In article k1FXd.109090$tl3.58979@attbi_s02,
wrote:

I need to add a clarifier to this post.
If the phases and and magnitudes of the paired elements
are exactly the same, then radiation to the rear is zero.
In the real world this is difficult if not impossible to do.
It certainly cannot be done with a yagi unless possibly,
when elements are contorted to ensure pure resistance
feed at the appropiate frequency

You might want to take a look at systems which use two or more
directly-driven radiators (rather than a driven radiator and a
parasitic element), with a chosen physical offset and phase
offset between them.

This is a classic way to get a 180-degree-only pattern. It's often
used with vertical antennas. I took a quick glance at Arnold Bailey's
"TV and other receiving antennas" text (written in 1950) and he shows
a similar sort of antenna using two horizontal half-wave dipoles,
connected together via a transmission line and fed at the center of
the rear dipole. Very clean 180-degree pattern.

This sort of arrangement might be a good starting point for your "move
all of the energy from the rear lobes into the forward half" quest.

You might be able to combine this sort of dual-driven-radiator
unidirectional beamer with additional parasisic elements, to create
more forward gain. A bunch of directors out in in the front would be
the obvious choice for a first experiment

There's a gotcha to this, though... the 180-degree pattern from a pair
of phased radiators depends on the radiators being driven with equal
currents, which (in this simple arrangement) requires that they have
equal feedpoint impedances. Stick a bunch of parasitic directors out
in front, and the feedpoint impedance of the forward radiator is going
to change (drop, most likely) and affect the current relationship
between the two radiators, and thus mess up the pattern.

I can think of a couple of possible ways to compensate for this:

- Matching network at the forward radiator (maybe shorten it a bit and
use a hairpin inductor match)?

- Tapered transmission-line section between the two radiators?

- Try installing some parasitic elements behind the rear driven
element... possibly in a corner-reflector arrangement? This might
tweak the rear element's feedpoint Z enough to make it easier to
match the two, might also help suppress any rear lobes which
develop as a result of the mismatch.

I have my doubts as to whether all of this work will pay off with
enough of an increase in forward gain, F/B ratio, cleanliness of
pattern, reduction in lobes, broadening of bandwidth, etc. to be worth
the effort over a classic Yagi, but I'm certainly willing to be proven
wrong.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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