RFI Filters and Ferrite Chokes

In our previous post we introduced the reality of radio frequency interference (RFI) which can affect hams on occasion.

A potential remedy for RFI (in addition to shielding, as mentioned there) is some form of RF filtering.  Filters are used to either reject (attenuate) or accept (pass) signals over a range of frequencies. A couple of license exam questions down below exemplify this topic.

Filters are a fundamental concept in electronics but details can get complicated so we will share only basic info and give several references for your own research.

Before reading further the reader is strongly advised to review an excellent article, Introduction to Filters, from All About Circuits.  Credit to that site for the filter diagram below.

Filters can be categorized into four essential types:

  • Low-Pass: Passes frequencies below cutoff and attenuates higher frequencies
  • High-Pass: Passes frequencies above cutoff and attenuates lower frequencies
  • Band-Pass: Passes frequencies only within a specific range, attenuates others
  • Band-Stop/Band-Reject (Notch): Attenuates frequencies within a specific range, passes others

This diagram compares and clearly illustrates how these four operate:

Four filter types

Regardless of design and type, all filters will introduce some loss of the passed signal (insertion loss).  Good filter designs will minimize loss and allow the protected device to function properly.

In audio equipment the tone controls are typically high-pass and low-pass filters with adjustable cutoff for treble and bass .  More sophisticated tone controls may add a mid-range adjustment while a graphic equalizer adds filters for multiple narrow frequency ranges.

Many RFI filters are low-pass because hams may need to remove transmitted HF or VHF/UHF signals from low frequency (50/60Hz) AC power lines, audio (25Hz-25kHz) signals, VGA video (31kHz) signals, or lower-frequency (AM) radio.

Filters can be made using active circuits and/or software but more commonly, RF filtering is accomplished with simple passive electronic components: capacitors (C), inductors (L), and resistors (R).  Simple LC, RC, RL, or RLC filters can be made from combinations of these parts. The simplest low-pass filter is a capacitor across (shunt) a signal, an inductor in line (block), or both.

Besides electronic components, something completely different and maybe unexpected can also form of passive low-pass filter.  Ever wonder what that bulge is near the end of many audio, video, and computer cables?


Specifically, it’s a ferrite bead or choke.  If you cut one open, under that plastic cover you would find that the cable passes through a gray tube.



That tube is made of  ferrite material, a ceramic with high iron content and high permeability.  While more complex than this, an easy way to visualize how ferrite beads work is to say that signals passing through them are coupled into the material which becomes resistive over the intended frequency range and dissipates the unwanted energy as heat.


Seems a bit mysterious and magical, but it’s science.  Different ferrite compositions have varying frequency, power handling, and temperature characteristics so selection is critical.


Ferrite beads (chokes) are typically simple and inexpensive interference filters to install Continue reading

Radio Frequency Interference (RFI)

Radio frequency interference (RFI) can be a problem for hams.  We transmit radio waves which can sometimes cause problems for other electronics (radio, TV, telephones, etc.)  Sometimes other devices generate RF noise that interferes with our radio reception.


Also known as electromagnetic interference (EMI), there are several US license exam questions involving this subject.  Effects of RFI vary from nuisance audio noise or video snow to disabling electronics.


Sometimes even normal, legal RF signals cause RFI to sensitive devices in our homes or our neighbor’s.  Even with the switch from the old analog TV broadcasts to digital format, certain ham radio transmissions can scramble over-the-air (OTA) and cable TV video and audio reception.

One common problem with cable TV interference is poor quality or improperly terminated coaxial cables.


Right or wrong, hams get blamed for any form of interference and it’s been that way for the whole century of amateur radio’s existence.  You may run across jokes and cartoons about amateur RFI like this:

interference cartoon bw

Just be aware that hams get a lot of blame and extra scrutiny for interference, often baseless.  Still we need to be good neighbors and work with others if there is a problem.


This goes both ways.  Sometimes neighbors can cause interference with ham radio reception.


Common outdoor sources of RFI include arcing power line insulators and neon signs.  Indoor culprits to consider: lamp dimmers, LED lamp drivers, switch mode power supplies, PCs, battery chargers, appliance motors, grow lights, garage door openers and remote controls.     Some of the above are examples of FCC Part 15 devices.


Recall that US amateur radio is governed by Code of Federal Regulations, Title 47, Part 97 (47 CFR 97).  Separately, Part 15 regulates low power, unlicensed devices.  Nearly every electronics device sold inside the United States radiates unintentional emissions, and must be reviewed to comply with Part 15 before it can be advertised or sold in the US market.  Many electronics gadgets in your home have a FCC Part 15 marking on them.


In theory, Part 15 devices should not cause interference with amateur radio signals, and be functionally immune from ham radio.  In reality, they don’t all play well with our licensed service, particularly cheap gadgets which may not have legitimate certification.

RFI is more commonly an issue on the HF bands but can occur at VHF/UHF and Continue reading