Let's briefly review the two types of antennas we have discussed so far.
- The first is the dipole which you will recall is two equal length lines going out in opposite directions from a central feed point. This antenna is an inherently balanced antenna.
- The other type of antenna we discussed is the ground-plane antenna. It is a single line leading away from the center feed point perpendicular to a plane created by a solid conductor or evenly spaced radials. It is an inherently unbalanced antenna.
- Open-wire lines consist of two conductors (wires) kept running in parallel. This is an inherently balanced feedline. It is important to keep the distance between the two lines constant. Other names for this line are parallel-conductor and open-wire. The common types of open-wire line are Twin-lead, window-line and ladder-line.
- Coax cable consists of a central conductor (wire) surrounded by a conductive tube (shield). This is an inherently unbalanced feedline. Like the open-wire line, it is important to keep the tube at a constant distance away from the center conductor by keeping the center conductor exactly in the center.
The open-wire line achieves a low level of radiation because the current flow in the two wires is in opposite directions with equal magnitude or strength. The fields created by the currents along each line are equal and opposite and thus cancel each other. Now they don't cancel completely, even in a theoretically perfect open-wire line. That is because of the separation between the two lines. In order for the fields to cancel perfectly there would have to be no distance between the two lines, in other words, the lines would have to occupy the same space. In practice, what's important is that the distance between the lines should be very small compared to a wavelength. The ARRL Antenna book puts 1% of a wavelength as a maximum and says, "smaller separations are desirable." Several other factors go into the design of good open-wire line but I'm not prepared to go into them yet.
The coax cable achieves a low level of radiation in a very different way. With the coax, the current flows in one direction through the center conductor and the other direction in the conducting tube or shield. A crucial fact of this type of feedline is that due to skin effects, the current flowing "in" the shield, actually flows on the inside surface. In a theoretically perfect coax cable there is no current flow on the outside of the shield and none of the field on the inside penetrates the shield. So the theoretically perfect coax cable does not radiate at all. As with open-wire antennas, there are many factors that go into making a good coax cable and I won't be going into them tonight.
Generally speaking, balanced feedlines work well with balanced antennas and unbalanced feedlines work well with unbalanced antennas as long as the impedance is matched reasonably well. Things get interesting when you try to connect an unbalanced coax to a balanced dipole.
When you connect the coax to the dipole you have the center conductor connected to one side of the dipole and the shield connected to the other. The mismatch between the balanced and unbalanced elements causes a secondary current to flow on the outside of the coax. This secondary current is called common-mode current. This does two undesirable things, it changes the current flow in one half of the dipole, changing it's radiation pattern and even worse, it turns the coax shield into a radiating antenna itself! You may have already guessed how we solve this problem. Enter the balun.
The entire purpose of the balun is to eliminate common mode current. If you do that you will end up with a perfectly balanced signal. So how do you accomplish this? The answer to this would be very simple except that I have jumped ahead of Lee's series on impedance. So let's make this a teaser for his upcoming segments. The way to stop a radio frequency AC current on the outside of the coax is to create a high inductive reactance. After a few more weeks with Lee you will know what inductive reactance is but I will just say that it is a resistance to current flow that makes the current flow more difficult, the higher the frequency becomes. The important thing to note is that we are putting this inductive reactance on the outside of the coax so it will only stop the flow of the current on the outside of the shield which we don't want. It won't affect the normal flow of current on the center conductor and the inside of the conductor. A simple and effective way to make your own balun is to simply coil a few loops of coax near where it connects to the antenna. Of course you can also buy a balun which will do essentially the same job in a more compact space. Another benefit of the store-bought balun, assuming it is a good one is that it will be designed well to do it's job and not have undesirable side affects over a wide frequency spectrum. It is certainly possible to achieve that yourself with a homemade loop but you must take some care in the construction.