The worse thing about any debate is all the bad data that
pours out of well-intentioned but totally meaningless
efforts to prove or disprove points.
> A good ham friend of mine in former East Germany, together
with a very
> reputable German antenna specialist, many years ago made
some measurements
> of 432 MHz yagis fed with split dipoles and coax looking
for pattern
> distorsion without baluns. Plots were first made with
baluns fitted, then
> they were taken off and new plots were made.
The problem with making plots at 432 and using them with
lower frequency antennas should be well known.
The scaling to 432 of ~14 MHz antennas was the cause of
cubical quad antenna gain vs Yagi gain errors made years ago
that haunt us to this day! To this day many people wrongly
cling to the idea that a quad has 2dB gain over a Yagi of
similar boom length. (In the quad antenna range studies,
measurements indicated the quads being tested had 2dB gain
over Yagis.) It does not work this way. The elements
diameters, feedline characteristics, earth effects, and many
other things were ignored the variables.
Look at a simple balun test at UHF and HF. At HF, the
feedline is a very small number of wavelengths long if even
as much as one wavelength. The common mode impedance goes
through large gyrations with length and frequency changes.
At UHF, the feedline is probably many wavelengths long. This
moderates the impedance at a reasonably high value, and the
radiation from the feedline has a very much different
pattern when it is electrically very long.
The elements are generally very thick in fractions of a
wavelength at UHF, much like using very large pipe for
elements at HF would be. The diameter of coax is very large
in terms of fraction of a wavelength.
Another problem is common mode impedance. In any balun test,
there are worse cases and best cases. A coil of coxial cable
in air has a VERY bad worse case effect, and a moderately
good best effect. The coil of coax is OK at best, and can at
worse destroy the system worse than not using it at all!
Low-Q baluns have a very good overall effect without
possibility of a bad worse case effect on the system. They
pretty much always help.
Just a cable itself with any balun can be a sweet length
that does not require a balun, or a sour length that
destroys balance. A 432MHz test using whatever common mode
problem occured by sheer luck cannot prove anything at all
in other situations.
The reason I can say this with certainty is even a six meter
VERTICAL antenna I measured had severe effects from feeding
with coaxial cables and no balun, and it was a four radial
groundplane antenna with a wide pattern we would expect to
be more immune than a dipole-fed Yagi. Yagi's I have tested
are just as bad.
How we make the test has a huge effect on results. The
problem with NOT using a balun is not that every system
suffers problems. The problem is we have no idea if the
system will be OK or not, and probably to way to check it.
I would never use a few turns of coax in the air and pretend
I was building something useful or reliable without
understanding what is required. To work as a balun:
1.) Design the balun to have a very high impedance at mid
range. That means a TA33 needs a high impedance balun at
21MHz, and that would require nearly 8 feet of coax in a
single layer maybe 4" in diameter. That's an 8-turn single
layer solenoid.
2.) The coax shield MUST be grounded to the boom and
mast/tower connection on the transmitter side of the balun
right at the balun. This is to be sure the coax common mode
impedance is reasonably low. Without that the coax could be
presenting a -j impedance that is the conjugate of the balun
impedance, and unbalance would be maximum!
> The tests were pretty conclusive, there was not much
pattern distorsion at
> all without the balun. The measurements were made at the
official German
> PTT antenna range, so we can indeed trust the integrity of
the plots. I
> have seen copies and the pattern distorsion was indeed
minimal.
"Official" is nearly meaningless in the context of "valid".
People make invalid measurements all the time, even when
they use the correct equipment.
I watched the HP sale representative do a demo at a radio
club that was an absolutely improper use of the analyzer
because he did not understand the problem he was measuring.
He understood the results. He understood the source. He
didn't understand the variables.
He plugged a small whip antenna in the analyzer jack and
looked at an HT from a distance of a few inches away and
said "the harmonic is XX dB down". That was wrong, because
the analyzer antenna was short and the frequency response
was not corrected. He also was in the near-field of the
antenna and the HT. He wasn't measuring anything meaningful
at all, and the print-out he gave the fellow was accurate to
"officially" accurate to .05dB but totally useless for
forming any conclusion.
We never could test one antenna especially at a greatly
different frequency and say with any accuracy "you never
need a balun" unless we fully understood how to obtain the
worse case and best case for antennas.
I can say from direct experience that Yagi patterns can be
severely compromised with improper feed isolation.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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