Heathkit box + more

[William Ward]

Rob,

The following excerpt may offer you a bit of theory and rationale for using a balun vs feeding a dipole directly.  It is taken from the URL http://www.cqcqcq.com/baluns.html

"An antenna such as a horizontal dipole, fed at its centre, and therefore electrically symetrical, is said to be a balanced antenna with respect to earth potential. In this situation to maintain the symetrical balance, it would seem best to feed with a balanced feeder such as 75W twin feeder.
However most transceivers today have 50W co-axial outputs, making the use of co-axial (the centres of the centre core,and outer shield are concentric with each other, therefore co-axial) cable more convenient.
Co-axial cable is not symetrical and not a balanced feeder, it is unbalanced, as one side (the screen) is connected to earth potential. It would seem that feeding the antenna with coax would therefore upset the symetry of the system.
In theory feeding with coax will cause current flow along the outside of the cable, which could cause the cable to radiate with resultant interference problems, inefficiency of the antenna (the antenna should be receiving all the power to radiate - not the cable), and r.f. in the shack itself.
The severity of the problem is less, the greater the ratio of the antenna length, to the coax diameter. At the lower bands, 1.8MHz, 3.5MHz, this ratio is very large, therefore the problem is small, but it will increase at the top end of HF and into the VHF bands.
As the frequency increases, so will the problem, and it may need to be addressed on the higher bands.
So, is there not a problem on the lower bands? Well, so far we have assumed that your dipole antenna is mounted fairly high, and far away from buildings. Sounds ideal, but not always so easy in practice. If one leg of your dipole runs near to the house, while the other is well away in the clear, your aerial will be slightly unbalanced. If one leg of your dipole runs over the house, while the other is well away in the clear, your aerial is unbalanced. If the dipole is mounted low, and the returning feeder runs parallel to and not far below one leg, it will be very unbalanced.
In the first instance, a balun at the feed point of the dipole will cure the problem and help prevent interference. In the second instance a balun may help to keep RF off the coax. In the last instance a balun is unlikely to have much effect, and a co-axial cable choke about 10" in diameter made from 7 turns of coax placed at the shack feed point would be more beneficial.

So which balun should we use? At HF, baluns are generally fairly broadband. They fall into two main categories: Voltage and Current.

In voltage baluns, equal and opposite voltages appear at the output terminals. If the antenna is balanced, the current flows will be equal and opposite, and therefore no current will flow on the feeder, and the coax will not radiate. Under these circumstances the current balun will perform the same. However if the antenna is unbalanced, by a nearby building, unequal currents will flow at the voltage balun output, and current will flow on the coax, causing problems. Because the windings in a voltage balun appears across the feed, it is less effective at lower frequencies than the current balun.

Whereas in a voltage balun, equal and opposite voltages appear at the output, in a current balun equal and opposite currents appear. Even if the antenna is unbalanced this has the effect of choking off any antenna currents. The current balun will also operate over a wider frequency range and with greater power levels as the windings do not appear across the feed. "

A quick internet search on "current balun theory" or "voltage balun theory" will net you a wealth of more theory, most of it written by other hams with professional and practical information, theory and anecdotes.  Very enjoyable reading for the most part, too.

Bill, W5QI

Robert Myers <rsmyers at ROGERS.COM> wrote:

Steve,

Does this contribute to RFI that may be experienced by any neighbours (I mean by NOT using a BALUN)? Whether the BALUN is (1:1:), or say (3:1) --- and even when the SWR shows as being quite good WITHOUT a BALUN in place?

Suppose my antenna is an inverted "V," and that it is pretty close to 50 Ohms (down from its 72 Ohms) due to its adjusted slope. Should you still use a (1:1) BALUN? You are still going from the coax (UN) to the inverted "V" (BAL), so maybe you should put in a BALUN anyway?. But why? (Again, with emphasis on how this relates to any RFI --- if at all.)

If you are having slight RFI problems with neighbours, is the addition of a BALUN a possible solution --- I mean does it "in theory," at least, cause a few dB less interference? What's the physics/electrical engineering behind it? It certainly wouldn't hurt, would it? (Would it?)

I hear a lot of anecdotes (and I'm sure they are all quite true for the particular case they stem from), but I wound like to hear some theory referred to, if possible (just to help me perhaps), as to why there's a need for a BALUN, and/or why it works, etc., or when would it really NOT matter at all, perhaps (if such a conditions even exists) ----again, with some electrical engineering theory from someone who understands what's going on from that standpoint.

Thanks,
--- Rob

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