Talk:Choke Balun
why use a balun?
Well, the canonical answer is that the balun converts the dipole (BAlanced) to coax (UNbalanced). But what does that mean?
For transmission lines (twin-lead or coax) to not radiate, each conductor must carry equal and opposite currents. It's these equal and opposite currents canceling each other that results in zero net field away from the transmission line. If the currents are not equal and opposite, then they don't cancel, and the difference results in an external electromagnetic field, and your transmission line radiates like an antenna.
This is generally undesirable. When transmitting, it can mean high RF in the shack, messing with speakers, digital electronics, etc. If attached to a directional antenna, it causes problems transmitting and receiving because the actual radiation pattern is a combination of the intended antenna, and the unintentional antenna formed by the feedline, which probably isn't directional at all. If you've modeled your antenna, the model assumes a feedline that does not radiate.
OK fine, you need equal and opposite currents in coax to make it not radiate. If you attach it to a dipole, you should get that. It looks pretty symmetrical, right?
Well yes, the dipole is symmetrical, but the coax isn't. Usually, the shield is attached to ground somewhere. Even if it isn't, the geometry of the shield and the center conductor are not identical, so the shield will have a different impedance with respect to the dipole than the center conductor.
Regardless of the coax, the dipole may not be really symmetrical anyway. Consider not just the antenna, but everything in its near field. Is it perfectly level to the ground? Is one end closer to a metal gutter?
The result of all these asymmetries is that each side of the dipole presents different impedances to the feedpoint. Ideally, the antenna would accept all the power sent to it by the transmitter. The equal and opposite currents put into the coax by the transmitter each go down opposite legs of the dipole where the EM energy is radiated away. But when the dipole is not balanced, the currents accepted by each leg of the dipole can't be equal. But, all the current must go somewhere, so the difference in current between the dipole legs is reflected back down the coax, back at the transmitter. However, this current doesn't have an equal and opposite current on the center conductor, so it radiates off the coax, like an antenna.
If you operate a dipole fed with coax without a balun, this is happening to you, though you may not notice. The coax shield radiates just fine, and if your signal still finds its way to the other station and you don't give yourself RF burns, then there's no problem. A good RF ground at the feedline's entrance to the shack can reduce the RF re-entry. Even without any special attention to grounding, at 100W, the RF in the shack probably won't do more than make some speakers buzz, which might be considered "normal".
By adding a balun, you are avoiding this issue. By isolating the feedline, you won't get RF in the shack, and your dipole will behave like a dipole described in textbooks. Your antenna tuning and performance won't be altered by other objects near the coax, because the coax won't be part of the antenna. If your dipole is in a good location for an antenna and your coax isn't, then you will have overall a better antenna.
A dipole is a particular machine for creating EM fields. The idea is to set up a voltage between the two halves of the dipole in such a way that an EM field is created and efficiently radiated away.
Remember, voltage is a difference between two things. When we consider just an dipole, with no feedline or anything else around it, it's very simple to see that the dipole is symmetrical. The currents in one leg create voltages against the other leg, because there's nothing else.
If we feed the dipole with twin-lead, it's still easy to see how the dipole can be symmetrical. Each leg is now working against the feedline, but since both leads of the feedline are very close to each other, they are affected equally by whatever the antenna is doing, and there is no net effect. Of course, everything else (the Earth, trees, gutters, towers, etc) around the antenna would also need to be symmetrical, and this is unrealistic.
If we feed the dipole with coax, the situation is very different. The shield intercepts any EM fields, effectively hiding the center conductor from any external fields. You don't have a symmetrical arrangement anymore: one leg of the dipole is connected to the shield, so really, it's an L shape. In fact, the shield is just as much part of the antenna as the dipole it's intended to feed.
So now consider what happens to the currents traveling down the coax. The signal from the transmitter consists of $I_A$ and an equal and opposite $I_B$. $I_A$ must flow down one leg of the dipole, because it has nowhere else to go. However, at the feedpoint, $I_B$ has two options: it can flow down the attached leg of the dipole, or it can reflect back down the coax shield.
Thus, $I_B$ will split, with some of it ($I_C$) returning on the coax, and the difference ($I_B-I_C$) going down the dipole, as intended. The degree of the split will be determined by the relative impedances of these two options.
This isn't often a problem, because the coax shield makes just as fine of an antenna as the dipole. It distorts the radiation pattern horribly, but since a dipole wasn't a directional antenna to begin, it hardly matters. It could mean that you get a lot of RF in the shack, but if you are transmitting with 100W this is unlikely to cause any serious problems.
There are however, some more subtle problems: if you are transmitting RF in the shack, you are also receiving RF from the shack (reciprocity). You might be able to get your antenna far away from the powerlines, switching power supplies, computers, and other noise sources in the shack, but if your feedline is actually part of the antenna, then you are receiving all this noise anyway.
By employing a balun, you are somehow forcing the currents on each leg of the dipole to be equal, so that no current returns on the coax, so that the coax does not radiate and act as part of the antenna. There are many designs, but one of the most common and effective is a simple common mode choke, also known as a 1:1 current balun. This clever device has the effect of inserting a large impedance between the feedpoint and the outside of the shield ($I_B$ and $I_C$ in image above), making it relatively more appealing for the current to go down the dipole
Note that the mismatch of 50Ω coax to a 75Ω antenna isn't relevant to any of this. If your feedline and antenna are mismatched, you will get some power reflections, but those reflections would be inside the feedline, and not cause the feedline to become part of the antenna. There are balun designs that also perform impedance transformation, but this is not the function denoted by balun. On a practical note, 75Ω is a good enough match to 50Ω, and typical feedline losses at HF low enough, that in my opinion most attempts at impedance matching in this situation are unnecessary, and likely waste more energy through inefficiencies than they gain in obtaining a better match.
answered Nov 5 '13 at 14:17
Phil Frost
source: http://ham.stackexchange.com/questions/538/using-a-balun-with-a-resonant-dipole
antenna matcher just wastes signal!
Most people appreciate the need to use an antenna tuner at HF, so that their tranceiver transmits into a 50 ohm load and sees an SWR of close to 1:1. Most hams today have an antenna tuner sited right next to the transceiver, or even built into it. What is often not appreciated is that using an ATU at the transmitter doesn't solve the problems caused by a high SWR. It merely fools the transmitter into thinking that the antenna is matched to 50 ohms. Sure, this is pretty important for most solid-state finals, which don't like working into a high SWR, and which may even fail if there is a big mismatch. But whilst an ATU may help your PA keep its cool, it won't do a lot of good for your radiated signal.
Whatever mismatch exists at the antenna, whatever SWR is measured without the ATU in circuit, this mismatch still exists whether your ATU has "matched" the antenna or not. Transmitted power will still be reflected back from the antenna instead of being radiated from it.
If your antenna is designed to be nominally 50 ohms on the bands you operate on, if the SWR is, say, 3:1 or less, then it's true that the loss through mismatching will be fairly small. However, since most modern PA's aren't fazed by an SWR of 3:1 or less, there's really little point in using an ATU in this situation, apart from satisfying a need in the operator to see a 1:1 reading on the SWR meter! It will make little difference to the radiated signal (and you should bear in mind that losses of up to 10% can occur within an ATU itself.)
