Roy Lewallen wrote:
Mike Coslo wrote:
Roy Lewallen wrote:
Mike Coslo wrote:

. . .
I don't doubt that someone might come to a different conclusion,
but I think my reasoning is pretty sound. I modeled an equal length
version of this in 4nec, and it just doesn't work very well. OTOH,
turn it into an OCF dipole, and it is a different story. . .


How did you possibly determine what the balun input impedance was
when terminated with the impedances the antenna presented on the
various bands?

You are correct, I couldn't. But what I got was enough to tell me
that with the two sides of equal length, there wasn't much need to go
any further. Cannot some antenna characteristics be modeled without
the entire system in place?

Yes, but you have to at least include the whole antenna -- you can't
tell much about a two-element array by modeling a single isolated
element. In the case of an OCF dipole, unless heroic efforts are made to
keep common mode current off the feedline (which the balun doesn't
achieve), the feedline is part of the antenna so, like the second
element of an array, it can have a major impact on the both the pattern
and impedance and has to be included in the model. The balun, feedline
length, and feedline orientation all play a role in determining how much
current goes down the feedline part of the antenna and where that part
is. So you have to know at least that much to get a meaningful result.

I'm no expert, so I'll ask the question: Is
there some Balun that will make a 135 foot equal length antenna
perform on 80-10 meters?

"Perform" is one of those binary terms that depend on where you put the
dividing line. But the answer is that the only practical way you can
achieve a reasonable impedance match to a coax feedline on all bands
with a center fed 135 foot antenna is to introduce a fair amount of
loss. This could be in the form of a resistor at the feedpoint, for
example. Then you'll have an inefficient antenna at least on some bands.
Alternatively, you can have low loss at the feedpoint but a lousy
impedance match. Then you'd have a lot of loss in the feedline if you
fed it with coax. The bottom line is that you'll have poor efficiency on
at least some bands if you feed it with coax, no matter what you do --
short of putting either an adjustable or very elaborate fixed matching
network at the feedpoint.

Nearly any ham can measure the SWR but almost none can measure the
efficiency. So many antenna manufacturers have produced lossy antennas
which exhibit a low SWR. This is perfectly acceptable to many amateurs,
as evidenced by glowing reviews for a number of antennas which can be
shown to be quite inefficient such as the B&W T2FD or the Isotron. Those
amateurs would positively say that such antennas "perform", and this
can't be disputed since the judgment is entirely up to them.

. . .

Others may take it at all at face value, disregard all the
evidence to the contrary, and assume that the sellers are just trying
to hoodwink a gullible public into buying a non-working product.

That doesn't make sense to me. YMMV

"Working" is like "perform" -- the threshold is different for different
people. But a quick scan of reviews for the Buckmaster and Alpha-Delta
OCF antennas (the latter apparently manufactured by Buckmaster) show
high satisfaction from at least the users who have taken the time to
post reviews. Whether you or I would be happy with one depends on our
personal criteria.

As a *very* loose definition of work, I would say that I would expect
an antenna that advertises itself as an "all band" would allow a modern
Transceiver to put our something like full power, that is, to not fold
back on power. Oh yeah, and to radiate something. I don't see a tuner in
that diagram, so take your pick whether they are assuming an internal tuner.

Certainly there are a lot more stringent criteria. But that would seem
to suffice for many hams. 8^)


- 73 de Mike KB3EIA -