Coaxial Cables
95 Slides 35 minutes – no presenter narrative
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Coordinates
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SWR
Batteries for Amateur Radio
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Mobile Antennas
Presented March 2023 with the presenter’s narration
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How Do Ferrite Beads Work and How Do You Choose the Right One?
Ferrite beads are generally used for high frequency EMI noise suppression
We know EMI is present when we hear it in our audio circuits or it raises hell with the performance of our equipment. One possible fix for that kind of interference is a Ferrite bead. Ferrite beads (also called a ferrite choke, ferrite clamp, ferrite collar, EMI filter bead, or even a ferrite ring filter) can be a bit of a mystery. They can be a fast and affordable fix to EMI and common mode currents.
The Ferrite core function resembles that of an inductor, but the Ferrite core frequency response deviates from this functionality at high frequencies. Additionally, different types of beads, such as wire wound Ferrite beads and chip Ferrite beads, provide different responses to noise reduction. For example, wire wound “doughnut” Ferrite cores operate over a wide range of frequencies but offer less resistance in direct current setups.
This image shows why a ferrite bead is sometimes called a ferrite ring or ferrite choke
What is a Ferrite Bead and How Do Ferrite Beads Work?
Ferrite beads are passive electronic components that can suppress unwanted currents on a power supply line or coaxial cable. They are normally placed around a power/ground line pair that is incoming to a particular device, such as the power cord for your laptop, or on your coaxial cable where it enters your station . These beads work according to Faraday’s Law: the magnetic core around a conductor induces a back EMF in the presence of a high frequency signal, essentially attenuating the ferrite frequency response.
Ferrites are magnetic materials, and placing this material in a ferrite clamp around the power supply/ground or coaxial feedline line allows a source of inductive impedance for signals passing through the line. That might tempt you to think of them as a standard inductor, but they’re more complex than that. In reality, a Ferrite bead is a nonlinear component; the impedance it provides changes was the load current and voltage drop across the ferrite change.
What are Ferrite Beads Used For?
Because Ferrite bead impedance is inductive, they are used to attenuate high-frequency signals. When a Ferrite bead choke is placed on the power line or RF carrying cable connected to an electronic device, it removes any spurious high frequency noise (current flow) present. This Ferrite clamp use is one of many approach to noise suppression, such as that from a switched-mode power supply, household appliances and outside power lines. This application of Ferrite beads as a Ferrite filter, sometimes called a “choke”, provides suppression and elimination of conducted EMI.
A common mode current suppressing balun, which has a Ferrite core and is rated for a given wattage will be damaged if you exceed it’s rated wattage capacity. As DC current flowing in the antenna system exceeds the rated capacity of the Ferrite core, the Ferrite will “saturate” and lose inductance. This would be a rare occurrence with a Ferrite bead.
Clamp-on Ferrite beads can be quite useful in attenuating unwanted common mode currents in your station, but only if you understand exactly how they work.
The diameter of the cable affects the performance of the ferrite shield. More than a single cable may be inserted through the core opening as every circuit interacts with the ferrite substance independently. Ideally, your goal should be to ensure a snug attachment that also boosts the attenuation effect significantly, owing to the greater magnetic path length.
To create the best magnetic path and maintain a relatively higher attenuation ratio, the outside dimensions of the cable and the inside dimensions of the ferrite should create a tight fit. However, a looser fit is required in some cases. For example, if you want a bigger ferrite or a purpose-fit slack cable for flexing, and tension control, it is necessary to opt for a looser fit.
Ferrite beads are normally situated near the cable termination where it exits the electronic enclosure. In fact, you might have to install a suppressor on both ends if the cable connects two separate enclosures containing radio frequency sources.
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Print Your Own QSL Cards at Home
Here is a link to an easy to use free online QSL card creation & printing program
http://www.radioqth.net/qslcards
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Wilderness Protocol by W7PJ
(printable version)