Hi Gang,
I'd like to toss in my two cents worth on the subject...
Two years ago, I was with a group of QRP'ers who built a top loaded 2 el
80M phased vertical array, with hopes of making a 250mW two-way QSO
between the NC Outer Banks and a group on the German/Yugoslav border.
We set the array up in my horse pasture in central NC (clay soil,
not-so-hot ground conductivity). Sixty radials were used per vertical
in order to reduce the ground losses to an acceptable level. The
resulting feedpoint Z was around 6 ohms per vertical. Broadband
matching ununs were used to transform the value to 50 ohms. The F/B was
quite good--about 30 dB, and many EU stns were heard, --and a few were
worked while running 2W output.
The next weekend, the array was transported to Core Banks, south of Cape
Hatteras, and erected on the beach about 75 feet from the high tide
line. This was necessary in order not to block the beach from vehicles
used by fishermen.
We ran less radials this time due to three factors:
1) A Nor'easter was brewing. The wind and rain eventually tore up three
of our four tents, but left our verticals standing tall.
2) We assumed that since we were setting up the verticals NEAR salt
water, the increased ground conductivity would negate the need
for as many radials as were needed back home in central NC.
3) Since we ran several radials into the surf, we reasoned that we MUST
have had an almost perfect, lossless ground plane.
The good news is that despite the howling storm, we were successful in
pulling off the two way 250mW QSO with the Germans on 80M. At times,
the Germans were an honest 559! The lowering of the takeoff angle
thanks to the salt water made a significant improvement in signal
strength over the W4/DL path.
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.
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.
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).
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.
72/73, Paul AA4XX
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