> The following morning after the storm had passed, we started making
> feedpoint measurements to determine why the SWR on each vertical was so
> much higher than back home. Imagine our surprise at finding the feedpoint
> Z of each vertical reading 40 ohms instead of 6 ohms! We then started
> laying out short radials, about 10 feet long, to augment our 16 long
> radials that had been previously layed out. After laying out a total of
> fifty radials per vertical, the feedpoint Z for each antenna was close to
> the expected 6 ohms.
>
> Many lessons were learned that weekend, but most importantly:
>
> 1) Substantial ground losses can occur near salt water unless a
> sufficient number of radials are used extending from the base of a
> quarter wave (or shorter) vertical. This can be readily determined by
> using an antenna analyzer or noise bridge. Running a few radials into the
> salt water doesn't get the job done if the base of the vertical isn't VERY
> close to the waterline.
It won't get the job done even if the base of the antenna is OVER
the salt water.
Salt water is not copper. It is a few hundred times better than good dirt,
that's all.
> 2) The lowering of the takoff angle near salt water can make an
> astounding difference in the ability to work DX, especially with QRP
> power levels. From the beach, 80M was alive with EU DX, and we worked
> almost everyone we heard with QRP power levels.
Maybe, maybe not. What were conditions like at that SAME
moment back at the other QTH?
> 3) Phased arrays for the higher bands (10, 15, 20M) can be made to
> utilize half wave base fed elements to virtually eliminate ground loss and
> to negate the need for radials if located right at the salt water line.
> (I know some of you are gonna' beat me up for that one-HI!, but repeated
> tests from Coastal North Carolina have borne out the efficacy of this
> concept).
If you handle the feed system right, it will work, but 1/4 wl lines
need to be changed to 1/2 wl lines when phasing voltage fed
antennas. The entire feed system will be wrong if you use the
concepts for a normal current fed element, and you can run into
phase shift problems in matching systems.
> 4) Personal tests comparing inverted vees with verticals near salt water
> have shown substantial (consistent) gain in favor of the verticals. Back
> home 125 miles from the coast, the vees win out every time. The antenna
> models over salt water vs. average ground bear this out, but it's a lot
> more fun and interesting observing it firsthand.
Long term tests here have show my verticals in Georgia far from
the coast, with a path to Europe that exits the mainland up around
the northeast USA and is over land, show my verticals beat dipoles
at 300 feet on 160 meters.
I have a real radial system, not a few elevated radials.
73, Tom W8JI
w8ji@contesting.com
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