[TowerTalk] rotatable dipoles

[L. B. Cebik]

A recent discussion of an add-on 40 meter dipole element certainly
confirmed something that George Grammer and Ed Tilton noted 50 years ago:
a rotatable dipole is roughly 2/3rds as good as a beam relative to a fixed
dipole at the same height.

Consider:  when we aim a beam, we are rejecting QRM from 3 sides.  When we
aim a rotatable dipole, we reject QRM from 2 out of those 3 sides.  The
rejection can range from 8-20 dB depending on antenna height (the heigher,
the deeper the side nulls), which is far superior to the gain of most
small Yagis over a dipole broadside.

So one loses some gain and rear rejection--and that hurts.  But one still
can lay the max signal lobe on the intended station and null out 2/3 of
the competition.  At any frequency, these are worthy advantages.  

Hence, if the beam collapses but the driven element is salvagable, use it
in the interim.  If you use wire, try a triangle of antennas to maximize
the chances of getting the best combination max signal strength to QRM
nulling.

Consider these options:  a fixed wire 2-element beam (even bidirectional
with electronic switching) with two quadrants permanently nulled (the
sides) vs. a rotatable dipole with access to all quadrants, but a little
less gain and no QRM suppression to the rear. Depending on where you are
and what you want, I can imagine either option being the superior.  (Of
course, if your mechanical, fiscal, and maintenance resources are
unlimited, the question becomes irrelevant.)

Most commercial antennas using tubing employ a matching scheme that
permits bleeding off static charge.  Please use some such system.  It will
not remove external noise (from power lines, etc.), but it will reduce
noise from the antenna itself--and that may make a difference in some
locations.

Although this note may state the obvious, please store it away and drag it
out when catastrophe strikes your beam 3 days before the contest.  It is
the antidote to hopeless depression. You may have to work harder with only
a rotatable dipole, but you can work successfully.  And if you are moving
up to a rotatable dipole from a fixed dipole (at the same height), you
will have made a significant improvement in operating capability.

Hope this provides a little perspective on the place of rotatable dipoles
in the general span of antenna capabilities.

-73-

LB, W4RNL




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