Perhaps one of the greatest confusions in the hobby of ham radio is wrapped around the subject of baluns and ununs. What are they, where should they be used and what do they do?
At Ham Radio Outside the Box several antennas that need a 4:1 impedance matching device have been described. Since I had a handy-dandy 4:1 Guanella current balun in the drawer I figured it would get the job done. I applied it to my Rybakov antenna and it worked fine business. I applied it to my Simple, Low Profile, Multiband Antenna and it also worked.
In theory a 4:1 Guanella current balun (properly wound on 2 cores) would also attenuate common mode current and avoid the necessity to add a separate common mode current choke. Like I said - in theory! To be honest, I didn't experience any common mode current issues using this device. But then I read an article written by Joeri Van Dooren, ON6URE that explained why using a Guanella 4:1 balun is often not a good idea. The reason is quite obvious once it has been explained. If it used with a vertical random length antenna - like a Rybakov - it expects to find balance where there is none. It struggles to do its job and creates a risk of core saturation or overheating. Overheating you say? Maybe that's another one of the benefits of being a QRP or low power operator, it ain't gonna happen - at least it's very unlikely.
What is a Balun?
A balun can be described as a device used to match a balanced antenna to an unbalanced transmission line. Its name derives from BALanced to UNbalanced.
The accompanying schematic diagram here shows an example of a 4:1 balun. This device will match an antenna with an impedance of 200 ohms to coax with a characteristic impedance of 50 ohms. To further complicate matters, the diagram shows a voltage balun, also known as a Ruthroff balun.
It has two separate windings on the same core. The two windings are "bifilar" meaning they are wound together, side by side. Unlike the 4:1 Guanella current balun, there is no expectation that a Ruthroff balun will attenuate common mode current. A separate common mode current choke is needed.
Note that the balun has no ground on the antenna side. The two arms of the balun are balanced. The Ruthroff balun forces equal and opposite voltages on each side of the antenna. A Guanella balun, on the other hand, forces equal currents on each side of the antenna. If we were to need a device to match an antenna that is unbalanced - such as a vertical - we would need a different device called an unun.
What is an Unun?
An Unun can be described as a device used to match an unbalanced antenna to an unbalanced transmission line. Its name derives from UNbalanced to UNbalanced.
The schematic shows how it is wired. A 4:1 unun is shown but it could also be built as a 9:1 by adding a third winding creating a trifilar construction (i.e. 3 wires wound together, side by side).
Note that the configuration is quite different to the 4:1 balun. Instead of the two arms of the antenna being balanced, one side is now at ground potential. This suits a vertical antenna with a system of ground radials.
Isn't a Balun just a fangle-tangled Unun?
Strangely, considering their different functions, there are a lot of similarities. If we wanted to convert one to the other, a few minutes spent with a soldering iron would get the job done.
Referring to the accompanying diagrams here we can see there are three main components:
- The antenna connections - either balanced or with one side grounded
- The bifilar (or trifilar in the case of a 9:1 unun) inductor wound on a toroidal core
- The coax connector for connection to our transceiver
If we follow the next two diagrams showing how to assemble an Unun or a Balun we can see the similarities in the two devices.
How to choose the correct device?
Once I had made the decision to "do it right" and replace my Guanella current balun with a Ruthroff voltage device, I had to choose between a balun and an unun.
Since I was out of spare toroid cores in my shack component drawers the Guanella balun had to be sacrificed. It had given good service but, in the interest of adhering to good RF physics, it was laid to rest.
The two cores each had a bifilar winding that was identical so I decided to build both a balun and an unun. My intention was to evaluate what would happen if I tried each one. The antenna on which the trials were performed was my wire version of KJ6ER's Challenger antenna.
The science was telling me that the unun was the correct choice. Call me 'ornery but I prefer to find out for myself. I like to test the alternatives in the big outdoor lab I call my backyard.
With both devices in my pocket I erected the antenna - clearly not a balanced antenna because the vertical wire is 22 feet long and the counterpoise is only 5.5 feet long with its far end laying on the, as yet, snowless lawn. So, first up was the balun. I didn't expect this to work at all, but I did want to observe what the result would be. Would it be smoke, rising up into the sky, signaling my ignorant disregard for what the physics was clearly telling me?
Gadzooks!
To minimize the size of any potential mushroom cloud arising from my scientific folly, I chose a very low level of RF - you could call it "QRPpp" perhaps. The source was my antenna analyzer. To my astonishment, hitting the big button that sent milliwatts of RF coursing through the balun into the antenna produced no fireworks. Instead, with a little minor adjustment of the length of the radiator and counterpoise, the antenna analyzer showed a surprisingly good SWR. Hitting another button on the analyzer showed the antenna was resonant just below the 20m band. I like to operate CW near the bottom of the band so that would be just fine.
Next up was the scientifically anointed device, the Chosen One, the device whose destiny was ordained to become part of my field kit - the Unun. Of course it worked too.
Now let's summarize the experimental results here. The Guanella 4:1 current balun worked - but it shouldn't have. The Ruthroff 4:1 balun worked - but it shouldn't have. The Ruthroff 4:1 unun worked as expected. I earned my physics degree a long, long time ago but studying for it gave me a lifelong habit of following the scientific method - as verified by my own experiments.
Of course a low SWR tells us nothing about the efficiency of the antenna system. It only indicates that it won't drive the magic smoke out of our transceiver. The wrong matching device may result in less of our transmitted signal being radiated.
So the end of the story is I will be using the unun, confident that it is backed up by antenna physics - and my own, white lab coat wearing independent verification!
The final image shows the unun mounted on my wire Challenger antenna during its backyard trial. The larger toroid is a common mode current choke. The small one is the unun.
Note I used an FT140-43 ferrite toroid core even though many sources recommend a powdered iron core such as type 2 material. Type 43 ferrite material is not the optimum mix for the 20m band and up but it is what I had. I may have wound too many turns on the core, risking core saturation. When I tidy up the construction I may reduce the number of turns from 10 down to 8.
NB: I have been unable to find a source for type-2 powdered iron toroids. Can anybody tell me where to buy them, preferably in North America?
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