T7B: Common Transmitter and Receiver Problems – Ham Radio Technician License Study Guide

Every amateur operator will eventually encounter interference — either causing it or experiencing it. T7B teaches you how to recognize the most common problems that affect transmitters and receivers, understand why they happen, and apply the correct fix. Being a good neighbor on the airwaves and in the community means knowing how to diagnose and resolve these issues promptly and professionally.

This group covers over-deviation, fundamental overload, RF feedback, distorted audio, interference to consumer electronics, and how to work cooperatively with neighbors when interference occurs.

Causes of RF Interference

Radio frequency interference (RFI) can have multiple causes. The exam identifies three: fundamental overload (when a strong signal on its fundamental frequency overwhelms a receiver not designed to reject it), harmonics (integer multiples of the transmit frequency that the transmitter radiates unintentionally), and spurious emissions (other unintended signals the transmitter produces). All three of these can cause RFI — and when the exam asks which causes interference, the answer is all of them.

Over-Deviation on FM

FM transmitters encode audio information by varying the carrier frequency up and down — this variation is called deviation. When you speak too loudly or too close to the microphone, you over-deviate: the frequency swings wider than the channel allows. This causes your signal to splatter into adjacent channels and sound distorted on the receiving end.

If you are told your FM handheld or mobile transceiver is over-deviating, the fix is simple: talk farther away from the microphone. This reduces the audio level reaching the transmitter, reducing deviation. Distorted audio through an FM repeater can also be caused by being slightly off frequency, having low batteries, or being in a poor location — any of these could be the problem.

RF Feedback

RF feedback occurs when RF energy from the transmitter finds its way back into the audio circuitry — through the microphone cable, through the power supply leads, or through the station ground system. When RF gets into the audio circuit, it modulates your transmitted signal with RF artifacts, producing garbled, distorted, or unintelligible voice transmissions. This is the characteristic symptom of RF feedback.

A ferrite choke placed on the microphone cable can cure distorted audio caused by RF current on the cable shield. The ferrite choke presents high impedance to RF while passing audio normally, breaking the RF path without affecting the audio signal.

Fundamental Overload and Filters

Fundamental overload happens when a strong amateur signal overwhelms a nearby non-amateur receiver — a broadcast AM or FM radio, a television, or a neighbor's radio — even though your signal is perfectly clean and on a completely different frequency. The affected receiver simply cannot reject signals that are strong enough and close enough geographically. The receiver's front end is being saturated by your fundamental frequency.

The correct fix for fundamental overload is to install a filter at the antenna input of the affected receiver — a high-pass filter for TV and FM, or a band-reject filter tuned to your operating frequency. This filter prevents your strong signal from entering the affected receiver. Installing a filter on your transmitter would not help because your signal is legitimate; the problem is the other receiver's inability to cope with strong out-of-band signals.

A VHF transceiver can be overloaded by a nearby commercial FM broadcast station. The correct solution in that case is a band-reject filter — a filter that rejects the commercial FM band frequencies specifically.

Interference to Neighbors

When a neighbor reports that your station is interfering with their TV or radio, the correct first step is to verify that your own station is functioning properly — confirm it does not interfere with your own radio or television on the same channel. This establishes whether the problem is with your station or with the neighbor's equipment.

If something in a neighbor's home is causing interference to your amateur station, the right approach combines all of: working with your neighbor to identify the offending device, politely informing them that FCC rules prohibit interference-causing devices, and making sure your station meets good amateur practice standards. For cable TV interference from your amateur transmission, the first thing to check is whether all the TV feed line coaxial connectors are properly installed — loose or corroded connectors are a very common source of cable system interference pickup.

T7B Practice Questions