Practical Antenna Theory - Part 3
One of the most often discussed topics in Ham Radio (and the most misunderstood) is SWR. In this series of articles we will learn some basics, some theory, and dispel a few myths that are generally perpetuated in the Ham Radio community.
In our first installment we learned some terms used in discussing antennas and feed lines. We also dispelled the myth that a high SWR sends RF back to your finals and can cause damage from the RF. If that is what you still think, reread the first article and think about it some more!
Last time we took a look at the tuner and the role it plays in the transmitting system. We crushed yet another myth - a low SWR reading at your tuner in the ham shack, is an indication that the antenna is tuned.
We also provided guidelines for an efficient transmitting system that should remain trouble free (given there is no component failure involved).
In this issue we will look at a common component in some transmission systems – the balun. It is quite common to see a balun in wire antennas of all sizes, production quality and brand name. Well, what is this thing and why do we need it? Is it good for anything else or is it just for wire antennas?
The term balun is formed from two words: balanced and unbalanced. It can be correctly inferred that this device services both balanced and unbalanced components.
Most hams think of a balanced component as something like a loop antenna wire. The truth is that any dipole antenna (antenna with two connections not connected to ground) is a balanced component. Balanced being defined as a device that demands relatively equal currents from both feedpoint terminals. Examples of balanced feedpoint antennas are; Beams, Yagi’s, wire dipole, wire loops and quaggi’s.
When one feedline connection is to ground, the antenna is generally considered to be “unbalanced”. That is to say RF tends to flow on the radiating element and the current for the other connection is in the ground. Examples of this type antenna are: Vertical monopole and trapped vertical antennas or single wire vertically erected antennas such as the inverted “L”.
There are some types of antennas that seem to defy this simple definition. Take for example the so-called Carolina Windom. The Windom dipole is an off-center feed, flattop dipole. That is, one side is generally longer than the other measured from the feedpoint connections. This particular antenna takes advantage of a phenomenon whereby feedpoint impedance increases as you move away from the center of a dipole. This off-center placement allows a balun to have a higher matching ratio than the normal 1.5:1 used on most dipoles (caused by the mis-matched nominal 72 ohms of the diple and the characteristic 50 ohms of the coax). Currents do flow equally in both antenna feedline connections, necessitating the use of the balun to connect to the unbalanced coaxial feedline.
The G5RV antenna is similar with the exception that a length of twin wire feed line is used to create a broadband feedline balun before connecting to the coax. Both types of antennas are world renown for their unique characteristics in the 40 meter and 20 meter bands. There is even a 17 meter DX version available from some suppliers and homebrew designs are online.
So what about the vertical antenna? Does it need a balun? The short answer is no. But it is more involved than that. Vertical antennas carry RF currents just as beams and dipoles. The major difference is that one of the feedpoints is ground. Quite often, vertical antennae are directly connected to the coaxial feedline. However, it is possible to isolate the antenna from earth ground and the transmitting system by use of an unbalanced to unbalanced device known as a unun. Just as the balun provides its’ effect by isolation, the unun separates the unbalanced antenna from the unbalanced coax cable through transformer action.
Last time we talked about the practice of not trying to operate on frequencies where the VSWR is greater than 3:1. Aside from the reasons mentioned last time there are a few more where balun and unun devices are concerned.
Transformers are constructed with insulating properties between turns and between windings rated at a given voltage. In the days of old when vacuum tubes were common, modulation was accomplished by passing the plate current through one winding of a modulating transformer. The other winding of the transformer was for the modulating audio circuit. These two circuits operated at thousands of volts difference. The windings in this type of transformer were specially designed to insulate the very high plate voltage of the finals (2000-3500 volts) from the relatively low audio circuit voltage (150-300 volts).
When a balanced antenna experiences a high SWR, the resulting voltages (especially at high power) could be very high. The maximum voltage rating for most affordable coax is in the neighborhood of 3500-5000 volts. Using Ohms Law for power circuits we can predict that 50 ohms at 1000 watts and very high SWR, can (under certain circumstances) produce upward of 20000 volts at the antenna terminals.
It is not hard to see that the voltage or current ratings of a balun or unun can be easily exceeded under high VSWR situations.
The same thing is generally true of unbalanced antennas, albeit at lower voltage expectations.
Is it possible to work a dipole that is directly connected to coax? Sure. However, using this configuration builds in many system inefficiencies that manifest in numerous ways (some of which may be undesirable or dangerous).
Let’s add to our rules of good practice:
Rule #4:
When feeding a balanced antenna, use a balun. The possible exception might be a beam or yagi. In recent years beam design has begun to include a quarter wave coax balun at the feedpoint in order to minimize RF current on the coax shield. Some have even gone to the expense of choke baluns constructed of quarter wave cylinders over the outside of the coax, even toroid broadband feedline chokes. Many hams also include a makeshift coaxial loop choke by coiling up a few turns of coax just below the balun. In a well designed and conservatively used antenna system, this serves as little more than meager lightning protection.
Rule #5
Be particularly careful when using high power supplied to an antenna that has a balun. This care could avoid internal damage due to voltage breakdown of the balun windings and provide many years of faithful service.