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Types of Radio Frequency Interference (T7B03)

The 2022-2026 Technician License question pool includes a question about various types of radio frequency interference, or RFI:


T7B03: Which of the following can cause radio frequency interference?

A. Fundamental overload

B. Harmonics

C. Spurious emissions

D. All these choices are correct

Woman with fingers in ears making bad face.

Interference is the undesired reception of RF signals produced either properly or improperly by another electronic device. Interference received by your transceiver may reduce your ability to hear another station with which you wish to communicate. Interference produced by your transceiver may interrupt communications between other stations. See Chapter 12 of the HamRadioSchool.com Technician License Course book for more information about interference and how to reduce or eliminate it.


Fundamental overload is a type of interference in a receiver caused by very strong signals. The very strong RF signals induce currents on receiver components, overloading them and making it impossible to properly process weaker signals received via the antenna. You may experience fundamental overload with your transceiver if another operator is transmitting very near your station on the same band to which your radio is tuned. Frequently this will produce loud and distorted audio from your receiver because your receiver is unable to reject the strong interfering signal. So, fundamental overload may be a cause of radio frequency interference.


Harmonics are undesired transmissions that are multiples of the intended transmission frequency. Almost all RF transmitters produce some harmonics at low power levels, as compared to the fundamental frequency that is the desired transmission frequency. The 2nd harmonic, double the frequency of the fundamental, and the 3rd harmonic, triple the frequency of the fundamental, are usually the strongest harmonics produced by a transmitter. Harmonics produced by an ill-functioning amateur radio transmitter may lie well outside of the amateur frequency bands and cause interference to other radio services or electronic devices. Harmonics are another potential cause of radio frequency interference.


Spurious emissions are a more general term for radio frequency emissions that are not desired or deliberately transmitted, such as harmonics. A common term for some spurious emissions is splatter, meaning that a transmitter is improperly transmitting outside of the necessary and proper bandwidth for the mode and/or frequency band being used. For instance, if a single sideband transmitter is emitting a band of 15 kHz in transmission, rather than the proper 3 kHz (or less) necessary for communication in that mode, the transmitter is producing splatter that will interfere with other stations tuned to frequencies nearby on the band. Check your transmitter for off-frequency spurious emissions if you are informed that your station is causing splatter or interference with nearby frequencies. Thus, spurious emissions such as splatter and harmonics are also a potential cause of radio frequency interference.


The answer to Technician question T7B03, “Which of the following can cause radio frequency interference?” is D: All of these choices are correct.

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