Topband: Deep earth rod vs radials

[Bill Tippett]

From: DAVID CUTHBERT <telegrapher9 at gmail.com>
Date: Tue, 17 Mar 2009 06:46:32 -0600

I have set up verticals above sea water and above the Great Salt Lake of
Utah.

For the ocean ground connection we used the keel of a 30' sail boat. Think
of the keel of the boat as a fat ground rod that extends several feet into a
very good conductor. The skin depth of ocean water is about one foot. The RF
current would remain very near to the keel until reaching the surface of the
water. Being that the wire antenna was fed with a tuner I have no base
impedance measurements.

At the Great Salt Lake I have set up two different balloon verticals. This
water has a much higher salt content than the ocean and so one skin depth is
8 inches. The measurements were taken with a VSWR meter. I did take the
AIM-4170 along, intending to make some good measurements, but after each 160
meter contests I quickly pulled up stakes and drove home. Using the SWR
meter confirmed calculations that show a ground loss of a few ohms. This is
the 'spreading resistance' out from the 1" diameter ground rod. A 12" metal
disk on the surface of the water would provide a sub 1-ohm ground.

The ground rod was placed 140' from shore, next to a road that cuts across
the lake to Stansbury Island. The water is 8" deep at this point with over
10 miles of water to the East and a good shot in all directions but North,
where Stansbury Island is. VSWR measurements were made during zero wind
conditions, when the balloon vertical was vertical. When the wind came up,
performance dropped - more as the antenna got closer to the water. Each time
the insulated wire dipped into the water the SWR meter would peg and the
received signal dropped by over 40 dB.

The predicted decrease in the pseudo-Brewster angle was very,
very noticeable. The prediction was a 6 to 11 dB increase in signal,
depending on the take-off-angle. Running 5 watts things worked more like 100
watts. Received signals were noticeably higher in signal strength than the
home antenna and with greatly decreased nearby manmade noise I could hear
well.

To sum things up, the advantage of salt water is more in the decreased
pseudo-Brewster angle than in the local ground loss. Local ground loss can
be fixed using miles of radials but your signal is at the mercy of the
far-field ground.

     Dave WX7G