On 9/20/2011 6:07 AM, ZR wrote:
> With commercial field strength meters being fairly available on the used
> market it would be one way to end the arguments with things that are purely
> speculative.
You're certainly right that comparative field strength measurements are
the only good way to determine how well a given antenna is working, and
I agree that one of these AM broadcast FSMs would be a good way to do
it. The K3 can be set to read the strength in dB of one signal relative
to another, and it can also be calibrated to a signal generator. Last
summer, I added a pair of sloping verticals to my 120 ft tower, and
wanted to compare it to my omni vertical, an 86 ft tee, which, by the
way, is top loaded to the point where a series cap matches it to 50
ohms. The omni has 70 irregularly spaced radials varying in length from
about 70 ft to about 130 ft. The slopers have eight elevated radials,
and the tower has eight radials on the ground.
EZNEC predicts that the slopers should beat the omni Tee by about 2 dB
in their favored direction, and that their vertical patterns are the
same. So far, in about a dozen ground wave tests with K3s on the other
end, I've only seen that twice.During several contests, I did a lot of
switching between the antennas and noting relative signal strength. The
slopers are about 6dB quieter on RX, but they rarely had greater signal
strength than the omni. I've also tried watching signal strength
readings from Skimmer spots while QSYing a few kHz when switching
antennas. And tried running on the slopers, but always went back to the
omni because I had much better rate, even in the favored direction of
the slopers.
Even carefully done comparative antenna evaluations like this are tough,
thanks to QSB.
Another point. I've modeled a vertical, center-fed half dipole for 40M,
and built it using the coax feedline as the lower half of the dipole and
ferrite chokes (two 5Kohm chokes in series) as the lower end insulator.
It's hanging from a pulley in a redwood next to my house, with the
bottom of the antenna at about 40 ft. I modeled the dipole at various
heights above ground, and nowhere does the pattern or relative field
strength look very good. On-the-air performance isn't very good either.
When I first built it, I did a number of comparisons with horizontal
half wave dipoles at about 100 ft, and the horizontal dipoles beat it by
several S-units. There are no radials under it.
73, Jim K9YC
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