# Tag Info

33

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....

24

"Balun" is a portmanteau of "balanced" and "unbalanced". Anything made to interconnect a balanced and unbalanced load can be called a balun. A common-mode choke (like a length of coax wound over a ferrite ring) works as a balun because it inserts a high impedance in the common-mode without affecting the differential-mode. By action similar to a voltage ...

16

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 ...

12

Baluns are a source of much confusion in the hobby, and are probably one of the most common causes of RFI, common mode noise reception, and poor antenna performance. Folks try to shoe-horn a lot of very complex values and characteristics in to a very linear way of thinking, and most of it is just flat wrong. Starting from the top: That design should in ...

9

Another answer mentions the paper by K9YC, Jim Brown. This is the best reference on baluns in amateur radio. See: http://audiosystemsgroup.com/RFI-Ham.pdf Chapter 6 is about baluns and antennas. He says, "The primary function of most baluns, at least in our ham stations, is to minimize the interaction of our antennas with the transmission lines that connect ...

8

Balanced lines (of which twisted pair is a special type) really have an upper frequency limit; you can't use them to transport 1 GHz (well, you can, but the smallest variation in direction or distance would have catastrophic effects, and the conductor distance would get pretty small). This can be seen in technical practice: 100 Mbit/s Ethernet (Fast ...

8

A balun matches a balanced load to an unbalanced line, but it can also do other useful things. A current balun can present a high impedance to common-mode signals, which will help reject noise. Common mode signals are the same on both conductors, so are not "balanced" or differential. An unun is an impedance transformer, usually 4:1 or 9:1, which matches an ...

7

I'll explain the operation of that balun very briefly: for the differential mode (which by definition has equal but opposite currents on each conductor), each conductor induces an equal but opposite magnetic flux in the core. These magnetic fluxes cancel, and so the differential mode sees no inductance: it's as if the core and the windings aren't there. ...

6

You don't need a balun, in that it will work and it won't hurt you. But your antenna will not work as well as it ought to. When you are transmitting, the effect of a balun used to connect a coax (unbalanced) feed line to a dipole (balanced) antenna is to prevent the RF from the transmitter from returning down the outside of the feed line and go entirely ...

6

In most multi-band balun applications, there is rarely a need to maintain a perfect 50 ohm impedance within the balun. The feedpoint or input impedance is varying widely so another impedance bump in the mix typically has no detrimental effect. I highly recommend the use of coaxial wound over bifilar style for a 1:1 balun. Comparatively, the coaxial and ...

6

It's a confusing diagram. Some of the voltages are differences between points you might intuitively assume. For example, the $2V$ at the right is the voltage across the entire load (which also confusingly, is modeled by two identically named series resistors). But the other voltages are implicitly relative to the ground symbol attached to the connector ...

6

Many baluns will work just fine in either direction, though there isn't just one kind of "4:1 balun". This kind is wound on two cores, and works as a common-mode choke: simulate this circuit – Schematic created using CircuitLab A common-mode choke works in either direction, so it matters not which end is balanced and which is unbalanced (or if both ...

5

It's easier to see what's going on with a bit of rearrangement. Imagine this built of tubing: The feedpoint is still where it would be on a dipole without the balun. The feedpoint sees the dipole as usual (blue). In parallel with that is a twin-lead transmission line formed by the two parallel sections of the balun (red). This is a quarter-wave section ...

5

OK, let me try to answer this, but this answer may also be qualified as unqualified. If you have a 50 Ohm receiver, and connect a perfectly (Z=R) 75 ohm antenna system, then your VSWR would be 1.5, and the "load mismatch attenuation" will be about 0.177dB. (with antenna system I include feedline) I doubt that you would actually notice this. However, you ...

5

The ideal number of turns depends on core material, geometry, and frequency. This is why you find such variance in how many turns should be used. More turns increases the choking impedance up to a point, but decreases the choke's self-resonant frequency (SRF). Once the SRF goes below the operating frequency, adding more turns increases the distributed ...

5

How do I convert the above data to provide the impedance of my antenna, at this point, for this tested frequency? "Z (Ω) = 167.19 - j63.91" is telling you the impedance. The impedance of your antenna is $(167.19 - j63.91)$ ohms. Then, how many ohms, increase or decrease, would be required to “transform” from the antenna impedance, so it becomes the 75Ω ...

5

1. You need to re-think your balun. At UHF we don't use toroids and wire, it's always done with transmission lines of varying impedance, on PCB or maybe thin coax. There are transformers for UHF, mini-circuits sells them, but they're tiny - the cores are say 2 x 4 mm and the wires hair-thin. Power handling is 0.25 Watts. Here's a photo/rendering - the pads ...

5

Depends on how much current you mean by no current. But in general, unless the dipole is actually perfectly symmetrical, in a perfectly symmetric environment, above a symmetric ground, with the feed line at exactly 90 degrees all the way into the far field, and/or with the radio equipment and their ground connections also exactly 90 degrees perpendicular ...

5

Since you say "Terminated Folded Dipole" and mention a resistor, it sounds like you are talking about a T2FD. This is a significantly different antenna from a vanilla folded dipole, which does not have a resistor. A T2FD can indeed provide a flat-ish impedance over a wide band, however this comes at the expense of efficiency. A significant fraction of the ...

4

Perhaps some of my confusion comes from an audio background, where one has "balanced" and "unbalanced" cabling in a way that is easier for me to understand: an unbalanced cable is simply a signal and ground wire, while a balanced cable actually has a ground wire and two conductors whose voltages are opposite. That's not entirely accurate. What makes a ...

4

I'm no expert on balun designs, but I think they mean to construct a folded balun, as follows. The coax's outer conductor is soldered to the same part of the antenna, but it is left on the coax, not peeled away, so the final balun structure includes a length of the coax. (The interpretation you proposed in your question is unlikely because if the outer ...

4

Baluns are designed to be transformers (like 1:1 4:1, 6:1, etc.) or choke baluns, and both. For an antenna, the purpose of a choke balun is to create a high-impedance to common mode currents that would flow on the outside of coaxial cable shielding. These common mode currents can cause all kinds of problems such as RF in the shack, matching problems, and ...

4

A "Guanella balun" is really just a common-mode choke. simulate this circuit – Schematic created using CircuitLab For differential-mode currents, the current on one half of the transmission line is equal but opposite the current on the other half. Likewise, the magnetic fields due to these currents is equal but opposite, so they cancel, and so there'...

4

There are. I suspect you're confusing a couple of different things when it comes to balance. Common mode current is current which appears to flow on both conductors in the feed line with the same polarity and magnitude. If you put RF in to an ideal coax and from an ideal source, the current on the center conductor is precisely equal in magnitude and ...

4

Short answer: No, and in fact you'll often get more bang for your buck by avoiding coaxial baluns. The description of the balun's purpose in an antenna system you quote is an oft repeated bit of "common wisdom" in ham radio, but it is such an over-simplification that it is at best drastically misleading and at worst, just plain wrong. The purpose of a ...

4

TV coax typically has foam dielectric and very low loss. On receive it does not matter if you use 50, 60 72 or 75 ohm cable. With short cables, losses are also not important. The cheap TV balun could be problematic, particularly if you want to transmit. It is trivial to make a 1:2 transformer to get the 4:1 impedance transformation you need, but it is hard ...

4

There are balanced feedlines (twin-lead), and balanced antennas (dipoles), and similar things all described as "balanced". In this sense, balance means the two halves of the thing have equal impedance relative to ground. Here, ground does not mean whatever is called ground on the schematic. Rather, it means "the environment". Consider twin-lead for example. ...

4

How do I connect the coaxial cable to the dipole Using a balun. That's its job. when only the inner conductor transfers the signal This is an incorrect assumption. Current always flows in loops, so there is always an equal and opposite current somewhere. In a coaxial cable, the signal is carried by both the inner conductor and the outer conductor (or, ...

4

I believe it's a misprint. Comparing the drawing showing the construction of the balun with its schematic representation, it does appear the dot on the lowermost winding (terminals 2 and 5) is backwards. If we build it like the schematic suggests, consider the application of a differential mode input on the balanced side: simulate this circuit – ...

4

The advantage of using coax for a 1:1 choke-balun is the constant Z0 of 50 ohms. As can be seen in the following graphs, the only time a 1:1 choke-balun accomplishes a 1:1 transformation is when it sees 50 ohms at its output. Any other impedance at the output causes an impedance transformation because the SWR is not 1:1.

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