Baluns

In the second article Keypounder has generously blessed us with, the issue of Baluns are covered. Baluns are most often (but not always) used as dipole centers to match BALanced antennas to UNbalanced feed line. (see why it’s called Balun?) While his article explains the nuts and bolts, in this abstract I’ll state that the effects are improved radiation efficiency (through impedance matching) and cleaning up noise that otherwise would be heard in the receiver coming from a variety of sources. Keypounder, as always, does an excellent job explaining the why and how through enabling the aspiring homebrewer. Take notes, knowledge can never be taken away.

Baluns and impedance transformers 101

By Keypounder

So, the NVIS article posted recently mentioned baluns, and in that article I mentioned that I homebrewed my own baluns and impedance transformers. So, what does a balun do, and why do I spend my time making my own? Hmm, complicated question. Looks like we’ll be peeling some more onions again!

The word balun is a contraction of the phrase “balanced to unbalanced;” the essential function of a balun is to provide a balanced output. The reason that you want balanced output is that many, but by no means all, common antennas are balanced antennas. Any center fed dipole (inverted vee, vee, flat-top) is supposed to be a balanced antenna, and so is anything made up of arrays of dipoles (Yagi antennas and wire beams like the Lazy H or the W8JK come to mind.)

Back before coaxial cable was common, most ham operators used balanced feedlines to send power from the final stage of their transmitter to the antenna. The idea was that since the antenna was balanced, and the feed line was also, with both legs of the feedline connected to the same impedance on each side of the antenna the RF signal from the transmitter would be the same on both legs and the feedline would not radiate RF. Although both the antenna and the feedline were balanced in theory, in the real world, minor differences in antenna length, proximity of the antenna to houses, trees, and other antennas, as well as differing ground conditions caused some degree of imbalance, with different impedance on each leg of the antenna, and resultant difference in signal on the feedline. Usually, as long as these imbalances were small, and the rules of thumb on keeping antennas and feedlines separate from conductive surfaces were obeyed, the impact of these imbalances was minor, and nobody worried much about common mode on the feedline.

With the advent of inexpensive coaxial cable after World War 2, amateur operators moved more and more to using coax and as they did they began to notice that their antennas did not work as well as expected. The outside (shield) conductor of coax actually carried two currents; one on the inside of the shield, 180 degrees out of phase with the center conductor, the signal being fed to the antenna by the transmitter; and the other, on the outside of the coax, picked up from the RF radiated by the antenna itself, called common mode. All the books tell you to route the transmission line directly away from the antenna at a right angle to the antenna, to reduce this pickup. Easy to say, not so easy to do in the real world. The leg of the antenna that was connected to the center conductor got an undistorted signal as intended, but the other side, which was now sourced to TWO usually out of phase signals did not. All sorts of problems resulted, including RF in the shack and on the audio, and distortion of the antenna pattern among others.

Rather than give up the convenience of coax cable, which unlike balanced feeders (ex: twin-lead, window line and ladder line,) is insensitive to proximity to metal and can be run virtually anywhere, even under ground or under water, amateurs sought to find a way to prevent the common mode current from becoming a problem. At this point, those seeking a more detailed description of the problem and the solution are invited to take a detour to K9YC’s website and check out this presentation-

http://audiosystemsgroup.com/CoaxChokesPPT.pdf

It will take about an hour to go through this, but it is worth your time to do so. Go, read, and check back!

To summarize the information K9YC presents so well, what was needed was a way to stop or “choke off” the common mode radiation on the outside of the coax, and ideally, to force equal currents to be fed to the antenna regardless of the environmental effects on the antenna, which induce different currents on the two legs of the dipole.

There are a number of ways to do this, some better than others-

  • Make a coil of coax, either scramble wound or a single layer solenoid, to create inductive reactance to inhibit the common mode current induced on the outside of the coax shield layer.

    • Pluses- all you need is some extra coax, and possibly a form on which to wind the coax. This is a decent field expedient.

    • Minuses- The impedance of the coil is relatively low, and may not be effective in correcting significant common mode imbalance. Weight and bulk can also be an issue, especially on the lower bands where you need a pretty big coil to get enough inductance to make a significant choke. This also only addresses the common mode on the outside of the coax and does nothing for environmental imbalances.

  • You can put a series of small ferrite toroids or ‘beads’ on a section of coax, which will provide some inductive reactance to reduce the common mode current.

    • Pluses- Such chokes are commonly available commercially, which saves time, and they are simple to make; you string the beads on until you run out of beads or coax!

    • Minuses- The impedance of a string of beads is simply additive. If you get 10 or 20 ohms of impedance from each bead, you’ll need a lot of beads to get over a thousand ohms of impedance, which can get expensive, and heavy! Again, this only deals with common mode on the outside of the coax.

  • You can place a ¼ wave conductive sleeve on the outside of your coax.

    • This is common at VHF and UHF where the physical length is short, and the high isolation provided by a ¼ wave sleeve is easy to get.

    • At HF these dimensions can be very long; at 80 meters the sleeve would be over 60′ long, so this is not usually feasible.

  • You can run multiple coils of coax through one or more larger ferrite toroids. Inductance increases as the square of the number of turns, and the effect of the ferrite material multiplies that.

    • Pluses-This can provide higher impedance than any of the previous methods, and is relatively easy to do, although there is a limit to how many turns you can get into a 1.4” ID toroid.

    • Minuses- These chokes can be bulky and heavy, and the price of toroids purchased in small quantities can be fairly high. Coax with connectors already installed can further limit the number of turns, unless you purchase the clamp-on types, which are significantly more expensive. Again, these only deal with common mode imbalance on the outside of the coax.

  • Finally, you can use one ferrite toroid and some solid copper wire to build a 1:1 common mode choke. This is my preferred approach for both listening and transmitting antennas.

    • Pluses- capital outlay is low, even buying materials onesy-twosey from local shows and at Home Depot. It takes me about an hour to make one, and I can wind it to optimize which frequencies I’ll use it on, and whether it attaches to coax on both ends or to an antenna. I often use THNN for common mode chokes, and use enameled copper for some impedance transformers. Most importantly, this type of choke balun suppresses both interior and exterior common mode currents

    • Minuses- these baluns take some planning ahead; the ferrites are not found in most local shops, even most electronic shops.

As I consider the last option, the bifilar wound ferrite core 1:1 choke balun, to be decidedly superior that is what I am going to show you, in detail, how to build.

First things first; we have some essential design decisions to make.

  1. What frequency band or range do we want the choke balun to cover?

Most commercial baluns have a frequency range that they are rated to handle, but very few will tell you exactly what impedance they provide at different frequencies. This choke balun will be for 40, 80 and 160 meters.

  1. What power level is the balun going to have to handle?

This will affect what size toroid you’ll need and what size wire to use, too. I build most of my antennas and baluns, listening antennas, and 30 meter and backpacking transmit antennas excepted, to take the legal limit, so this one will be made to take 1.5 kw.

  1. What sort of enclosure do we want to use?

I have used everything from PE freezer cartons to PVC pipe to PVC electrical boxes. For the most part I have standardized on 4” x 4” x 2” PVC electrical boxes for my baluns, but there are times when other enclosures, or even no enclosure, are used. This one will be a 4” PVC box.

  1. What kind of connections are we going to install?

I’ve used stainless eye bolts for baluns that are going up as feedpoints for antennas, banana jacks for baluns connecting to window line, 10-24 brass screws for Beverage antennas or ladder line, and SO-239 or N connectors for coax connections. I’m making up some isolation chokes for some of my low band antennas, so this one will be coax to coax using SO239 jacks.

Material parts list

1 ea 4” x 4” x 2” PVC weatherproof box with cover and SS screws $7 ea $7

2 each SO-239 sockets silver/teflon (hamfest) $2 ea $4

1 each Fair Rite #31 material toroid (bulk) $5 ea $5

8′ approximately 14 ga THHN $0.09/ft $0.72

#6-32 screws nuts and washers, about net $1.25

Total cost ~$18

You will also need some silicone window or tub caulk, which any self respecting DIY type will already have about, or should, and some liquid electrical tape comes in handy, too. I have and use both of those.

If you shop around and buy in bulk, going in with friends, you can do a bit better than this, but if you allow $20 for components you ought to be able to stay in budget. Occasionally the large home improvement stores run a special, so much off a given sized purchase, or my wife will get double credit card bonuses in gift cards. Those are good times to stock up on things like wire and PVC boxes if you intend on experimenting with antennas. If you have friends who are electricians, ask them to keep you in mind when they are renovating; a little bit of used wire will go a long way in making choke baluns, and if you ask the superintendent and get permission to scrounge the dumpsters when a job is wrapping up you can find all sorts of treasures.

Here is what the parts look like:

balun-1.jpg

The first step is laying out the holes in the box; we’re making an inline choke balun, so I’m going to put a coax fitting on opposite sides of the box. You could put the fittings on adjacent sides, or maybe even on the same side, with no noticeable difference in performance, but I like my inline filters to be, well, in line! I drill a pilot hole on each side, a 1/4” or 3/16” works well, but use what suits you. You can see that I centered the holes L-R and up and down; there is some advantage in being a bit closer to the front of the box, but on balance I think this works a bit better in the field. Off center attachments seem to encourage the box to get all twisted up if there is any wind.

balun-2.jpg

Then I ream the hole using this 5/8 reamer. I think I paid about $4 or $5 for it a number of years ago, and it works perfectly for this. Don’t think I will ever wear it out.

Once I have the hole reamed, I put one of the SO-239 panel connectors on the box, mark one of the small holes and drill it, then I put the SO-239 and one screw on to secure it temporarily so I can drill the other hole in just the right spot.

balun-3.jpg

A 9/64” bit works well to drill this hole; do both sides at the same time, because it is a pain to have gooey silicone get on you when drilling the other side. For this sort of balun, I don’t typically drill more than two holes on each fitting for this purpose; for antenna baluns, where I’ll be suspending the coax for some distance, I usually use all 4 holes on the SO-239. Once you have all the holes drilled, go ahead and get your silicone out, and put a small bead around the large and the small holes.

balun-4.jpg

balun-5.jpg

You want a little silicone around every part of a hole to the outside. Set the connector in place, insert both 6-32 screws, and put the washers and nuts on the inside of the PVC box. Snug them up and repeat on the other side. It should look something like this-

balun-6.jpg

Set the box off to one side and let the silicone cure while you wind the toroid.

Cut two pieces of 14 gage THHN solid wire about 42 inches long. Straighten them out gently and tape them tightly together about every three inches. When you are finished, you will have a short piece of transmission line that ought to look like this, with the #31 toroid-

balun-7.jpg

Take the #31 material toroid and stick about 3” of line through the middle, then hold it there with one hand. Take the long end with the other and feed it through the hole on the other side. Best not to try to bend it too closely to the toroid; I have found it best to leave the wire in as large a loop as possible and draw it up as tightly as you can, in one motion. Here is what the choke looks like at the start:

balun-8.jpg

Note that the wire is inserted into the center of the toroid and pulled tight. Don’t bend it around the toroid as this will cold-harden the copper and make it much harder to get snug. Also, DO NOT overlap the turns on the inside of the toroid.

This is what it looks like about halfway wound:

balun-9.jpg

15 turns is all that is required for this choke, intended for 40 through 160 meters. (A turn is one insertion through the center of the toroid.) This is what it looks like when you are finished winding.

balun-10.jpg

Once the toroid is wound, then trim the ends to fit, strip them, and solder them into place. Here is what that looks like:

balun-11.jpg

And then install the cover, label it, and you’re done!

balun-12.jpg

After I attach the coax fittings, I use liquid electrical tape to seal the fittings and the outside edge of the box to keep moisture out. This one took me a bit longer than usual as I had to stop and take pictures, but if you allow an hour or so for your first one that ought to be plenty of time; they get quicker as you practice. Hope to hear you on the air.

Keypounder.

Bibliography-

http://k9yc.com/CoaxChokesPPT.pdf

http://k9yc.com/RFI-Ham.pdf

http://www.introni.it/pdf/Amidon%20-%20Transmission%20Line%20Transformers%20Handbook.pdf

https://archive.org/stream/fe_Transmission_Line_Transformers/Transmission_Line_Transformers_djvu.txt

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23 thoughts on “Baluns

  1. Chris

    Technically, what is built here is an “unun.” The coax in and out are unbalanced. Yet it could be employed at an antenna feedpoint – because what counts is its ability to choke common mode currents. And a pair of screw terminals could replace one of the coax connectors for easier connection to a wire antenna if you prefer.

    The sources of unbalance, as mentioned in the article, and references, are many. It is not just a matter of coax versus balanced line – in fact if the antenna and its environment were perfectly balanced, a direct connection of coax to antenna would work as well as using a balun.

    But antennas and their environments are rarely perfectly balanced. A perfect balun (or unun) would reduce the unbalanced current to zero – at the point it is inserted. But that won’t reduce it to zero everywhere. If the unbalanced current is a problem you may have to employ multiple “common mode chokes” to adequately suppress it. One case might be if you are trying for certain antenna pattern – the radiation of unbalanced currents from the feedline would distort the pattern. A more common example is “RF in the shack” where unbalanced current (usually current on the outside of a coax) couples to objects like microphones and keys at the transmitter, causing problems. In that case the common mode choke may need to be near the transmitter, or entrance point to a fixed station (an perhaps better grounding there, too). Again, depending on the cause of the unbalance, and the resulting bad effects, you may need one or more baluns or ununs in the feedline as a cure.

    Also, there is a lot more in the references – especially Sevick – on the use of baluns and ununs as impedance changing devices. It is a subject that could take many hours of study. It has for me.

    Liked by 1 person

    1. With respect, Chris, this is referred to as a choke balun, or a current balun. It is not an unun, which is an unbalanced to unbalanced transformer. The extremely high impedance seen by unbalanced currents in this device (over 5000 ohms) results in forcing the currents to be very close to equal in magnitude and opposite in phase, which is the definition of a balanced output.

      Liked by 1 person

  2. shocktroop0351

    Much appreciated information gentlemen! The visual step by step is very helpful. At what point in the feedline would you install an inline filter? I’m under the impression that the closer to the despite the better. Thank you again!

    Liked by 1 person

    1. I use similar baluns at the antenna feedpoint, but with different connections to the antenna, as mentioned in the article. This one will be used near the entrance to the shack from one of my low band transmit antennas, and is intended to eliminate the common mode current from the outside conductor of the coax. On particularly long runs some authorities suggest breaking the coax run up by inserting one of these choke baluns into the line. I’ve also seen it suggested that where coax drops to the ground from a high dipole that a choke balun like this one be used along with a ground rod. None of my coax runs are especially long; the longest run I presently have is about 150′, of which 65′ is up in the air.

      Liked by 2 people

  3. shocktroop0351

    I should have worded that better. How would the balun constructed in the article best be used? I understand that changing out one so-239 for some screw terminals would make a good antenna feed point, where would you use a coax to coax connection?

    Liked by 1 person

    1. Chris

      You would use a common mode choke like this at the entrance to the station, typically, in conjunction with a ground (which you should also have at the station entrance panel). If you have a very long transmission line run, and it’s picking up currents (perhaps by couple to the antenna), and the currents are causing a problem, you might break up that line with more common mode chokes.

      It should be mentioned – certain expedient antenna types such as Zepps, end fed half-waves, etc. are very likely to have common mode currents on feedlines. Do you have to put in common mode chokes everywhere? No. If your goal is simply to radiate (and you aren’t trying for a special pattern like a highly directional beam) – and if the common mode currents are not causing any problems – you don’t need to choke the currents.

      Historically, hams started using baluns when coax became popular after WWII. Coax connections to yagis were allowing too much common mode current, messing up the pattern they worked so hard to achieve. Since then, the theory and construction of “voltage baluns,” “current mode baluns,” “common mode chokes” and so on have come a long way.

      Liked by 1 person

  4. Pingback: Brushbeater: Baluns And Impedance Transformers 101 | Western Rifle Shooters Association

  5. dangero

    So timely! I just installed a new 20m wire dipole at my house and suspect I was getting common mode down my coax. I’m still working it out and trying to isolate sources of RFI in my house but I came across this site:
    http://www.kf7p.com/KF7P/Ferrite_chokes.html
    I’m not associated with them in any way and I have yet to purchase their products but I called the guy and he was super nice! He had some great suggestions for me to try before even buying his products. That to me is the mark of integrity in a vendor so I thought I would share.
    Thanks for this great write up! I’m definitely getting more and more comfortable with DIY projects when it comes to ham radio. It definitely saves money but it also gives me a much better level of understanding of how these component work than when I buy them off the shelf which is super helpful.

    Liked by 1 person

    1. Dangero:

      That is a reasonable price compared to some other vendors, and it is certainly a good idea to support vendors who support their customers. His markups on the cores he is selling is fairly low. He’s also got decent prices on transmission line, too!

      Before you buy toroids in any quantity, you owe it to yourself to check out the big industrial vendors like Allied and Mouser. I deal with Mouser a lot; they seem more friendly to mid-sized purchasers, and they have a large variety in stock. http://www.mouser.com/
      If you can get a group buy up you can save a lot of money; Mouser’s current price is $542/100 for the 2.4″ #31 toroid Fair-Rite #2631803802.

      It does help to have the manufacturer’s catalog number when you want to order from them as the interface is not user friendly; send off to Fair-Rite for their catalog, it will help immensely. They are great folks to deal with.

      With regard to your 20 meter dipole, I use #43 material for 20 to 10 meter choke baluns, it is slightly better than the 31 material, but only slightly. If I ran out of #43 toroids, I’d use the #31 without another thought.

      As regards RFI, the best suggestion I have heard so far is to turn the entire house off and then turn on the breakers one by one to trace the noise source. You’d be surprised how noisy some of the modern ‘high tech’ things can be…… from lights to TV sets to thermostats to furnaces…… Good luck and glad you liked the article!

      Liked by 1 person

  6. ApoloDoc

    Good stuff! I am putting up an 80m doublet fed by homemade ladder line (4″ spacers down the 14g THNN). I know that some folks will directly feed that into a coax connector at the radio, but my plan is to use a 15-20′ piece of coax and was investigating what to use. Balun Designs is nice but expensive. I believe that this1:1 current choke is just the ticket, only swapping screws for the coax connector for leads on the antenna side. The low-loss nature of the balanced ladder line is just too useful, especially when I am trying to improve my reception.

    Excellent timing, my friend. Thank you!

    Liked by 1 person

  7. Aloha Scout,
    The Emergency Amateur Radio Club here in Hawaii has plans for an end feed matchbox antenna that when coupled with an antenna tuner will work from 6 to 40 m. You are welcome to re-post the plans on your site with reference to our site. We have had great feedback for operators across the country regarding its use.

    Marshall – NH6TD

    Like

      1. Aloha again.
        Here is a link to the club’s web site that has the plans in pdf form. We sell the assembled ones as a fundraiser to keep our repeater system up and running. I can build an antenna but can’t figure out how to attach a simple file. sigh

        http://earchi.org/proj_homebrew.html

        If you send me a P.O. Box I’ll send one to you for evaluation. Though I did see you just did one on the LNR end-feed.

        A hue huo,

        Marshall

        Liked by 1 person

      2. Marshall Ness

        I’ll need to know where to send. Evaluations of this nature tend to be more effective with the actual item in hand. Then again, I’m new at this radio thing……. 😁

        Liked by 1 person

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