Hi Peter,
This is all a very important discussion!!
> Tom, if I say that the test were made at 432 MHz and they
were pretty
> conclusive, they were. On 432 MHz !
No. The test was only pretty conclusive for that exact test
setup including cable lengths and types and that antenna on
432MHz.
The test did not tell anyone a balun is not needed as a
general rule on 432MHz or any other frequency, unless they
tested with all variables changed and the effects produced
no changes. For example, what if the cable length was
altered or the antenna type was altered?
The electrical characteristics are what determines the need
for a balun, and the rules do not change. A proper test
demands we know what we are measuring and altering, and we
need to know the likely worse situation and likely best
situation.
A random test of "something" does not prove anything except
an exact duplicate of that certain something. That is why we
do multiple tests with different things changed. At UHF, the
trend is that feedlines are electrically longer in
wavelength, bigger in diameter in wavelength, andtennas have
thicker elements and booms in wavelength, and so on.
Consider the 49 MHz groundplane I tested. In the testing, I
altered the feedline common mode impedance by altering the
ground point of a shield to a second groundplane. Worse case
condition of common mode was with 1/2wl electrical feedline
shield length between the GP feed and the reference ground.
If I did not add that variable in the test I used a
favorable combination for not requiring a balun. When the
antenna gets shipped out in the real world, no one would
know if the situation was at one end or the other of the
extremes.
I do know a good decoupling methods made the system totally
immune from where the feedline was "grounded", and I could
be sure the SWR and pattern would always be good on a mast,
tower, or support of any type as long as the antenna itself
was in the clear.
> If relevant to HF is another matter, and therefore I
stated nothing but 432
> MHz, clearly remembering the F/B discussion we had a
couple of weeks ago..
Still doesn't matter Peter.
>The mere fact that the antenna measurements were valid and
carried out by
> people with excellent knowledge was just information added
so that we would
> not end up in an endless discussion regarding wether the
test range was a
> "proper" antenna range or not.
What was the test protocol?
If they tested only one antenna and did not look for worse
case combinations we can be sure the test does not mean
anything at all.
My point is not if the test range is good. I assume it is.
Asking me to believe all UHF Yagis do not need baluns based
on one test is asking a great deal, because I can easily
prove a UHF antenna could require one.
Here are the requirements:
Coaxial cable requires the shield to have ZERO volts of
potential difference along it's length. This means the
antenna terminal connected to the shield must have zero
volts with respect to the boom and everything else. A
dipole does not do that and still have balanced currents in
each half.
The current flowing into or from the shield must exactly
equal and be at any instant of time exactly opposite current
flowing from or into the center conductor.
If either of the two conditions are not met, the feedline
will radiate. This is true on 432 kHz and 432GHz also.
73 Tom
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