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.

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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.

rfi1

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.

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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.

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

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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.

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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.

Part15

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 upwards (including GHz microwave bands).

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Locating sources of RFI can be tricky and beyond what we can suggest here.  One time-tested suggestion for zeroing in on noise sources at home is to switch off power to the house and listen for the noise with your receiver on batteries (that’s an Extra Class exam question).  Refer to the links below for other ideas and research the web for others.

The two main ways of reducing RFI are shielding and filtering.  Shielding reduces EMI using conductive or magnetic barriers. It is typically applied to enclosures (Faraday cage) and interconnect (shielded) cables.  Filtering reduces EMI by blocking or bypassing RF signals.  A Combination of the two may be required to solve tough RFI problems.

We will present some information on filtering in a separate post.


Useful web links

Radio Frequency Interference (RFI)– ARRL

Electromagnetic interference– Wikipedia article

Interference Handbook– FCC Compliance & Information Bureau

What Is RF Interference? – Com-Power Corporation

Understanding and Solving RF Interference Problems (PDF)- Jim Brown K9YC

Part 15 Devices– Ham Radio School

 

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