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 on....how 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|
Take a look at the bend radius of 8x… I believe you are exceeding it with that choke. That runs the risk of the center conductor migrating and touching the braid, with interesting results if you are transmitting…
Good morning Dave, very good point and I was thinking about that but did see some vendors use RG8X. But then again I may look at RG142, 174 or 316 coax and rewind the choke because as you know things will fail at the most important of times.
Dave thanks for the input it gets the old gray matter working!
Tnx for the interesting article. I had RF feedback problems with my 5BTV vertical a year ago and this really helped. Will keep this info on file for new Hams in my area to see. 73, DE K0RDS
Good evening, Richard I found the extra choke at the radio end did help with the last bit of RFI I was having.
Richard thanks for stopping by and taking the time to leave a comment,
Thanks for the info Mike! I’m having a RFI issue right now only on the 75/80 meter band. The RFI is getting into my Dell Latitude E6540 laptop PC I use for digital modes from my Yaesu FT-991A that uses a USB-A to USB-B cable to the audio codec comports and as soon as I transmit Ham Radio Deluxe not only disconnects, it shuts the program completely down. I run a 130 feet multiband doublet, (it used to be twice that long, but I never operated on 160m so I cut it in half), being fed through about 30 feet of RG-213 coax out to a Balun Designs model 4115t 5-Kilowatt rated 4 to 1 current balun to the 450 ohm ladder line/window line going on up to the center feed-point. Not sure what to try next other than trying to get the antenna and balun further away from the house. Right now the antenna is up only about 35 feet on the ends sagging to around 25 feet at the center feed point. I’d try winding a 8 turn choke in the coax using a 2 liter soda bottle for the form, but as it is now I’ll have to add some coax first because after I left enough sag in the coax for a rain drip I’ve got the length maxed out already. I don’t have the issue on any other band besides 75/80 meters. “Very 73 de Cliff, KU4GW”
Good morning Cliff very nice to hear from you and I hope you are doing well. As for the issue, it definitely sounds like RF is playing games with your setup. Regarding the USB A to USB B cable from the laptop to the radio have you tried any good quality ferrite chokes on that cable? I had similar issues with some rig to PC cabling and wrapping the cable multiple times through a split ferrite did the trick. You can also try a 1:1 choke where the RG 213 coax is connected to the radio.
Keep me posted on the progress.