I am assuming you have accurately ruled out more in-your-face
problems, such as water-related problems in the loading coil, or the
unun, or unintentionally bypassing some required element of the
antenna. Is the unun actually a unun or is the shack facing winding
isolated to remove feedline current, making it an isolation
transformer? If the later you need to fix it.
The specifications for your antenna on the DX engineering site
indicate only 0 dBi gain with IDEAL ground and MINUS 6 dBi with
typical. That's 3/4 of power lost in the ground. Depending very much
on the dirt, particularly if it's acidic, fully dry to sopping wet can
be a very large change in ground conditions. Again, assuming there is
NO problem in the antenna itself...
The change in SWR with the dampness is an indication of the
insufficiency of your radial system compared to a dense, commercial
grade radial system.
26 buried radials would not be sufficient in any event, to insure
complete independence of the antenna from the wide variation in
ground-related effects. For there are either too few of them if they
are quarter wave, or too short if 26 radials are properly dense close
to the antenna. Even if all your radials are 1/8 wave which borders on
dense, use of only 1/8 wave in dirt as variable as you describe would
not prevent seasonal variation, as you would still have significant
ground interaction beyond the 1/8 wave. Further, if the radials are
not equal and evenly spaced, some portion of the ground field
cancellation benefit will be lost, the exposure to ground losses
increased, and therefore the variation is increased.
Further yet, given the shortened (30 to 36 feet) antenna, the
performance basically is 95% determined by the ground treatment, which
is why you can have huge variation with ground changes. Worse yet,
empirical evidence continues to accumulate that in typical ham
situations, and with reference to sky wave, that even the mighty NEC4
modeling methods ***UNDERESTIMATE*** ground losses UNTIL a DENSE,
commercial grade radial field is in use. Model results seem to play
truer to performance if one always starts with POOR ground assumptions
until a more optimistic estimate can somehow be PROVEN. Guess or hope
doesn't cut it.
The minus 6 dBi on the DX engineering page could easily be minus 9 or
even minus 12.
There is a simple rule that is emerging with respect to on-ground or
buried radials: If you can't do dense and quarter wave, don't do
on-ground or buried. I realize that creating such can be beyond
reach, but less than the full monty on ground based radials is far
lossier than most realize. If that's all you can do, you're stuck
with it, and you will be victimized by the dirt, high losses when the
SWR is good and broad, and lower losses when the SWR is poor (and
matching is difficult).
Solving your SWR will not solve your performance. Solve the ground
and then match that.
73, Guy.
On Mon, Jul 18, 2011 at 8:31 PM, BRYON PAUL n0ah VEAL <bryonveal@msn.com> wrote:
>
> 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>?
>
> 73 Paul N0AH
>
>
>
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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