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
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-
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.
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.
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.
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.
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:
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.
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.
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.
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-
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-
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:
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:
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.
Once the toroid is wound, then trim the ends to fit, strip them, and solder them into place. Here is what that looks like:
And then install the cover, label it, and you’re done!
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.