Hi, Top-Banders:
After I posted a message comparing gains and radiation angles of
various-size verticals, I received a message from John, ON4UN, pointing
out that the radiation angle is largely affected by the type of ground at
the antenna site, and he is quite right.
I modeled those antennas using W7EL's EZNEC v1.0 and using "average"
ground, with no radials. I also checked all models using ELNEC and
K6STI's MN. Results were the same (+/- one degree) using any of the
software packages.
After receiving john's message, I again modeled a 1/4-wave 160m vertical
to see the effect that different types of ground would have on the
radiation angle. Here are those results, from worst to best (all for a
1/4-wave vertical):
Ground Type Cond (S/m) Diel Const Radiation Angle
--------------------- ----------------- ---------------
-----------------------
very poor (cities) .001 5 28
deg
poor (rocky) .002 13 25
deg
average (clay) .005 13 23 deg
fresh water .001 80 22
deg
very good (rich) .0303 17 17 deg
salt water 5 81 7
deg
perfect inf. inf.
0 deg
These results were with zero number of radials. I then began to add
1/4-wave radials to the 1/4-wave vertical over "average" soil. I first
tried 4 radials, then 8 radials, then 16 radials -- the radiation angle
remained at 23 degrees. When I went to 32 radials, the angle dropped to
22 degrees. 64 radials dropped the angle to 21 degrees, 128 to 19
degrees, 256 to 17 degrees, 512 to 14 degrees, 1024 to 11 degrees, 2048
to 9 degrees, and finally, 4096 radials yielded an angle of 7 degrees,
the same as salt water.
John mentioned that he once tried a 5/8-wave vertical on 80 and that it
was the worst antenna he ever had for that band. I suppose that the
longer verticals require a good far-field soil conductivity. I say this
because when I was W5RTQ in Houston, I shunt-fed my 64-foot crank-up
aluminum tower with a TH6DXX at 65' on 80m and that antenna (approx
1/2-wave) was outstanding - almost always #1 in pile-ups - a real
performer! When I moved out here to the Mojave Desert in Calif, I did
the same thing with only mediocre results. The only difference was the
soil conductivity - in the Houston area it was purported to be 30
millimhos/meter, while here it is said to be 2 millimhos/meter.
Unless you have very good soil conductivity, stick with a vertical less
than 3/8-wave. Longer verticals than this will also exhibit more
critical tuning and will be very narrow-banded (over any ground). John
suggests that a vertical longer than 3/8-wave will perform poorer than a
1/4-wave vertical unless it is over salt water.
As he also points out, longer verticals require longer radials. (My 80m
shunt-fed vertical in Houston had only about 16 short radials under it,
further evidence of the fantastic soil conductivity there.)
The next thing I wonder about is what is the optimum angle of radiation
for DX on 160? EZNEC showed that while a 1/4-wave vertical over salt
water has an angle of maximum radiation of only 7 degrees, there was
still plenty of signal in that lobe at higher angles, down only 3 dB at
40 degrees.
73, DX, de Earl, K6SE
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