# Tag Info

38

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....

25

"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 ...

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 ...

14

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 ...

11

You'll find voltage baluns in plenty of circuits. For example, the QRP Labs receiver module: HPSDR Pennywhistle (twice, T1 and T3): The Elecraft KX3: A voltage balun can do some things a current balun can not. Firstly, the turns ratio can be varied to provide impedance transformation. Some of these circuits use something other than a 1:1 turns ratio to ...

10

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 ...

10

Without having the ability to measure this particular one, I expect that it's to counteract the inductive reactance introduced as a side-effect of the balun design/implementation. Edited to add: check out this semi-related answer. In constructing his balun, he used a prototype with a variable capacitor to find the value with lowest SWR for his application, ...

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

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 ...

7

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 ...

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. ...

7

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 ...

7

Think of it first without the feedline: The half-wavelength of transmission line provides a 180 degree phase shift, and the (short) connection between the shield ends is "ground". That means it's at 0V relative to the environment, and also the voltage at the open ends of the coax center conductor will be equal in magnitude but opposite in sign relative to ...

6

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 ...

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 ...

6

The other part of me thinks that the DC blocking capacitor will be a low impedance at RF so that the RF is actually "pushing against" GND or VCC but I don't know which one. This appears to be a kink in your understanding. RF doesn't "push against" one or the other. Let's be more specific about what we mean by "pushing against": ...

5

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 ...

5

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 ...

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

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

You could rewind it, but the result may not necessarily be better. This is because the core must be optimized for different needs in each balun design. In what you're calling a "current balun", the idea is to maximize the impedance in the common mode. Ideally, that impedance is mostly resistive, meaning high loss. Saturation current isn't so important, ...

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 ...

5

When making a coax choke balun, does the size of the ferrite matter, and if so, why? Simplest answer: If the choke impedance is low enough to allow some common mode power through, then there is a possibility of overheating. The bigger cores either dissipate heat better or provide higher impedance. Another way to say the same thing: As long as the choke ...

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

A choke is an inductor which is used to block high frequencies while allowing DC to pass. All chokes are inductors (though sometimes more than one inductor), but not all inductors are chokes: to be called a choke the application must be to block high frequencies. Counterexample: an inductor in a matching network is not a choke, but the same inductor, used to ...

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