Why do you need a balun? (Amateur radio)

Whenever you go setting up your antennas, you often read or see online that you are supposed to have something called a balun, or an unun, or a choke, or choke balun, or voltage balun, or some other doo hickey between your feed line (coax) and your antenna.

But why? And what is this bal-un thing-a-mick-bob?

If you look up "balun" online, you'll probably get even more confused, as explinaitions online ususally show circuits with three wires on one side and two or three on the other... And that doesn't make sense for a two line feed line feeding a dipole antenna, which literally means "two poles," not three or five or whatever.

So, to understand this, let's start with why we need something between our feedline and antenna. And the reasons are

1. Because you may need a transformer.
2. Because of something called "common mode current" that we need to get rid of by what we call a "choke."

Let's start with number 1. As you learn about radios, feedlines and antennas, you soon learn of the importance of impedance matching. If your radio is 50 ohms, your feedline and antenna need to be 50 ohms. If everything is 50 ohms, then it all works like a big 50 ohm resistor. That means for every 50 volts, we'll get 1 amp of current, and everything works just fine and dandy. The same with 100W of power, which would translate as 70.7 volts and 1.414 amps.

If everything is perfectly 50 ohms we get that perfect 1:1 (one to one) SWR that you'll hear a lot about in ham radio. Basically a high SWR, like above 2:1 or 3:1, is bad because a lot of power is being reflected back to the radio. But 1:1 is good because no power is reflected back to the radio. But let's say we try forcing that 70.7 volts at 1.414 amps into a 100 ohm impedance circuit. If we push 70.7 volts into a 100 ohms it will only let 0.707 amps go through. That's half the amperage! Now where will that amperage go? The fact is that we end up with both voltage and amperage reflected back to the radio and only part of our power actually makes it all the way to the antenna. We need some way of turning that 70.7 volts into 100 volts, and that 1.414 amps into just 1 amp.

• 70.7V x 1.141A = 100W
• 100V x 1A = 100W

When might something like this occure? Well, it depends on your antenna. Most antennas are NOT 50 ohms. Your typical center-fed half wave dipole is around 73 ohms. And what if you want to feed your dipole off-center, or on the end? Yes, off-center-fed and end-fed dipoles are a thing and have many advantages. But the problem is the farther you go away from the center of a dipole, the higher the impedance gets. This is because towards the ends of a dipole the voltage goes up and the amperage goes down. So how do we solve this?

Easy! With a transformer. All baluns are transformers, and I'm not sure why the ham community is so obstinate about calling them baluns instead of transformers. Well, in reason two we'll get to that, but as you may well know, a transformer is a device that exchanges voltage for current. An ignition coil in a car, for an example, transforms the high current 12 volts from the battery into a low current but high voltage spark for the spark plugs, usually somewhere in the range of 50:1 to 200:1 turns on the secondary windings when compared to the primary. This is why you see baluns designs identified as 1:1, 9:1, 25:1, 85:1, etc., etc., etc. Those are the transformer characterisitcs, the ratio of turns from one side to the other. Now baluns aren't the only type of transformer for antennas, but regardless, if you are using an end-fed or off-center-fed dipole antenna, you almost inevitably will need some sort of transformer in order to match the impedance of your circuits.

So what is it with the terminology "balun" and what is this common mode current choking thing we apparenlty were told to address? Well, when you look at coax you might only see two lines and think that there's only one way current can flow through a coax cable, down one conductor and up the other. That's called "differencial mode current", because current is going in opposite (different) directions down each conductor.

But what if current came down both conductors in the same direction? That's called "common mode curent" and that is bad, we don't want that. When can we end up with common mode current? Well, remeber that in a center-fed dipole antenna we are trying to feed two elements, the two legs of the dipole. But if we have a coax cable hooked up, in reality would could have three elements, the outer conductor of the coax being the third element. You can see this could mess up our antenna design completely! How do we solve this?

Well, with some sort of transformer. If we're not changing impedance a 1:1 transformer would work. This is why people use 1:1 baluns. The 73 ohm impedance of a dipole is close enough to work, we just need to keep the coax from becoming  a third element on our antenna. So we wind up a 1:1 balun.

Now, you may note that a 1:1 balun doesn't isolate the antenna from the coax. And the reason is is because it doesn't have to. All we need is a transformer that make sure that the current going down one conductor in the coax is the same as the current in the other. We can do this by either coiling up the coax on itself making one big inductor, or by coiling it around a ferrite ring. You could also put ferrite beads on the coax, although that's not as good and isn't a true balun. The idea is that if you make a coil with a high enough inductance then it will choke out all common mode current. Your coax can no longer fuction like a third element. So how does our RF make it through the balun? Well, if you have the same amount of current going one way as you have it going the other, they cancel out and experience no inductance. Therefore our RF signal to and from our radio will pass through, but the RF signal of just one of the dipole elements will not be able to pass through and make the coax work like a third element.

Well I hope that explains it. I will try to draw up some pictures and update this later on to make it easier to understand. If you have any questions feel free to ask me. Thanks!