Jim, harmonic resonance of a loaded radiator is no longer related to
wavelengths like a full sized radiator. The loading inductance is lumped and
has
a reactance that is linear with frequency. The equivalent radiator length
in degrees that is replaced by the inductance is more of a tangent function
of the number of degrees than being linear.
I have not studied this but seems like something that could be modeled with
proper software.
A direct measurement like you are asking for will give some very quick
answers and hopefully someone will do it and report the results.
73,
Gerald K5GW
In a message dated 10/11/2010 9:11:41 A.M. Central Daylight Time,
jim.thom@telus.net writes:
If anybody owns either a F-12 Sigma 180-S or 280-S.[ both 80m ants]..
or knows somebody who does own
either one, perhaps you could stick a few watts into it on 20m, and see
what kind of swr
you have on 20m. If you have the time, perhaps sweep it with a
MFJ/etc, and see where it
does resonate. Not all these ants will resonate on their 3/5th
harmonics. The F-12 340-N [3-el 40m yagi]
does not resonate on 15m.... it resonates just below the 12m band.
What baffles me is 3.5 x 3 = 10.5 mhz. 10.5/14.2 = .74 The
80m ant, on it's 3rd harmonic is
26% lower in freq than 20m. I have heard several horror stories now of
80m arrays completely
screwing up 20m yagi's that are in close proximity. Rotating the 80 + 20m
arrays , so their booms are
90 deg to each other does not always work. The 35' booms used on both
the 280-S and 340N will screw
up 20m real fast.
I noticed that Optibeam uses decoupling rods on their 4-el 40m yagis.
They won't screw up a 15m yagi, nor
another HF array. Does anybody know how this is done ? Can this
technique be adapted to an 80m array ?
Jim VE7RF
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