Part of our Building Your First Ham Radio Station guide.
If you’ve ever gotten a tingle off your microphone, watched your SWR shift when you touch the coax, or picked up noise that seems to ride in on the feedline, common-mode current is the usual suspect. A 1:1 choke (sometimes called a choke balun or line isolator) stops RF from flowing on the outside of your coax shield, where it doesn’t belong. It’s one of the cheapest, most worthwhile things you can build.
This is a natural add-on to a basic wire dipole, a homebrew EFHW with a 49:1 transformer, or really any wire antenna running a counterpoise. If you’re still chasing noise after adding one, my guide to hunting down RFI covers the rest of the troubleshooting process.
What you’ll need
- One FT240-31 toroid core (best for 80m/40m, roughly 1-10 MHz) or FT240-43 (broader coverage, better on the higher HF bands), for QRP builds, an FT140-43 works fine
- Coax: RG-58 or RG-316 for up to about 100W, RG-213 or RG-142 if you want headroom for higher power
- A weatherproof enclosure
- Two SO-239 connectors (or just run the coax straight through with PL-259s on each end and skip the connectors on the box)
- Cable ties or clamps to keep the windings from shifting
Step 1: Choose your turns count
The number of turns of coax around the core determines both how low in frequency the choke is effective and how much suppression you get:
- FT240-31: 9 turns gives resistive choking from about 3.5-30 MHz; 12 turns extends effectiveness lower (into 80m/160m) but narrows the top end somewhat
- FT240-43: 10-12 turns covers roughly 5-30 MHz well
- FT140-43 (QRP/portable size): 8 turns of RG-316 or RG-174 makes a solid ~100W choke in a smaller package
If you’re not sure which to pick, 31-mix is the better choice if you mostly operate 80m and 40m; 43-mix if you spend more time on 20m and up. Stacking two cores of the same mix increases both power handling and choking impedance if you want to go bigger.
Step 2: Wind the choke
Leave yourself a foot or so of coax as a lead on each side before you start winding. You’ll need it to reach your connectors. Wind the coax through the center of the toroid the number of turns you chose, spacing them evenly around the core rather than bunching them on one side. Each pass through the center hole counts as one turn. Secure the windings with a cable tie or two so they don’t shift around in the box.
Step 3: Terminate and box it up
Install PL-259 connectors on the coax leads, or connect them to SO-239 bulkhead connectors if you’re building it into an enclosure. If you use a metal enclosure, make sure neither SO-239 body is directly grounded to the case in a way that shorts across the choke. The whole point is to keep the two ends of the coax shield isolated at RF, and a metal case bridging them defeats that.
Step 4: Where to put it
The choke works best right at the antenna feedpoint, where common-mode current originates. If you’re still getting noise or RF feedback issues in the shack after that, a second choke where the coax enters the building (or right behind the radio) often finishes the job.
How to tell it’s working
- SWR readings stay steady when you touch, move, or reroute the coax
- No more tingling or RF feel on the mic, radio case, or nearby metal objects
- Cleaner audio reports and less injected noise on receive
It’s a small, inexpensive build, usually under $15 in parts if you already have some coax on hand, and it’s one of those upgrades that quietly fixes problems you might not have even realized were coming from the feedline.