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!