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When using a balanced antenna such as a half wave dipole, with an unbalanced feed line such as coax, an RF choke can be used to block RF current emitted by the transmitter from splitting up at the antenna feed point and traveling along the antenna and also down the outside of the coax. Without the choke, the outer shield of the coax becomes a radiating part of the antenna, which is usually not desirable.

Why do amateur radio operators refer to the RF choke in this instance as a balun?

To me it seems that the RF choke isn't a balun and doesn't convert an unbalanced transmission line to a balanced system at all, but rather just removes one of the effects of connecting an unbalanced transmission line to a balanced antenna, that being the unwanted current traveling on the outside of the shield of the coax, by blocking it.

Isn't a real balun for example a ferrite ring with wires arranged such that common mode current is cancelled out in the same way that a transmission line operates while at the same time allowing differential mode current to pass?

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"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 divider, most of the current will then favor the differential-mode.

Note that a common-mode choke is not quite the same thing as an RF choke. An RF choke is more or less just an inductor, albeit one with the intended application of blocking RF current. A common-mode choke is a four-terminal device which looks like an RF choke to common-mode current, but to differential-mode current it looks simply like a transmission line.

schematic

simulate this circuit – Schematic created using CircuitLab

Typically the differential-mode impedance will be something like 50 or 75 ohms: it's simply the design feedpoint impedance of the antenna. A decent common-mode choke can provide a common-mode impedance of 4000 ohms. Because the differential-mode impedance is so much lower, the common-mode current will be very low, practically negligible.

A common-mode choke like this accomplishes the objective of converting between a balanced and unbalanced load. It's easy to manufacture, has low loss, high power handling, can be effective over a wide band, is inexpensive, and works even if the "balanced" load isn't quite perfectly balanced.

This is by no means the only way to make a balun, but for HF and lower this particular design has a lot of advantages, making it very popular. It is very much a proper balun, and with an appropriate design and application can be a superb solution.

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I can agree with the answers above, but I see radio Amateurs just coiling up the coax feed cable at the base of the antenna, without the ferrite ring and this does choke off the RF in both directions (No transmitted power or received signals).

I tried this with a half-wave long wire on a field day and demonstrated that with the coli present we could not be heard on an SDR receiver station monitored using a mobile phone app only 100 miles away, whereas once the coil was removed we could be heard.

Am I correct in saying that a coil of coax cable is an RF choke and not in any way a Balun?

Clive

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    $\begingroup$ Hello Clive, and welcome to ham.stackexchange.com! Our site is a question-and-answer site, rather than a forum-style site, and we're particular that posts typed into the "Your Answer" box should answer the question. Your post doesn't answer the question, and will eventually be deleted if it isn't heavily edited. Please have a look around and see how things work here; the tour helps. We have a chat room that you can use when you have 20 reputation points. That's a great place for less-structured chats. $\endgroup$
    – rclocher3
    Sep 1 at 23:58
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    $\begingroup$ Of course, you are very welcome to post your question as a question. :) :) $\endgroup$
    – David Hoelzer
    Sep 2 at 18:26
  • $\begingroup$ If you search questions for "balun", "coax balun" or "ugly balun", you may find your answer. $\endgroup$
    – Mike Waters
    Sep 10 at 19:06
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Everyone knows that balun stands for Balanced to Unbalanced and is an AC electrical device which can be used as an impedance transformer and is also used to convert an unbalanced or single ended AC system to a balanced or differential AC system and vice versa.

A common example of usage of this device is to enable an unbalanced coaxial transmission line to be used to feed a balanced dipole antenna split in the middle with the feed point in the center.

Very quickly, there are a few different types of balun :

  1. Voltage balun which behaves like an isolation transformer and removes the DC ground or common connection between input and output and allows the output to 'float'.

  2. Current balun which allows conversion from balanced to unbalanced but with a ground or common connection between input and output thereby referencing one side of the balanced system to the same ground as the input.

  3. Delay line baluns which used 1/4 wave length sections of coax cable as impedance transformers.

One of the desirable effects of using a voltage balun with coax and a dipole antenna for example is that because the balun is a low impedance device which blocks common mode current, all of the current present on the inside of the coax is transferred through the balun and no current can split up at the end of the coax braid and flow down the outside of the coax.

This prevents an undesirable effect commonly known as "common mode current on the outside of the coax" which basically makes the outside of the coax part of the antenna.

Because an RF choke can prevent current on the outside of coax just like a voltage balun does, confusion has arisen and there is a widespread belief that when used with an antenna an RF choke is a balun.

Unfortunately while both devices can produce one same result amongst many variables, this common misconception also confuses anyone trying to understand how antennas and transmission lines work.

An RF choke is in fact not a balun and doesn't convert a balanced to unbalanced system at all, but rather is an inferior band-aide solution to get ridding of common mode current.

An RF choke can't be used on it's own as an impedance transformer, and if you use coax with a balanced dipole antenna and an RF choke in line to stop the common mode current on the outside of the coax problem, you still end up with an unbalanced dipole with one side which has a DC connection to the ground of the input, which is not the same result you get with a voltage balun which provides DC isolation between input and output.

Also it's far easier to get an acceptable common mode rejection using a proper balun to get rid of undesirable common mode current than it is to get the exact impedance required in an RF choke to block the same current, especially since the effectiveness of an RF choke is highly frequency dependent whereas a balun isn't as much.

Saying that an RF choke is a balun because both things block common mode current is the same thing as saying that a hammer is a brick because you can use both to bang in nails so let's all build our houses out of hammers, or sea water is the same as tap water because there are fish in both so it's ok to drink both, etc. !

The result is that there are probably many antenna installations out there with a dipole for the driven element that don't work as well as they could because one side of the antenna has a DC connection to ground causing an unbalance in what is supposed to be a balanced system, and with RF chokes which don't actually block that much common mode current, thereby messing up the radiation pattern and being more susceptible to local interference, where a real balun wouldn't have these problems.

Hope that all makes sense !

RF CHOKE

enter image description here

VOLTAGE BALUN

enter image description here

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  • $\begingroup$ @Mike Waters In my opinion the point system on sites like stack exchange don't work when many people believe something that's false. The perfect example of this is the widespread belief by ham radios operators that inductive and capacitive reactances cancel at resonance for a dipole antenna, which is simply not true. When everyone agrees that a false idea is true and then down vote correct answers the result can be called Propagation of Misinformation. Then anyone who reads the question and answer can become confused because it all doesn't make sense. $\endgroup$
    – Andrew
    Sep 11 at 22:34
  • $\begingroup$ When users of this site down vote an answer without explanation, this can be an indication that the author of the down vote isn't able to intelligently comment on the subject being discussed, so unless there is an explanation i don't take these down votes seriously. I wonder if the Meta site has any comments on these ideas ... $\endgroup$
    – Andrew
    Sep 11 at 22:34
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    $\begingroup$ I'm happy to downvote and explain. There are many issues so I'll have to be brief. A DC ground for half the antenna does not present any practical problem. A common-mode choke can provide an impedance transformation. Whether a voltage balun or a common-mode choke is easier or more effective depends on the situation. Common-mode chokes can be very effective. But most importantly you're just arguing against the commonly understood meaning of a word. If you want to argue voltage baluns are superior, you can do that without asserting everyone is wrong. $\endgroup$ Sep 12 at 17:04
  • $\begingroup$ @PhilFrost-W8IIv I don't agree with most of what you say. You are saying that a balanced dipole antenna with a DC connection to ground on one side is still balanced ? I don't know if a choke can be used as an impedance transformer, but i'm not arguing the meaning of a word, i'm arguing that a choke is not a balun, which it isn't, i can't understand how this simple idea is so hard to understand. $\endgroup$
    – Andrew
    Sep 13 at 4:03
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    $\begingroup$ "You are saying that a balanced dipole antenna with a DC connection to ground on one side is still balanced" Yes, that's what I'm saying. The implicit assumption, by strong convention, is when we say "this antenna is balanced" we mean "this antenna is balanced at the operating frequency". For example we might also say "this antenna is a good match" but no one expects it to be a good match at every possible frequency. It certainly isn't 50 ohms at DC. $\endgroup$ Sep 13 at 13:40

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