Hi Hardy,
Everyone's soil is different, and what works well in one location or
even at one time of the year might not apply to another location at
another time of the year.
I'm a believer in measurements, and I certainly appreciate your
work. I make the test in a slightly different manner. I am not
interested in SWR on the feedline, but rather standing waves on
the antenna because I want to maximize F/B ratio, not power
transfer.
With that in mind, I sweep the Beverage input and watch for SWR
(or impedance) variation with frequency. When the impedance (or
SWR) remains basically constant as frequency is varied, I know
the antenna has the lowest standing waves. That clearly tells me
the antenna is properly terminated. A very small impedance
change with frequency, and/or minimum standing wave changes
with frequency, also results in about the best F/B ratio.
We should all be cautious and not expect results at one location or
even tests at one time of the year to agree with results everywhere
every month of the year!
For example, I initially accepted the fact that I could tolerate a
single ground rod on my Beverages, based on the fact that I could
adjust the termination resistors for a very small SWR change
between 1.7 and 5 MHz. (BTW, this measurement MUST be made
right at the Beverage, and not through a transmission line of more
than a few feet).
There seemed to be little difference in wet and dry weather
performance, and I used a 390 ohm resistor.
However, a year later and we had a drought. F/B seemed worse,
and I measured the antennas. Now I needed 270 ohms, and could
never make the antennas quite as flat. I added two radials on each
ground rod, and the resistor moved to about 450 ohms while SWR
was once more mostly unchanging with frequency changes.
Since them it has rained quite a bit, and the antennas remain in
adjustment.
I would never run a ground wire from end-to-end, I think that is a
silly concept, but I learned the hard way my antennas are more
stable with weather changes if I have more than just a few ground
rods at each end.
By the way, there are always frequencies where the SWR barely
changes even if the antenna itself develops standing waves
(indicated by standing wave changes with frequency changes). We
have to be careful, and check SWR at several frequencies.
What might be needed at some locations some times of the year
might not be needed at other locations or at other times of the
year, but a good ground will ALWAYS work and never be harmful.
A single driven rod is lucky to be below 100 ohms on 160 meters in
some soils, and that value will change considerably with soil
conditions.
73, Tom W8JI
w8ji@contesting.com
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