Antenas

Creating a balun

Written by G0KSC
What is a Balun and why do I need one?

Within this section we will look at the various methods and requirements of a balun in the hope that you will be better placed to make the right choice for your antenna. There are many different types of balun and some incorporate impedance transformation too and it is this areas which leads to someconfusion.
For 50Ohm direct feed antennas, just a 1:1 balanced to unbalance transformer is needed. However, other antennas need transforming from 12.5 to 50Ohm up to (and beyond) 200Ohm to 50Ohm. These types need the impedance to be transformed in addition to the feedpoint balancing effect. So why do we need any form of balancing at the feedpoint in any case?

Coax Cable
Coax is a wonderful thing.It's invention has allowed the feeding of antennas through a manner of different substances, conditions and places the a balanced feed line could not. However, there are issues that need to be over come when feeding an antenna with coax.
It has become commonplace that the Radio Ham or even commercial entities use coax to feed antennas due to the convenience of so doing. So much so, commercialmodern day transceivers have an unbalanced output designed to feed and receive directly from coax. Coax can be feed through walls, under-ground, up the side of our towers or metal poles without any drastic affect on the Antenna or it's tuning - or does it?
The best way to feed most antennas is with a balanced feeder. The reason being, most antennas (including Yagis which are of interest to us here)are balanced antennas. If we feed a balanced antenna with an unbalanced feed line, issues occur at the feed point; namely common mode currents which appear as a result of this balanced/unbalanced miss-match. These currents travel back down the coax and radiate and thus, the coax becomes apart of our antenna system and radiates.
In instances where we are feeding a multi-band antenna, it may notmatter too much to the operator as long as he is radiating a signal from his antenna (despite that there is more likelihood that he will cause interference and have RF in the radio shack due to the coax being apart of the radiating antenna). However, when we are feeding a Yagi, we need to ensure only the antenna itself radiates. Having the coax feeding our Yagi radiating will act to distort theradiation pattern of our antenna. No, this problem will not cause a high SWR. In fact, it may even reduce the SWR seen in the shack giving the ham a false sense of security that things are OK when they are not.
Balanced line feeders
The most common balanced line feeder in Ham Radio today is the 300 Ohm ribbon used within the very common G5RV antenna. Any twin line can be used as a balanced linefeeder; speaker wire, bell wire, even mains flex. They will all have different characteristics but will all provide a balanced feed to an antenna if used that way. The twin feeder will not radiate (or very little anyway) as a result of the phase of the RF in each feed line which is 180 degrees out of phase in each leg. This means, one side of the feed line cancels the other out so no radiation occurs.Disadvantages of a balanced line
Firstly, we will need a balun to use a balanced line feeder in any case as our radios today are not presenting a balanced output. The next point is the effect that any close by objects have on the feed line, walls, buildings in general, towers, all metal objects, ground, everything! We need the feed line to be in as much open space as possible in order to ensurethe balance feed line can perform as it should. Beginning to understand why a balanced feed is not in mainstream use for the Ham?
I will not go any further into balanced line theory as we are not going to use one. We are going to feed our Yagi with coax and minimize the losses and likelihood of common mode currents on our feed line at the same time.
The Dipole Centre
The dipole is an...