Common Mode Chokes

A common mode choke can be made by winding turns of coax round a ferrite ring. This is also known as a current balun. These are often used when you have currents running on the outside of your coax feeders. If aerial systems are unbalanced RF can get back into the shack, in the worse case you can get RF burns and frequently modern equipment can malfunction. If a choke is put on the feeder before it enters the shack, it can stop these problems.

How does the current get onto the feeder in the first place, probably it is caused by balanced aerial systems being fed with coax. It is not always obvious if a system is balanced or not, take the case of a vertical with a counterpoise or even a ground plane with sloping radials. These systems do not have the coax outer connected to earth and currents running down the feeder are likely. A balun is frequently not used as these are associated with centre fed dipoles etc. A common mode choke will act as a balun with very little loss, a few turns of coax round a ferrite ring does the job.

There is much written about which ferrite mix to use but this is more important when power is being transferred via the ferrite core, in the case of common mode chokes you are providing a high impedance to discourage the energy from travelling on the coax outer. At higher frequencies the capacitance between the turns becomes important so a special method of winding is used, if not the coax should not be wound all the way round the ring.

The RSGB website has good information about common mode chokes but it is hidden under EMC and discusses their use for eliminating RF breakthrough into electronic equipment. They also supply the smaller ferrite rings with a suitable mix.

https://rsgb.org/main/technical/emc/using-emc-filters-and-ferrites/
https://www.rsgbshop.org/acatalog/Ferrite-Ring-654.html

Larger rings are often used, these are more suitable for higher powers and you can get more turns on but they are also easier to use as you can put them on to coax without removing the connectors.

Interesting stuff on this link from DK7ZB, I found the bit about testing the performance of the choke as a balun useful, just take two resistors that are half the output impedance, joint them in series and connect the centre point back to the outer at the unbalanced side. If the SWR is good, all is working well, if things are not in balance you will get a SWR of up to 2:1.

https://www.qsl.net/dk7zb/Baluns/current_balun.htm

If you like to spend lots of money here is a link to a site with interesting info. They are very coy about the losses in their impedance transformers.

https://palomar-engineers.com/antenna-products/1-1-balun-kits

73s
Derek G4HSY