[TowerTalk] 80 meter beam
jimlux at earthlink.net
Sat Apr 1 14:21:22 EDT 2017
On 4/1/17 10:37 AM, Richard (Rick) Karlquist wrote:
> Not mentioned in the discussion:
> For 2 elements, a driven array can be tuned up for really high
> F/B ratio, like 20 to 30 dB, maybe even 40 dB on a good day.
> A 2 element Yagi will never do more than 10 dB or so F/B.
> On receive, a 2 element driven will have a pattern very close
> to a 3 element Yagi. Often, receiving on 80 is the limiting
I wonder how often you need that null "in the back" as opposed to
somewhere else? With a 2 element array, you can form a single null, but
most directions, you get two main lobes and two nulls (mirrored around
the line connecting the two elements. That might be ok, though.
On the other hand, I can think of cases where what you want is a
generally low response to some wide range of azimuths (say you're on the
outskirts of a noisy city). It's "lower average sidelobe level from 100
to 260 degrees" ...
On receive, I would think that *gain* isn't as big a deal since
background noise is higher - that is, if your goal is to place nulls,
smaller antennas work just fine for the "null forming" signals.
So some sort of switched array for Tx (picking phases and spacings that
are "easy") to squirt your power in the generally desired direction, and
then a separate receive array (perhaps sharing elements from the Tx) to
optimize the receive.
On Tx, it seems that horizontal pol is what you want (unless you live in
the proverbial salt marsh) for best low angle radiation. But what about
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