"Jerry Martes" wrote in message
...

How can I determine how effective a balun is?? I am working at 137 MHz
and cant consider a radiation pattern range of any kind.
I have been trying a coil of small coax in a relatively low loss ferrite
toroid and then slipping a high permeability tube of more lossy ferrite on
the coax to the receiver. I thought the higher reactance with a
reasonable
Q might minimize the current conducted along the feed line. Then, any
currents that do get by the reactive coil might get disipated in the lossy
and high permeability ferrite tube section.

Since I'm working without much knowledge and almost no test equipment,
I'd
sure appreciate any information about how to evaluate baluns at VHF.

===================================

It all depends on what YOU mean by "effectiveness".

First ask yourself why, in your case, you think you need a balun.

If you are referring to common-mode current on the feedline then you have to
state your requirements in numerical terms and find some means of measuring
it as a fraction of the total line current, without disturbing the antennas
normal environment.

But that's only half the job. Assuming you have some objections to common
mode current you then have to numerically relate common mode current to the
adverse effects it may have on ALL other aspects of antenna performance. And
unless you have some idea of the MAGNITUDE of side effects you don't want,
you can't sensibly talk about it.

Just a caution, nobody ever talks sensibly about power being radiated
specifically from feedlines. There's no such stuff. ;o)

The best way I can think of of discovering what a balun is actually doing is
to entirely remove it from the antenna and see what happens to antenna
performance in terms of the all-important radiation pattern and gain. If
within your limited means of measurement you can detect very little or no
difference then, of course, don't bother to replace it. The usefulness of
baluns is often overated.

Similarly with your ferrite coil and tube experiments. If by doing
something, nothing happens, then don't do it.


Praps you could make a very simple current detector (a current transformer)
which will fit around the feedline. A pair of suitably-shaped ferrite blocks
plus a few turns of wire plus a diode plus a 500pF capacitor plus a 100-ohm
resistor plus a DC microammeter are all that's needed.

But making measurements at 137 MHz is fraught with error even by experts.

If your antenna is for receiving purposes only, then its 99.9 percent
certain you don't need a balun anyway. And a few turns of a few inches of
small diameter coax on a small ferrite ring can't possibly do any harm even
if you decide to use one. It is sure to work as intended even if you've no
idea how well it's supposed to work.
----
Reg.