.
According to the shunt feed equation, that value equates to a 22 ohm
matching  condition at the antenna feedpoint.  With the ground now slowly
drying out and freezing, the resonance changed to 1816 khz and the SWR is
1.4 to 1.  Using a multi-tap unun, I can get a 1:1 match in the 36 ohm
position.

Is that telling me my ground losses have increased by 36-22 = 14 ohms going
from saturated wet to near frozen ground conditions??  Is it worth going to
a shunt inductor with multiple tap points, like one for 80m and perhaps two
for 160m ??  It seems easier just to match with the unun in the shack. 
That certainly is more flexible on the other bands as well. I could get a
dozen or so ground radials in a 100 by 50 foot area by one side of the
antenna, but no symmetrical arrangement is possible on the ground. I've
been pleased with its performance on 160/80/40m so far.   Any thoughts
would be appreciated. Mike WA3TTS  

PS: I also note that it is easy to QSY the antenna resonance by adding or
subracting 15 degrees or so of feed line to shift 35 khz or so from the 1/2
wave line length value on 160m.