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off the back

Here's an observation I went through some time ago.  I never hear this
mentioned on the bb or else where, perhaps everyone knows it.  My
terrestrial beam is a tri band cubical quad, and it has 4 elements on 10
meters.  It was cut for approximately 28.35Mhz, my measured F/B is about
18db.  For mode 'A' reception it's adequate off the front but off the
back there's noticeable gain.  The reason- at 29.5Mhz my first director
is actually acting like a reflector, the reflector is acting like just a
random piece of nothing, the SWR is off course sky high.  Jerry-K5OE once
ran these numbers through his modeling program and confirmed 6db gain off
the back at 29.Mhz, and nulls off the back.  I'm sure this would be
applicable for any quad, or yagi beam cut for the lower part of 10M.  As
I said perhaps this is common knowledge but I'll bet there are a few out
there that will be surprised when you flip it around.  
My thanks to everyone who gave me advice on coax I have some LMR on the
way.  My station should be back up in a couple of weeks, on mode 'A' &
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