In other OnAllBand entries, we’ve covered what ferrites are, how their behavior varies with frequency, and some of their applications in the ham station. This time, we’ll focus on their use as chokes in the antenna system, one of their most popular uses.
Common- and Differential-mode Current
Let’s start with a little refresher. Figure 1 below shows the difference between common-mode (CM) and differential-mode (DM) currents on a coaxial cable. DM signals are what we often think of as “balanced” in that the signal consists of identical currents flowing in opposite directions along two closely-matched paths, neither of which is connected to a ground or grounded enclosure. CM signals flow equally on all conductors of a multi-conductor cable or on the common (shield) conductor of a coaxial or shielded cable.
A single cable can support both DM and CM signals at the same time. In fact, due to the skin effect, at RF the outside of a braided or foil shield is electrically independent from the inside! That’s why we use shielded cables — to keep noise and radiated signals from being picked up by the conductor inside the shield.
DM currents are equal-and-opposite so the fields they generate oppose each other and cancel. The fields from CM currents don’t cancel at all — after all, CM current on your antenna creates your radiated signal! Because antennas work the same in “both directions,” a radiated signal can cause CM current on any conductor it encounters. That’s how your signal causes RFI (RF interference) when an audio or control cable picks up your signal. CM can flow on multi-conductor cables, too, like on your rotator control cable. None of the conductors needs to be grounded for RF to pick up and/or radiate RF common-mode signals.
Ferrite Chokes in the Antenna System
A transmitting choke creates a high impedance in the path of common-mode (CM) RF current that a feed line would otherwise pick up from a transmitted signal. That interaction is called coupling, and blocking the current is called decoupling. Coupling between a feed line and an antenna can distort the intended radiation pattern, upset the feed point impedance, and presents a great path for RF to flow into your station!
The most common place to put a transmitting choke is at the point where a coaxial feed line connects to the antenna. The outer surface of the coax shield is a great path for RF! You want all your transmitted signal current to flow on the antenna, not the feed line, so decoupling the feed line at that point is definitely a good idea.
The usual technique for creating a transmitting choke is to wind several turns of coaxial cable on a ferrite toroid or clamp-on core. This creates an impedance of several hundred to several thousand ohms in the unwanted current path. Because of this resistance, current from the coaxial cable flows equally into each antenna terminal — it doesn’t have a choice! — these chokes are often referred to as choke baluns because they serve as a transition for RF power between a balanced load (the antenna) and the unbalanced feed line. You can learn more about this type of balun in W7EL’s classic paper, “Baluns: What They Do and How They Do It” at www.eznec.com/Amateur/Articles/Baluns.pdf. Knowing when you need to use a balun and how to make one is an important piece of radio know-how!
The simplest form of choke balun is a coil of coax but a few turns of coax on a large toroid core, such as the DXE-TC31-24-4 in Figure 2 will do a better job over a wider frequency range. Notice that I’ve covered the core with electrical tape to keep the ferrite’s sharp corners from damaging the cable jacket.
How many turns do you need and on what type of core? A very good question! The answer depends on what frequency range you plan on the choke being effective and how much power you are using. The answer to those questions can be found in the ARRL Handbook or ARRL Antenna Book and online in resources like Jim Brown, K9YC’s excellent paper, “A New Choke Cookbook for the 160–10M Bands” at k9yc.com/2018Cookbook.pdf.
A receiving choke is very similar but it has a somewhat different purpose. It still blocks RF current but instead of doing it right at the transmitting antenna, the receiving choke prevents CM current from getting into the cable at an antenna or connection to equipment. If the RF gets in at these points, it turns into a regular differential-mode (DM) signal and mixes with the signal you want. This allows noise picked up on the outside of the cable to cover up weak signals. Because it isn’t located at the antenna, a receiving choke can use smaller, less expensive cores than a transmitting choke but they are just as valuable.
Getting and Taking Care of Your Ferrites
Like batteries, screws, and coax connectors you always seem to need another ferrite bead, core, or snap-on and when you need them, you really need them! The DX Engineering core assortments will keep your ferrite box well-stocked. For most HF RFI problems, the DXE-CSB31-COMBO is a good starter set of clamp-ons. For transmitting chokes you’ll need beefier cores, such as the DXE-TC31-24-4 set of 2.4-inch cores for winding larger coax through. Receiving chokes for low-band antennas also work as AM broadcast chokes, so the DXE-AM-RFI combo pack is a good set. As you gain experience in using ferrite cores, you’ll find yourself using them regularly, so having a few extra is a good idea. An old tackle box is a great way to keep track of your RFI Extinguisher collection!
Once you start building a collection of ferrites, take care of them and don’t throw them all in a box unprotected. Ferrites are brittle – they chip and crack easily. While small beads or cores aren’t big enough to damage each other under most conditions, larger toroids and clamp-ons certainly can. A little foam wrap or paper wrapping will protect your investment. Although a chip or two won’t affect performance very much, once a core is broken, the magnetic path is interrupted and is probably unusable.
Summary
This has been a “30,000 foot” overview of using ferrite chokes. Hopefully you’ve found this encouraging enough to start reading up on ferrites so you can use ferrite chokes in your station. Once you find out how useful ferrite can be and what problems it can solve, you find lots of places to put its “core technology” to work for you!
