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In another question, commenters recommended that I look into adding a 1:1 balun to a dual-band dipole to eliminate the feedline as a radiator. As I understand it, a 1:1 balun also functions as a choke, but is more efficient because... reasons?

When I try to find plans to build a balun, they all seem to specify a frequency range of 1.6-30MHz. Why is that and how can I make a balun that will work on VHF and UHF?

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  • $\begingroup$ Must it be both, or can it just do one or the other? $\endgroup$ – tomnexus Feb 15 '17 at 21:21
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    $\begingroup$ I don't have the time to give you a good answer, however Fair-Rite #43, #61, and #67 nickel-zinc (NiZn) ferrites are intended for 30 MHz-1 GHz. A bead or two of the appropriate ferrites around the coax at the feedpoint is one way to choke off common-mode currents . See audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf. More info about these ferrites can be found at fair-rite.com. $\endgroup$ – Mike Waters Feb 15 '17 at 22:23
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Let's clarify some terminology: a balun is any device used to convert between a balanced system (ladder line, dipoles) and an unbalanced one (coax, vertical monopoles). There are a lot of ways to make a balun, and a common-mode choke is one of them. A choke can also be used in balanced to balanced, or unbalanced to unbalanced connections where suppression of common-mode currents is merited.

The choke approach (or "current balun" as you'll sometimes hear it described) is nice because it's small, easy to fabricate, broadband, and low-loss. "Small" is especially an advantage on HF where other balun designs would be huge.

Ferite-wound chokes can be done on VHF as well. Jim Brown, K9YC has some measured data on ferrites. Look for page 49. His data show that Fair-Rite mixes 31, 43, and 61 can all be made to work. He shows for a T240 core of any of those mixes, you can get about 600Ω on 2m with 2 turns, and 300Ω with one turn on 70cm.

However, the effective permeability of ferrites drops with increasing frequency, so it becomes difficult to get a sufficiently high choking impedance for the choke to function effectively as a balun. 300Ω isn't great but might be sufficient for some applications. You can always measure and see.

Alternatively, VHF wavelengths are small enough that other balun designs become practical.

One approach is to keep the system unbalanced all the way. If you have an unbalanced coax feedline, then put an unbalanced monopole antenna on it.

Antenne gp vhf 3.jpg
By F1jmm - Own work • Self-photographed, CC BY-SA 3.0, Link

The three drooping wires are the "ground plane". Drooping them increases the feedpoint impedance from the 37Ω of a monopole on a flat ground plane to 50Ω for a better match. Since the ground plane presents a very low impedance to the ground currents, that's where most of the current will flow. The coax common-mode is a relatively high impedance.

Some antenna designs which aren't simple monopoles use a "skirt" which looks like radials on a monopole. As in the monopole, the skirt looks like a low impedance ground plane, and the feedline is "hidden" under it.

Other designs are the folded balun, and the folded dipole. These designs work by getting the coax to attach at a ground potential point, while creating such a point through some clever arrangement.

A sleeve balun is another possibility. It uses a quarter-wave stub to insert a high impedance in the common-mode.

There some antenna designs out there that claim no balun is necessary. Sometimes it's true (the balun is intrinsic in the design) and sometimes it's not. Look for an antenna model that contains a feedline such as W8JI did with the J-pole. Or build the antenna and measure the common-mode current yourself.

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  • $\begingroup$ "However, the effective permeability of ferrites drops with increasing frequency" is the key thing I was missing. This is due to skin effect, right? $\endgroup$ – William - Rem Feb 16 '17 at 7:41
  • $\begingroup$ @William-Rem No. Sounds like a good follow up question :) $\endgroup$ – Phil Frost - W8II Feb 16 '17 at 15:22
  • $\begingroup$ Phil, you'll be happy to know I just ordered a large package of mix 61 ferrites from Mouser. Thanks again for your help! $\endgroup$ – William - Rem Feb 27 '17 at 19:30
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What you really want is a choke (often called current balun), as a voltage balun (i.e a transformer) will not isolate the feedline unless the antenna is in book-ideal conditions. The minimum misbalance in the dipole and the isolation is broken.

Don't have experience at those frequencies but I'd try a several series toroids as other have suggested to get a minimum of 2K commonn impedance at the frequencies of interest. If you can make it 5K much better.

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  • $\begingroup$ Welcome to ham.stackexchange.com! Please consider taking the tour to get the most from the site. $\endgroup$ – rclocher3 Feb 24 '17 at 20:58
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There is one easy answer, a balun is a transformer! - a choke is a choke!

Forget you ever heard the term "choke balun".

I'm sure you know balun stands for BALanced to UNbalanced. A 1 : 1 balun can be used to match a coax to an equal value impedance, balanced line.

A simple choke (one to eliminate common mode currents - CMCS) can be made by winding 25 feet of coax around a one gallon ice cream container. They have a slight taper, making it easy to remove the coil after winding and securing with wire ties in 4 places or so. Note the number of turns is not the critical factor here; rather the length of coax used. This should be effective from 1.8MHz and up.

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    $\begingroup$ A lot of misinformation here. There are many balun designs which are not transformers, and "choke balun" is used in professional literature. When coiling coax around some form to make a balun, you are making an inductor. The impedance of that inductor is what matters, and the diameter, number of turns, height, coax size, and spacing between turns are all factors, and the required parameters depend on the frequency in play. Simply winding 25 feet of some unspecified coax for some unspecified frequency around some loosely specified form is not a recipe for success. $\endgroup$ – Phil Frost - W8II Feb 22 '17 at 17:21
  • $\begingroup$ Welcome to ham.stackexchange.com! Please consider taking the tour to get the most from the site. $\endgroup$ – rclocher3 Feb 24 '17 at 20:58
  • $\begingroup$ It's two wires (center conductor, shield) passing through a core. Sounds like a single turn transformer to me. Does it "choke" common mode current? Yup. Is it a choke or a transformer? Yes -- both! Denying the terminology is valid is a sign you haven't studied why that terminology was selected and accepted by the industry. $\endgroup$ – user10489 Jul 31 '18 at 4:37

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