[TowerTalk] B&W folded dipoles

[Tom Rauch]

> Cebik models that these get 6dB hit over a dipole.  If
they're that lossy,
> it's no wonder they get a good match: heck, any antenna
with a 6dB pad in
> the feed line will get no worse than 1.66:1 VSWR.
(nothing says that "any
> antenna" will actually radiate, though).


Actually the loss varies quite a bit by band. On bands where
feedpoint Z would be low, termination resistance is shunting
a low impedance and loss is modest. On other bands loss can
be very high, over 10dB.

What you have to do is model the antenna in EZnec 4 and look
at average gain. You could also look at dissipation in the
termination. Either gives an approximate idea of efficiency.

Look at peak gain in the lobes and comparing kit to a dipole
isn't a valid data point. The reason is the antenna forms
multiple nulls on higher bands a dipole does not have, and
so it squeezes radiation into smaller lobes than a reference
dipole or isotropic radiator would have. The absence of
energy in null areas that a dipole would fill means the
antenna is less useful for overall communications, unless
you could rotate the antenna or were working fixed
directions and somehow managed to align narrow lobes with
the desired directions.

You have to either use average gain or resistor dissipation
to get a good efficiency comparison.

That particular antenna "design" actually came from someone
in Hudson Ohio. He got it out of some old antenna article.
It was sold to B&W by that person after he tried to market
them for a while. Resistor dissipation was measured, and the
dissipation figures tracked pretty well with models I made
in Eznec. (I still have one of the original termination
resistors used in the measurements). On some bands loss is
more than ten dB. On some bands less than 3 dB.

A second variation of that antenna was produced by an
amateur antenna company called Sommer. It is a ~25 foot
folded vertical that is terminated. It's another efficiency
dog. It's less than 5% efficient on 80 meters.

A third variation is the Max Con or Max Com automatic tuner
(not sure which name spelling). It employs a resistance to
shunt whatever load is across the "tuner". It's claimed to
have instant "automatic tuning", but in fact it's just the
same old game of shunting off a large portion of applied
power as heat in order to bring SWR into reasonable areas.

73 Tom