I finally got around to some modeling and rereading of N6LF's work on
elevated radials to respond to Jim's post.
The comparison on 15m was being made between a crankIR on dry compacted
sand about 70' from and 6' above high tide line. Hence the assumption
that this was "typical earth" conductivity. It had one radial elevated
60" and pointing towards NA, where the reporting stations were located.
The vertical on the beach had wet sand below it at all times, thus sea
water conductivity is the assumption.
My NEC4 analysis of these two situations on 15m shows the crankIR had a
gain peak of 1.9dbi at 15deg and the v-o-b a gain peak of 4.5dbi at 6deg
and a broader low angle lobe.
Hence it is reasonable to me that 1 or 2 S units (or more) might be
heard in signal strength difference over a 6000km path.
The crankIR is not disadvantaged as a single radial implementation to
40m (with extensions which we take) since both the vertical and radial
are not doubled back. On 80m there is some small loss of gain with the
folded element. We followed the (correct) advice to aim the single
radial in the favored QSO direction and elevate it as much as
practical. There is a terminal on the crankIR to add more radials, and
more are certainly desirable on average or worse ground.
As a sanity check I referred to N6LF's work on elevated radials. Over
average ground, he modeled a single 40m radial at 48" and reported a
gain loss of 0.77db in the reverse direction of that radial vs 4 radials
at the same height. My model showed a "f/b" of 1.8db for the 15m single
radial at 60" which is consistent with his advice about the height of
elevated radials to be > 0.05 wl.
On two dxpeditions our teams have had good signal reports from two
crankIRs. However, putting a crankIR into/at the water isn't a good
idea (or possible with the necessary guys). So I've used a SS whip with
2 radials for 20 - 10m if the high tide line is accessible. One radial
models within 0.2db of two.
For a suitcase dxpedition there aren't many alternatives to a crankIR
that fit in a suitcase and cover 80-10m.
My rule of thumb is get within 1/4wl of the salt water for best
results. Then v-o-b's rule.
Grant KZ1W
2/13/2015 1:11 PM, Jim Brown wrote:
On Fri,2/13/2015 12:29 PM, Grant Saviers wrote:
At TX5D (FO-A), I was able to instant A/B a 15m vertical (two
elevated radials) at high tide line vs a crankIR tuned on 15m about
70' from high tide. US stations (5k to 7k km) reported 1 to 2 S
unit improvements with the antenna nearer to the lagoon salt water.
Received signals were at least that much improved.
What radial system did the Crank-IR have? The version that they demo
has only one. That could account for at least part of the difference.
A greenhorn DX trip used one or more of these antennas, and they were
piss-weak.
The nature of the earth under a vertical has a strong impact on its
EFFICIENCY -- that is, it burns some the TX power, so that fraction is
not radiated, thus never sees the earth in the FAR field, nor does it
excite skywave. It is this loss in the earth under the antenna that a
radial system reduces. While I'm sure that many of our readers are
aware of this, many may not be.
73, Jim K9YC
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