Or more accurately do you need a balun because I know the answer in my case!
I made my first HF antenna for SOTA on 16th December 2006 and I used it with great success on 18th December 2006… my first HF activation. It was a simple dipole cut for 60m. Having seen how easy you can tangle the component parts in the past, the dipole legs detached leaving a feeder and centre piece, all designed were possible to be assembled wearing gloves. After a while added some links so I could operate on 40m as well. After than some extensions some extensions so I could use 80m. Not rocket science at all. Then some new legs with links for 30/40 instead of just 40m.
Theory states you need a balun when feeding a dipole with coax. This is because the dipole is balanced and coax is unbalunced. When you connect the two you get the balanced current flowing on the inner, the matching balanced current flowing on the inside surface of the coax braid. All this is fine. You also will get SOME common mode current flowing on the outer surface of the braid. RF travels on the skin of a conductor and so you can have differing currents flowing on the same piece of cable. Cool! Or not depending on your viewpoint. The amount of common mode current you get depends… on a myriad of local factors. The effect this current has depends… on a myriad of local factors. In a lot of circumstances the common mode current has no observable effect. In which case not having a balun wont matter.
OK so do I need a balun? Well it depends! A simple balun can be constructed from simply coiling the coax near the feed point. This is often called a choke balun or current mode balun. It isn’t really a balun but it is a current choke. The word choke describes what it does, chokes off current flow. The current flow on the inside of the coax braid is not affected (it’s inside the screen. Only common current (which you don’t want) is affected. So when I made my antenna I added a few turns of feeder coiled up near the feed point. So it looked pretty ( because we all know pretty things work better ) I wound the turns using a CD pancake box. I used about 5 turns around this 13cm former and wound the coil like a solenoid i.e. a single layer with no turns overlapping with the coax input and output ends 180degs apart. All kept in place with a few bits of tape and tiewraps.
Job done, on the air, contacts made, summit activated, activator went home smiling. Addiction to HF SOTA now made!
200+ activations later I had a dead 817. Ah but old 817 PAs were flakey. Spherics said a famous tactical HF manpack designer (Barry GM4TOE), 817 PAs die because of abuse… SWR issues or too much heat. He should know… me, I have a hardware degree but have only written software for 30 years!
A few more activations showed obvious intermittency with the cable (just like the manpack dude suggested there would be) and so I made a new one. Fresh RG-174 and I was away. There had been discussion as to whether the choke balun of the size I had worked at 60m so I didn’t make one this time. Anyway using it on 60m/40m and the new antenna system worked with no intermittency issues. Pleased? Not half!
I was sitting in wonderful WX on Meall a’Bhuiridh and I couldn’t get the antenna OK on 30m. Booger, booger and thrice booooooooooooooger! I had to make do with 60m & 40m neither of which worked well. Yesterday I set up the antenna at home and bunged the MFJ-259 on it. Just like on the hill the resonance was at 9.8MHz so it worked as badly here as the day before. That was good… consistently faulty Any fix should be a fix on a summit. I altered the length of the feeder by adding some RG-58 in and the SWR dropped a little. Then as moved the spare feeder about the SWR dropped right down. Let go and the SWR jumped up. Hmmm… I dropped the fishing pole, wound a quick coil in the feeder at the feed point, pushed up the pole and on 10.118 the SWR was perfect at 1:1.2
To confirm I removed the coil and the SWR shot up. Coiled in situ and changing the feeder length made no difference. I did lots more tests to confirm that a choke balun was needed and 30mins later I could be sure on the fix. Back into the shack, neat solenoid wound and taped up and I’m back in business on 30m. Further checks showed that my SWR was flatter on 40m than before but no observable change could be seen on 60m. But that was immune to feeder position with or without the choke balun.
So do you need a balun? I don’t know! But I do know I need one!
On 30m with the arrangement I have then there are detrimental common mode currents that are reduced to a non-detrimental effect with 5 turns of 13cm diameter in the coax at the feedpoint. YMMV. Another thing worth checking if you have a link dipole is how the unconnected section(s) affect the section in use. On my antenna, the match is best on 30m when the 40m links are pulled apart as well. The parasitic effects of the 40m/60m section are greater when it is one wire than two isolated pieces. Worth checking. All you need is a sunny afternoon, a box of small bottles of Belgian beer left over from a BBQ, an MFJ-259 and to ignore the funny looks from the neighbours!