The 40m OptiBeam decoupling stubs apparently make the antenna resonate
outside the ham bands (except for 40m obviously) but they do not make the
antenna invisible and you still need to use reasonable spacing (8-9 foot) to
other HF antennas. The 80m shortened beam may already not be resonate on
any other ham band and therefore not need stubs. I had no noticeable
interaction between my homebrew linear loaded 80m dipole and a 20m beam and
Subject: [TowerTalk] Sigma 180-S / 280-S
From: "Jim Thomson" <email@example.com>
Date: Mon, 11 Oct 2010 07:11:14 -0700
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
kind of swr
you have on 20m. If you have the time, perhaps sweep it with a MFJ/etc,
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
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
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
be adapted to an 80m array ?
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