T7C: Antenna and Feed Line Troubleshooting – Ham Radio Technician License Study Guide

The path between your transmitter and your antenna — the feed line — carries your signal to the world. If that path is not working correctly, power is wasted, equipment can be damaged, and your signal suffers. T7C teaches you how to measure, evaluate, and maintain that critical connection, and how to test your transmitter safely without transmitting over the air.

This group covers SWR measurement and its meaning, dummy loads, antenna analyzers, what happens to power lost in a feed line, and the causes of coaxial cable failure. These are practical skills every station operator needs.

Dummy Loads

A dummy load is a device that substitutes for an antenna when you need to test your transmitter without radiating a signal. Its primary purpose is to prevent transmitting signals over the air when making tests. This is important for bench testing, adjusting transmitter power, and checking audio quality without occupying a frequency or interfering with other stations.

A dummy load consists of a non-inductive resistor mounted on a heat sink. The resistor — typically 50 ohms to match standard coaxial cable — absorbs the transmitter's RF output and converts it to heat rather than radiation. It is "non-inductive" because an inductive resistor would behave differently at RF frequencies. The heat sink is necessary because even a brief transmission produces significant heat in the resistor.

SWR: Measuring Impedance Match

SWR stands for standing wave ratio. It is a measure of how well the impedance of the antenna system matches the impedance of the feed line and transmitter. When there is a perfect impedance match between the antenna and the feed line, all the power travels from the transmitter to the antenna with nothing reflected back. That perfect match produces an SWR reading of 1:1.

When there is an impedance mismatch, some power is reflected back toward the transmitter, creating standing waves on the feed line. An SWR reading of 4:1 indicates an impedance mismatch — the higher the ratio, the worse the mismatch. A directional wattmeter is an instrument that can be used to determine SWR by measuring forward and reflected power.

Most solid-state transmitters automatically reduce output power as SWR increases beyond a certain level. This is not to lower the SWR — it is to protect the output amplifier transistors, which can be damaged by the voltage spikes that result from high reflected power.

Feed Line Loss

No feed line is perfectly lossless. Some of the power traveling from the transmitter to the antenna is absorbed by the feed line itself. Power lost in a feed line is converted into heat. This loss is inherent to all practical feed lines — the goal is to minimize it by using low-loss cable, keeping runs as short as practical, and maintaining a good impedance match.

Coaxial Cable Failures

Coaxial cable is durable but not indestructible. The most common cause of coaxial cable failure is moisture contamination. Water entering the cable — through damaged connectors, cracks in the jacket, or cuts — dramatically increases signal loss and eventually destroys the cable from the inside. This is why coaxial connectors must be properly weatherproofed when used outdoors.

The outer jacket of coaxial cable should be resistant to ultraviolet (UV) light because UV light can damage the jacket and allow water to enter the cable. Sun exposure degrades plastic jackets over time, causing cracking and brittleness that opens pathways for moisture.

Air core coaxial cable (where the space between center conductor and shield contains air rather than foam or solid dielectric) has lower loss than foam or solid dielectric types, but it has a significant disadvantage: it requires special techniques to prevent moisture from entering the cable. Unlike foam or solid dielectric types that physically block water, air core cable provides an open path for moisture to travel.

Antenna Analyzers

An antenna analyzer is the instrument used to determine if an antenna is resonant at the desired operating frequency. It measures the impedance of the antenna system across a range of frequencies and shows you where the antenna is resonant and what its impedance is at any given frequency. This allows you to trim an antenna to resonance or diagnose feed line problems without transmitting. An antenna analyzer is a more capable and specific tool than an SWR meter for antenna work.

T7C Practice Questions