Air-wound choke: any good?

Many years ago, I took delivery of my lovely 24MHz 3-ele LFA antenna, and was keen to get it in the air pretty quickly. 

I seem to recall, I hope corretly, that the instructions advised an air-wound choke was an acceptable solution to common mode currents, and that dimensions were given, which I followed.

The air-wound choke balun used in this test, as once used with my 12m LFA

 

Regardless, air-wound chokes are commonly used, notably when we are putting up something without much preparation or warning. But, what is their return loss?  In other words, should you use one?

Well, having moved from the QTH where my LFA was based, I now have an orphan choke of seven turns of RG8-X coax.  I thought I'd connect it up to the analyser this morning and see just what kind of return loss I'd been putting up with over all those years.

First, let's look at a carefully-wound 1:1 balun, which also has a choke effect, using a FT140-61 core and RG-316 coax, which I made a couple of months ago. As you can see, the return loss is pretty good, though it might be improved with other designs/cores; at 14MHz, it's around 26dB; put 100W in, and only around 0.25W will be reflected back.

A quality 1:1 ferrite-wound balun I made last year.

Now, let's look at the air-wound choke, which turns out to be pretty poor. Note that the scale is extended into the lower VHF range in this plot, by which time the RL is quite good. But for the HF bands, the RL is bumping around 16.5dB; put 100W through this, and a much more significant 2.5W will be reflected.

A neatly-made air-wound (on a PVC former) coax 1:1 balun about to go on test.

This is not to say that the air-wound choke is useless; it will do a reasonable job of reducing common mode current in most situations at what we could call 'normal' British levels of output, below 100W and you won't be at some huge detriment, relative to the operator with a decent choke/balun.

But, the next time you quickly wind a few turns of coax, especially if it really is purely air-wound and not around a plastic former, usually meaning the cable isn't a good, sequential set of coils, then you may want to wonder if, on the next outing, you might want to do things differently - and better.

Incidentally, a long while after this post was published, I checked a toroid-wound choke using 16 turns of RG-8X generic coax. This produced the following, reasonably good result across the bands that doesn't show any cause for concern; juse wind and use (and, preferably, enclose!)