BALANS .....  When And Where To Use Them


If you have transmatch that includes a good huskly balun, and
plan to use balanced open line, you should have no problems. Also, if
you are using coax, no transmatch, a balun at the antenna, and staying
well under a 2:1 swr, with modest power, still no problem.  But let's
take the fellow who has an 80 meter dipole, cut for the middle of the band
around 3750MHZ.   He tried it first without a balun...swr is low at
resonance but around 7:1 at the extreme band edges.   Next, he puts a 1:1
ferrite-core transformer type balun up at the feedpoint of the antenna.

What happens?

His swr comes down to 1.5:1 at the band edges.    Boy, that balun really
solved the problem.   Right?.....WRONG!

In this case, he would be much better off without a balun and use a
transmatch.0  The antenna hasn't changed at all, the swr is still just as
high as ever.  The meter is reading less reflected power, because the high
reactive currents on both sides of resonance are being absorbed in the
balun's ferrite core.  That means that both radiated and reflected power
are down, making the meter read lower in the reflected power mode.
You could replace the ferrite balun with a 50 ohm resistor and get the
same results.

This is not a blanket condemnation of transformer-type baluns, used
correctly they are often helpful and necessary.  However, you should
know the basics of how the antenna works so when you run into problems,
as I have describd here, you don't rejoice and assume that you got
something for nothing.

There are two considerations when using transformer-type baluns:

1. Operate them well within their power ratings (there are some
   transformer-type ferrite baluns rated at 3KW, 5KW and even higher).

2. Don't operate them in the presence of high swr.

To operate all the way across several bands, handling widely-varying
impedances and swrs, you need either the husky balun provided in a
good transmatch or, even better, a well-designed coaxial balun.

Whatever kind of balun you use for balanced open line it should be 4:1 or
higher.  It is quite practical to put the balun just outside the shack,
terminate the balanced line there, and come into the unbalanced transmatch
input with 10 feet or less of coax. This is convenient and simplifies
switching antennas.