During the Covid-19 pandemic I spent a lot of time in the shack. One of the issues I wanted 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 15 m long wire with top hat capacity, covering 160 m to 30 m. I needed to find a common mode choke, also called a 1:1 balun or RF line isolator, that is effective on this 10 MHz bandwith.
This article is about the different options for a common mode choke for ham radio bands.
Best solution for reducing common mode currents
Antennas like a vertical, dipole, Yagi, EndFed (EFHW), G5RV, ZS6BKW, inverted V, Windom, Doublet and Cobweb tend to produce unwanted currents running over the coax’ outer shield. The common mode choke prevents this.
The principal of a common mode choke is to present a relative large impedance on common currents that flow over the coax outer shield/screen. Ideally, a common mode choke should present a minimum of 20 dB suppression of common mode currents. There are several ways to achieve that.
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 10 cm (4″) diameter will do up to 35 dB on 20 m. Quite effective on one single band, but with 10-15 dB not effective enough on 80 m and 10 m.
Ferrite beads choke and balun
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 25 dB reduction at 60 m up to 30 dB on 10 m. The more beads, the more bandwith. But doubling to 20 beads will extend the 25 dB reduction only to 80 m and not cover 160 m. Keep in mind, the price of a single bead is about 3-5 EUR (USD). So this a significant more expensive choke than an air core type. It is quite heavy in weight too, so you might need to support the weight to prevent tension on the coax.
Ferrite toroid choke and balun with coax windings
A more broadband choke and not to difficult to build, is a ferrite core type. This variant uses a ferrite toroid with multiple coax windings, like RG-58 power up to 100 watts or RG-316 for a little higher power. Most used toroids for HF are type FT240. If you take a single toroid FT240-43 with 10-12 turns of RG-58 you will have a choke that is effective, with about 25 dB to 40 dB suppression from 80 m up to 10 m. An FT240-31 toroid will make it effective from 160 m to 20 m. Downside is that the coax needs to be as close to the toroid as possible for maximum suppression. For that, you need to wind the coax tightly along the toroid. But most coax types have a limited bending radius (25 mm for RG-58). Winding it tightly may very well damage the coax. In fact only RG-174 and RG-316 are okay for these tight windings. But there is another variant that is very effective on the ham radio HF bands.
Ferrite toroid choke with bisectional bifilar windings
The same type of toroid with bisectional bifilar windings provides more common mode current suppression than previous variants. Plus it, does it over a much larger bandwith. About 25 dB up to over 45 dB and more from 160 m to 10 m. It even produces almost 20 dB suppression on 6 m band. When using PTFE insulated AWG 18 or 0.75 mm2 wire (600 V), this common mode choke can handle up to 1 kW in SSB. Stacking two (identical) toroids will double the power rating.
The common mode choke in practice
The setup is a Wellgood active magnetic loop (by M1GEO) connected to a SDRPlay SDR receiver. The loop is powered through 9 m (30 ft) of 50 ohm coax through a bias tee power injector. The choke was placed between the SDR receiver and bias tee. Receiver tuned to 14074 kHz (20m FT8).
You can clearly see the many signals engulfed with noise in the first screenshot.
The common mode choke lowered the noise floor with 12-15 dB, equal to 2-2.5 S-points.
Use as a 1:1 balun, 1:1 unun and line isolator
This type of common mode choke is also effective as a 1:1 balun or 1:1 unun. It prevents your feed line from becoming an active part of your antenna, resulting in an unfavorable radiation pattern, but also causing RFI. It is therefore also called a ‘line isolator’.
A 1:1 balun is used to transform a balanced or symmetrical antenna to an unbalanced or unsymmetrical feed line. An example of a balanced antenna is a dipole or G5RV. Coax is an unbalanced feed line. So when you feed a balanced antenna with coax, always use a 1:1 balun to prevent the coax from becoming an active (radiating) part of the antenna.
The 1:1 unun is used to connect an unbalanced feed line to an unbalanced antenna like a vertical, J-pole, off center fed dipole (Windom or OCF) or inverted L. For example, even if you have a vertical with a lot of (tuned) radials, the antenna will also use the coax’ shield as a counterpoise. The 1:1 unun prevents this.
Commercially available products
There are a lot of commercially available chokes, line isolators and baluns. But on many you can doubt the claimed specifications. In particular, the maximum power. First of all, that number says absolutely nothing about how well the product works. For example, a 100 W common mode choke can be a lot more effective than a 5 kW rated. It’s easy and cheap to put in a piece of coax that handles 5 kW, but not so easy and cheap to use ferrite that suppresses common mode currents more than 20 dB and handle 5 kW of power. Manufacturers rarely give numbers about the suppression of common mode currents. Quite frustrating… like buying a transmitter and not knowing if it does 10 W or 100 W.
I recommend checking with other operators about their experience with a particular brand and model choke or balun. Did it do the job for them? But better, get figures on suppression and frequency range specified by the manufacturer.
There is also a lot of confusion about power rating. A choke or balun for which 100 W is claimed as maximum power, often cannot handle 100 W of continuous power, like when using phone modes like FM and digital modes like RTTY and FT8.
Some manufacturers advertise insane power ratings. I have opened up commercially available common mode chokes, claiming significantly higher power ratings than possible in reality. I have seen ads claiming a ridiculous 3000 W SSB for a single FT240 toroid… In general, the maximum power for ferrite toroids, for use in common mode chokes, type FT140 is about 200 W and for FT240 is about 1000 W in average SSB operations.
Keep in mind that contest stations have a far higher load. Furthermore, the applied coax or wire is an important factor for the power rating as well.
Here is a list of more realistic SSB power ratings per toroid, with coax or bifilar windings, used as common mode choke, 1:1 balun or 1:1 unun.
|Toroid||Winding||Max. power SSB (W)|
|FT140-xx||RG-316 PTFE coax||200|
|FT140-xx||AWG 22 wire PTFE bifilar||200|
|FT240-xx||RG-316 PTFE coax||400|
|FT240-xx||RG-303 PTFE coax||1000|
|FT240-xx||RG-400 PTFE coax||1000|
|FT240-xx||AWG 18 wire PTFE bifilar||1000|
This is for average usage, not for hardcode contesting.
For CW multiply by 0.5. For digital modes (RTTY/FT8), FM and AM multiply by 0.25.
I would not recommend using coax with a foil screen like LMR240 and Aircell types and clones. The bending radius on an FT240 type toroid is so tight, that you could rupture the foil and damage the outer screen.
Ferrite mixes and frequency ranges
Ferrite comes in different mixes. For use in a common mode chokes, 1:1 balun, 1:1 unun or line isolator, you can use this table as a guideline for these ham radio bands / wavelengths:
|Ferrite mix vs band (meters)|
Using a metal case; wait…
If you build your choke into a metal case, keep in mind that the casing cannot get in direct contact with both SO-239 sockets. If you do so, the common mode currents will flow over the metal case. Your choke will not work.