Saturday, November 5, 2022

Time to make a choke balun


In your ham radio adventures, you may have heard the words..choke balun, feedline isolation, common mode choke or a 1:1 balun. In my humble opinion different names for the same device. These devices for the most part are present in a system to control common mode currents. I am not an engineer just a simple ham guy trying to sometimes understand some very complicated and technical explanations. 

I hope to keep the next bunch of lines understandable. Here we go...what are common mode currents! Most ham use some form of 52 ohm coax as it's very easy to route, secure and can within reason be placed anywhere. Ideally, RF will flow out on the OUTER SURFACE of the coax INNER CONDUCTOR and flow back on the INSIDE SURFACE of the coax shield. This is what happens in a perfect world but if a system is unbalanced RF current will return on the OUTSIDE of the outer coax braid. It does not return but it radiates.

This is when we hams can experience RFI issues and these issues vary with the amount of common mode currents flowing on the outside of the shield of the coax. How does one control this common mode current? One very effective way is to introduce a common mode choke, 1:1 balun, choke, feed line isolation or whatever else it could be called.

In a previous post, I did go over how I installed a 1:1 balun at the antenna feed point of my Hustler 4BTV and that seemed to satisfy my RFI issues but further reading informed me that a choke balun at the radio feed point would be very advantageous. I ordered 2 FT240-31 mix toroids as I decided to make a choke balun instead of buying one. The two common mixes for HF toroids are 31 and 43. The difference being the 31 mix is effective from 1-300 Mhz and 43 is effective from 25-300 Mhz. I chose to go with the 31 mix and purchased 2 from a reputable dealer as not all toroids are created equal. I would be very wary if you find toroids at a very low price....just remember you get what you pay for. I ordered mine from Mouser Electronics Canada, part number 623-2631803802 made by Fair-Rite for 13.00 each.

I used RG8X coax to wind around the toroid but the issue with using a ring toroid compared to a split bead is I had to remove the PL-259 to wind the coax around the toroid and then solder the PL-259 back on. I intended to make 10 loops and for some reason, I ended up with 11 turns through the toroid. I then soldered the PL-259 connector back on but not before placing the barrel and coax spacer many of us have soldered a PL-259 on and then realized we forgot the spacer and barrel!
Another advantage to having a choke balun at each end of the coax is that it helps remove some RF noise that is picked up on the shield of the coax.

Dont forget these BEFORE soldering the PL-259


PE4BAS, Bas said...

Never seen this kind of coax here Mike. If I buy RG8 over here it looks like RG213 with a black outer shell. Could be RG8U? Never seen the coax you use, interesting. Yes, I made several baluns. Sometimes it helps sometimes not. The problem is that we as "just HAM guys" can make one but not analyze if it does what it should do. We have to trust on information others give. Makes my suspicious sometimes. 73, Bas

VE9KK said...

Good evening Bas very nice to hear from you, yes it's RG8X and I find it to be a very decent coax. When I put the choke balun at the base of my vertical Hustler 4BTV I did notice a large difference if the RFI in the shack. The choke balun there is from Balun designs but making one is great and it gets me to read and research things. I have decided to redo the balun with RG 316 as it's a smaller diameter coax and I can place the whole deal in a PVC box. Truth be told it allows me to tinker more with the balun project.
73 Bas and have a great week,