Choke
An RF choke (or common mode current balun) is a component or device that blocks or reduces RF (radio frequency) current from flowing in places you don’t want it — usually along the outside of your coax shield or through equipment wiring. It differs from a balun in that it's not about matching — just about RF isolation.
It acts like a "traffic cop" for RF, allowing desired signals to travel where they’re supposed to, while choking off unwanted RF paths.
f you're using an end-fed antenna, vertical, or poor ground system, your coax cable or even mic cord might radiate like an antenna.
Performance of different Ferrite toroids as common-mode chokes
🧪 Symptoms That Suggest You Do Need a Choke
| Symptom | What’s Happening |
|---|---|
| 🔥 RFI in the shack (e.g. RF in your mic, tingly key, weird audio) | Common-mode current is getting back into your gear via the coax shield |
| 📡 SWR changes when you touch the coax or radio | The feedline is radiating — it’s acting like part of the antenna |
| 📶 Noise level changes when you move or reroute coax | You’re picking up noise along the shield from your environment |
| 📉 Strange radiation pattern or reduced signal reports | Feedline radiation is distorting your antenna pattern |
| 🎙️ Hot mic or audio artifacts when transmitting | Especially on higher power or digital modes |
🧲 What Is an RF Choke Made Of?
| Type | Description | Common Use |
|---|---|---|
| Ferrite Choke (Line Isolator) | Coax wrapped through a ferrite toroid or bead | Blocks common-mode RF on coax |
| Air-wound Choke | Coax coiled into 4–8 turns, ~6" diameter | Lightweight, effective on higher HF |
| Ferrite Beads | Snap-ons or beads slipped over wires/coax | Useful for mic cords, power lines, USB, etc. |