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- 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.
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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- 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 often uses a ferrite toroid with multiple coax windings, like RG-58 power up to 100 watts or RG-316 for 250 watts. Most used toroids for HF are type FT240. If you take a single toroid FT240-43 with 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.
But most coax types have a limited bending radius (25 mm onetime bending for RG-58). Winding it tightly may very well damage the coax. In fact only RG-174 and RG-316 are okay for 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 the necessary common mode current suppression on a broad frequency spectrum. 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 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 in your shack, your home and at your neighbors. 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 25 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 hams 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’s also a lot of confusion about power ratings. A choke or balun for which 100 W is claimed as maximum power, often cannot handle 100 W of continuous power (100% duty cycle), like when using modes like FM and digital modes like RTTY and FT8. You can read more about PEP power and duty cycles in this article.
Power rating
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 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 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 FT240 toroid, with coax or bifilar windings, used as common mode choke, 1:1 balun or 1:1 unun. Good to mention that these values should be used as ‘safe’ guidelines for when your antenna operates within an acceptable SWR window.
I do 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. These recommendations are based on 12 turns on a single FT240 toroid.
Be aware that the more turns of wire or coax you apply around the toroid, the lower the affected frequency range and the higher the common mode suppression becomes.
Using a metal case; no 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.
Dear readers, I have been really busy working in training and EMI troubleshooting in the past months. I am happy to be here again with a new post.
This month we will see a topic from a real-world situation – very interesting – for electronic designers working in conducted emissions problems.
In one of the projects I was involved with several weeks ago, a product was failing in conducted emissions on an AC power supply line. Measuring with a LISN (Line Impedance Stabilization Network), the problem was related to common mode emissions.
The designer of the circuit tried to work with a common mode choke to reduce emissions (Y capacitors were not possible for this application).
He was using a low cost (this is important!), common mode choke using toroidal core.
For that component he had several samples for testing and, initially, the choke looked like an effective solution in the trials inside of the company. Let’s name the choke used CHKA.
With the promising result in the company, a prototype was specially prepared to go to an external lab (time to cross our fingers!).
But, in the external lab the product failed again (I think you have experimented similar situation), and a typical question comes to your mind: “How is it possible that the solution in the company was failing in the external lab?”
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The answer to that question lies, as usual, in discovering what is different between both scenarios.
Analyzing the problem, I discovered that the choke used in the prototype for the external lab was a different unit from that soldered in the original prototype. Same part number, same manufacturer, same samples box, but …. a different unit, not EXACTLY the choke used in the company. Let’s name the second choke CHKB.
Before explaining the reason for the failure, let’s review the basics of a common mode choke.
A common mode choke is a coupled inductor: two inductors are built using the same core. Note the winding strategy (Fig. 1) is very important to obtain a common mode choke.
Fig. 1. Ideal common modes choke for differential currents (left), common mode currents (mid), and symbol for schematics (right).
For this ideal choke, the magnetic flux in the core is because the differential mode currents iDM (Fig. 1, left) cancel each other resulting in a zero impedance. But magnetic flux caused by common mode currents iCM (Fig. 1, mid), is accumulated resulting in a high amount of impedance value. The symbol for this kind of choke (Fig. 1, right) uses two points to specify how the windings must be done to obtain that behavior.
Summarizing, an ideal common mode choke looks like a simple wire for differential mode signals while it looks like an inductor for common mode signals. One of the advantages of these kinds of chokes is they will not be saturated by differential mode currents.
For those coupled inductors, coupling factor k can be calculated from Eq. 1:
k = M/√(L1×L2) (Eq. 1)
and common mode and differential mode inductances can be obtained from Eq. 2:
LDM = 2×(L-M) and LCM = (L+M)/2 (Eq. 2)
where M is mutual inductance and L1, L2 are inductances for both inductors.
Considering the inductors are equal, L1 = L and for 100% of perfect coupling k=1, mutual inductance M is from Eq. 1 equal to inductance L (M=L) and common and differential mode inductances are from Eq. 2, LDM =0 and LCM = L.
So, it is confirmed that we will find not impedance effect for differential mode signals and some value of impedance for common mode signals.
In a real common mode choke the cancellation is not perfect. As a result, the differential mode impedance is not zero. This effect is sometimes called “leakage”. This is useful for filtering differential mode signals but the saturation effect must be checked in high current applications.
Let’s go back to our example failing in the laboratory. To analyze the situation, I measured the response of both chokes with my Bode 100 network analyzer (a really useful instrument if you are interested in frequencies up to 50MHz).
A simplified measurement of a common mode choke can be done as shown in Fig. 2:
Fig. 2. Simplified measurement of impedances for a common mode choke.
The choke working satisfactorily in our application (CHKA) was measured and results are in Fig. 03:
Fig. 3. CHKA simple characterization.
You can see how big the impedance of the common mode effect is when compared with the differential mode effect.
For the second choke (CHKB), the one failing in the laboratory, I was able to see a very subtle difference: one of the coils of the choke had ONE TURN missing (Fig. 4).
Fig. 4. The chokes used in our example.
CHKA had 14 turns for L1 and L2. CHKB had 14 turns for L1 and 13 turns for L2.
This is a very critical difference. If one of the coils is not exactly as the other, the common mode inductance will be reduced (poor common mode filtering) and the differential inductance will increase (perhaps the core can be saturated with the nominal current in high current applications).
This kind of cores are manually winded so human errors and/or low-quality tests can create this difficult to find problem.
The comparison of both chokes is included in Fig. 5:
Fig. 5. Comparing chokes CHKA and CHKB.
From the measurements, it is clear how important a perfect symmetry for the two coils in the choke. With only one turn missing in one of the coils the common mode impedance (Fig. 5, left) is drastically reduced, as for example from point A to point B at the same specific frequency. The result will be a lower effectiveness to filter common mode EMI signals.
In the same way, the differential mode inductance increases from A to B (Fig. 5, right) with a typical effect of saturation of the core.
Let me conclude this post with two important pieces of advice: 1) be careful with low cost/low-quality components; and 2) try to have a network analyzer or impedance analyzer in your laboratory to check how the component you are using in your design is. And of course, good luck in your next design!
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