The broadband common mode choke

During the Covid-19 pandemic I spent a lot more time in the shack. One of the issues I decided to tackle was RFI. Common mode currents running into the shack were the cause. Those currents traveled along the shield of the coax cable which feeds my CG-3000 autotuner connected to a 15m long wire with top hat capacity. This setup covers 160m to 30m. I needed to find a common mode choke (also called 1:1 balun or RF line isolator) that is effective onthis 10 MHz bandwith

Best solution for reducing common mode currents

Antennas like a vertical, dipole, Yagi, endfed (EFHW), G5RV, ZS6BKW, inverted and Cobwebb are usually used in combination with common mode chokes. It’s the most easy and best solution for reducing common mode currents.

The principal of a common mode choke is to present a big impedance on currents that flow in the coax shield. There are several ways to create such an impedance.

Air core choke

The most simple and cheap way is the air core balun or ugly balun. This type of common mode choke is just rolled up coax on a certain diameter. It’s moderately effective with limited bandwith. An example: 12-14 turns of RG-213 on a 10cm (4″) diameter will do up to 35dB reduction on 20m, but not more than 10-15dB on 80m and 10m.

Moxon choke balun detail
Air core balun 6 turns of RG-58 on 10cm (4″) diameter for 10m Moxon

Ferrite beads choke

A choke with more bandwith can be achieved by putting ferrite beads over the coax cable. If you put 10 beads type FB-31-1020 over RG-213, you create 25dB reduction at 60m up to 30dB on 10m. The more beads, the more bandwith. But doubling to 20 beads will extend the 25dB reduction only to 80m and not cover 160m. Keep in mind, the price of a single bead is about 3-5 EUR (USD). So this a significant more expensive choke than air core and quite heavy too.

Ferrite toroid choke with coax windings

A more broadband choke but more complex to build, is a ferrite core type. This variant uses a ferrite toroid with multiple coax windings, like RG-58 or RG-316. Most used toroids for HF are FT240. If you take a single toroid FT240-43 with 17 turns of RG-58 you will have a choke that is effective, about 25dB up to 40dB from 160m up to 10m. But there is another variant that also effective on 6m.

Common mode choke from ferrite toroid with RG-58 coax windings

Ferrite toroid choke with sectional bifilar windings

The same type of toroid with bisectional bifilar windings provides more attenuation than previous variants over a much larger bandwith. About 25dB up to 45dB+ from 160m to 6m. It even produces 20dB attenuation on the 4m band. When using PTFE insulated AWG18 or 0.75mm2 wire (600V), this common mode choke can handle up to 1kW SSB. Stacking two toroids will double the power rating.

Common mode choke from ferrite toroid with bisectional bifilar windings

Buy one

Don’t want to build this common mode choke yourself? I will gladly build one for you to buy.