Balloons and Onions? Ham Radio?

I thought this would be a fun post. In my implementation of transmission line transformation I was looking at the terms "Balun" and "Unun" and only thinking someone with no knowledge of baluns and ununs, or ham radio in general even, would hear those and think "Hmm, balloon and onion? What do those have to do with radios?"

So, to clear up any, uh, cornfusion?:

Balun - pronounced Bál-Un (not Ball-Un, Balloon)

This is a device that transforms balanced loads to unbalanced loads (hence the term Balun), or the other way around. If you have a coaxial cable and want to feed a balanced antenna like a dipole or doublet then you would use a balun here. For a mono-band dipole you don't have to. It is the simplest style of HF antenna to make, and has been used for ever fed straight with coax since the impedance isn't too far away from 50 ohms. However, since a dipole is a balanced antenna it would help if a balun was used. For a doublet you would typically feed it with a balanced line as it is a multi-band, non-resonant antenna and balanced line doesn't attenuate in the same magnitude coax does with excessive standing wave ratios (SWR). In this case you would use a Balun between an unbalanced tuner (all auto and manual T, L, and π networks) and a balanced feed line. Note that tuners with built in baluns use 4:1 voltage types most often (read my post here for more info on that).

Baluns can be made to not only transform from unbalanced to balanced loads but they can also modify the impedance of the load in that transformation. For example - if you have a high impedance antenna, say a folded dipole (theoretically) with around 250 ohms if impedance at the feed point, you could use a 4:1 balun to bring the 250 ohm impedance down closer to 50 ohms where you connect your coaxial cable. Keep in mind a normal dipole has a 72 ohm impedance at resonance, in the folded dipole case with a 4:1 balun it would be around 62.5 ohms - an even better match than a normal dipole to coax (1.25:1 vs. 1.44:1 SWR).

Unun - pronounced Un-Un (not Union, U-Nun, Onion)

This is a device that transforms unbalanced loads to unbalanced loads (hence the term Unun). So whats the point? If both sides are unbalanced why use an unun? There are two reasons. First, and similarly with the impedance transforming baluns, you can use a unun to transform impedances of unbalanced loads.

Say you have a short vertical antenna (such as a mobile whip for HF) that provides an impedance of 12.5 ohms at resonance (80 meter antenna with a lot of loading). You can use a 4:1 unun reversed for a 1:4 transformation. This brings the 12.5 ohm impedance of the antenna up to 50 ohms to match with the coaxial cable and radio.

The second reason is for isolation. Occasionally antenna systems create feed line radiation due to common mode current on the outside of the coax. Antennas are supposed to radiate and feed lines are supposed to carry the signal to the antenna to radiate, not radiate themselves. If your antenna system is resonant and the SWR is good you can use a 1:1 unun near the antenna as a choke for the common mode current.

Balloons and Onions

On the other hand, balloons (as well as kites!) can still be used in ham radio. They can be used as a means of suspending a vertical (or, near vertical) wire as an antenna. As for onions, I have no clue. If you come up with a use for onions in relation to ham radio let me know!

Baluns - Homebrewing

I decided to tackle a new aspect of Ham radio - transmission line transformation. One concept I have learned is a must, at least being in a very restricted environment from an antenna standpoint, as well as portable operating, is that impedance matching of non-resonant antennas is a necessary evil. This impedance matching is accomplished through the use of a tuner, the device that actually does the matching from whatever the antenna and feed line present to 50 ohms to make our radios happy.

Recently I bought an LDG Z-11pro tuner. The purpose of this is to match up with my FT-857D for portable operating. The tuner is fantastic - I am having a lot of fun using it here in the shack. However, it's purpose is to accompany the FT-857D in to the field - whether that is Field Day, traveling, emergency use, what have you. For matching unbalanced loads the Z-11pro is great. However, it doesn't have a balun.

LDG is doing a promotion right now where you get a rebate form for a free balun with the purchase of a new tuner. So I sent that in for a 1:1 balun. However, the LDG baluns are still a little big. When you consider the size of the FT-857D just about everything you match it up with is "big". So I figured I would research some balun theory and see what I could come up with for an even more compact balun.

I was over at Universal Radio yesterday and picked up the book "Understanding, Building, and Using Baluns and Ununs - Theory and Practical Designs for the Experimenter" by Jerry Sevick, W2FMI. There is a wealth of knowledge in the book, more so than one can digest in any short amount of time. It does take some technical knowledge from the start to make the most of the material, but the diagrams and schematics should be easy enough for any ham to understand.

I will say up front that I don't have a good way to test my balun at the moment. I need to get an analyzer, then I can make all kinds of measurements. At some point in the future I will invest in one.

The first balun I made is a 1:1 current type. This is probably the simplest balun you can make. There aren't any shorted turns or any weird winding methods to use. You just take parallel wires held together every so often with tape to hold them together and wrap them around a ferrite toroid.

I chose the 1:1 current type balun because all it does is transforms the match of the unbalanced tuner to a balanced feed line without altering the impedance. If I used a 4:1 balun the impedance that the tuner would have to match would be 1/4 of the impedance of the feed line and antenna. On high impedance loads this is ideal - the closer the impedance is to 50 ohms the better the power transfer is to the antenna. However, in the case of already low impedances chopping the impedance down by 1/4 more is REALLY bad. Tuners have a harder time matching low impedances than high impedances so it is always better to stay on the high side of 50 ohms.

For the one I built here I used a T106-2 toroid because it is relatively small yet should have enough capability for 100 watts. The type 2 material was chosen for the HF frequency range. There are 10.5 bifilar turns of 18 gauge enamel coated magnet wire. I mounted the balun directly to a PL-259 connector for ease of use with the Z-11pro (no coax jumper needed). Then I filled the hollow PL-259 with epoxy to set the wire in place. Without the epoxy I don't think this would hold together long.

Here are some pictures of the product:





The PL-259 is the unbalanced side that plugs in to the tuner and the empty wire pig tails are the balanced output (pretty self explanatory I think). I don't know if I am going to mount it in anything at this point. It is nice having such a simple, compact device. Though the durability might be an issue.

I have heard, from N5ESE, that using coax for the wires might be better. He states that he noticed a drop in the variation in SWR going from the bifilar wraps to the coax. By the way, I like his call sign. Send that one in Morse code - it just flows with a very unique rhythm. I have worked him on the air before too (CW of course), no mistaking that call!

Just for the heck of it I opened up my LDG 1:1 balun:





For about $30 that LDG balun is PRETTY EXPENSIVE for what it is! I am glad I didn't buy it. The big question is how efficient is the LDG balun vs. my homebrew one?

I know I am not the first to come up with this idea, but I am thinking about building a balun in to a film can. I am not sure how to get the PL-259 to attach to it, but its an idea. I would like to make it selectable between a 1:1 and 4:1 ratio, hopefully with up to a 50 watt power handling. The two ratios would require two toroids, of course.

On a side note, this is the adapter I made for the Z-11pro for use with random wire antennas (which is what I currently use as my main antenna). I was trying to come up with a way to attach my .015" stainless steel random wire antenna to the tuner and came up with this idea. It is a pretty slick method - and it allows use with multiple antennas real easy, like when I go back to my dad's place for a weekend I toss a wire up in the trees. Now all I have to do is put a spade connector on that one and I can make my time to get on the air that much faster!



The gray thing on there is a bunch of wraps of duct tape. I needed something to insulate the wire from the sleeve. I had not thought of my epoxy method when I made this one. I think the epoxy method would be better (so long as the 4 holes where coax shield is normally soldered in are plugged, otherwise the epoxy will ooze out).

By the way, the PL-259's I used for the balun and my random wire adapter are both recycled connectors. I shorted several of these out when I was installing a run of coax in my truck. I didn't want to throw them out because they had never been used so I held on to them and tossed them in my junk drawer. Since there was still coax soldered in to them I heated them up with a propane torch, unscrewed the UG-176's, and then pulled the wire out.