Transmitting Baluns

 

Related articles at

Balun Test 
contains model of “perfect” dipole currents.  

 Sleeve Balun
shows how a sleeve adds impedance, useful for VHF and higher baluns

Receiving Common Mode
Noise shows how lack of a balun can contribute to system noise (it applies
to transmitting antennas as well)  

Longwires, Verticals,
and Baluns shows how unbalanced antennas can have similar problems

Balun and Core selection
for transformers and baluns

 

Testing Transmitting Baluns

One of the most popular Ham-lore rumors is a balun’s performance can be
tested or evaluated by grabbing the coax and watching for an SWR change. This is probably one of the
worse test-rumors circulating! 

An observable SWR change when touching, grounding, or altering length of a
feed line (with the same Zo at the test meter) indicates severe common-mode
current problems. 

The opposite response, however, means nothing. Lack of SWR change does NOT
prove the balun adequate and the system free from common-mode currents.

I was first exposed to this wild idea when asked to evaluate a problem with a
commercially manufactured antenna. The antenna’s design engineer decided a bead
balun was adequate, based on this “grab and touch test”. The antenna/balun combination passed the
“grab test” with flying colors, even though the system had gross common-mode current levels.
Despite extending the bead balun from the original foot to almost three-feet of
beads, I couldn’t remove common mode excitation of the feed line by the grossly
unbalanced antenna.     

If we think about it, we would never consider
it likely that our hand would have significantly lower impedance than the shield
on a long length of coaxial cable! We also would never expect, just by dumb luck,
to always grab the high
impedance point of a random wire like the feed line.

A hand-grab-test will cause an SWR change only when the impedance of your hand
is very low compared to the common-mode impedance of the cable at the point
where you are “grabbing” the cable. The portion of impedance
attributed to feed line radiation (compared to overall antenna impedance) must be a significant portion of antenna terminal
impedance or the SWR will not change. 

It is possible, if we alter antenna feed-cable lengths, to observe SWR
changes as an indicator of common mode currents from poor system design. But
such methods are never nearly as accurate as an absolute current measurement,
and are actually only a little bit better than the useless “hand
test”. They also can give false answers if the feed line impedance is not
the same as the SWR measurement device impedance.  

An Accurate Test

The most simple and accurate way to test the effectiveness of a balun is to
actually measure the common mode currents on the feed line with a suitable RF
current meter that does not perturb the system. Such meters are inexpensive to
purchase and are also easily manufactured at home. The meter would consist of a
closed core that snaps around the cable, a terminated winding on that core, and
a meter that measures RF voltage across that winding. The coil’s load resistor
should be low enough in value that the impedance of the cable shield is not
perturbed.

By sliding the meter along the feed line, we could get a good idea of the
common mode currents. The meter should be moved along at least 1/4wl of
feed line. It normally would be sufficient to sample just four points along that
distance.

feed line radiation, like antenna radiation, is a function of the linear
ampere-feet of conductor. The important distance is the spatial or linear
distance, not the length of a coiled conductor. It is the charge acceleration
over a linear distance of space that accounts for the radiation. A one foot box
packed with 500 feet of cable is still just a one-foot antenna!

There are many cases where adding a balun will actually INCREASE common mode
currents. Be sure you either fully understand the system or have modeled the
feed line and antenna correctly.

Power Testing

A choke or current balun can be tested by inverting the ground on either end,
and using a dummy load.  Actual operation may produce more stress than this
test, or less stress, but such a test gives a reasonable idea of the power
rating under reasonable conditions.

The balun should not overheat or change impedance with application of full
power for the expected duty cycle in the expected maximum operating temperature
(be sure to include sunlight heating).

Impedance Testing

Using the above test configuration, SWR can be measured. SWR should be flat,
and should not change when the ground by the dummy load is moved from
side-to-side. This should be true at full sustained operating power.

Testing Voltage Baluns

While voltage baluns are the least desirable types in most applications, they
still have some useful applications. Using this configuration:

By moving the tap point along the dummy resistance, an idea of balance can be
obtained. The balun should show perfect performance with the tap-point at the
load center, and should show an SWR increase as the tap moves to the top or
bottom.

Crossed Split Windings on Toroids

Some articles and handbooks show a Split winding method. This method is supposed to reduce winding
capacitance by moving the ends of windings further apart. The proposed theory is
by reducing shunt capacitance that “leaks RF around the balun”, balun
performance is enhanced.

This method very often hurts the performance! Check
this article