That is a question many have had. Why consider a balun? What is it? Where do you install it? What does it do?

 

Whoever made up the word had quite an imagination. It comes from "balanced" to "unbalanced".

 

Why consider a balun?

 

Most radios have 50 ohm impedance outputs. Coax is normally close to 50 ohms. No problem so far. However, sometimes the antenna has a mind of its own and may not be 50 ohms, but 300 or 400 ohms. This is the case with certain loops. Therefore, you make a transformer at the antenna to solve the mis-match.

 

Some choose to use a ladder line rather than coax. Then the balun or transformer is at the transmitter end, usually through a tuner. The ladder lines range from 200 ohms to 600 ohms. This will feed a high impedance loop antenna. It can also feed a low impedance dipole by feeding it differently using a "DELTA" connection. This is where the dipole is one solid wire. The ladder line is connected at a point to the left of center and to the right of center. At the point where the impedance of the dipole matches the ladder line impedance.

 

Most installations are, radio, 50 ohm coax then 50 to 70 ohm antenna. Why use a balun?

 

It is due to the nature of RF on wire. "Skin Affect". RF traveling along a wire appears on the surface of the wire. Sort of like floating along.

 

Coax has a center conductor with RF floating on the outside surface of the wire. The other side is the shield. The RF is floating along the inside of the conductive shield. This is perfect for good cancellation allowing the coax not to radiate until it gets to the antenna.

 

However, sometimes when it gets to the antenna, the skin affect makes a problem. Instead of going to the shield side of the antenna, it takes advantage of the skin affect and goes down the outside of the shield of the coax. Then it says howdy to you while your operating. Giving you a little RF burn when you touch your equipment. Or, causing havoc in the station. Coming through speakers not even turned on, light up light bulbs, activating alarm systems and the list goes on. The amount of RF coming back on the shield depends much on the length of coax. Normally it is a fraction of the total power.

 

What can you do? A balun.

 

You can either construct or purchase a 1:1 balun. Some are called isolation transformers.

 

This puts a high reactance component between the shield side of the antenna and the shield of the coax. There are a variety of ways to do this.

 

Coiling the coax is an easy way to do it. There are toroid configurations and there are ferrite beads. Most of these will work well. However, caution is needed with the toroids and ferrite beads. It needs to be large enough to handle the RF power of the transmitter.

 

The coil of coax is quite affective. Listed below are some suggestions on coil size per frequencies.

 

 

3.5-30 MHz 10 FT, 7 TURNS, FORM=5 3/8"

3.5-10 MHz 18 FT, 9-10 TURNS

14-30 MHz 8 FT, 6-7 TURNS

 

Wind the indicated length of coaxial feed line into a coil (like an inductance coil) and secure so it holds its form. The balun is most affective when the coil is near the antenna.

 

So, to balun or not to balun? You will know when you fire up your radio.

 

73,

Ralph WD0EJA

 

 

137 MANCHESTER DR.

FLORISSANT, CO. 80816 U.S.A

PH/FX: 719/687-0650