T7C

SWR, feed lines and dummy loads

Explain what SWR and a dummy load are for, how to measure them, and what makes feed lines lose power or fail.

11 pool questions1 on the exam

Syllabus: Antenna and transmission line measurements and troubleshooting: measuring SWR, effects of high SWR, causes of feed line failures; Basic coaxial cable characteristics; Use of dummy loads when testing

A dummy load lets you transmit without transmitting

Testing into a dummy load keeps your test signals off the air.

A dummy load is a 50 Ω non-inductive resistor on a heat sink. It soaks up your power as heat, so test signals don't go out over the air.

RememberDummy load = test without radiating.

SWR tells you how well the antenna matches the line

When the antenna doesn't match the feed line, part of the power reflects back down the line. SWR (standing wave ratio) measures how much.

The farther the antenna is from the line's 50 Ω, the bigger the reflected wave.

Antenna impedance200 Ω

SWR4 : 1

Power reflected36%

Reading	Meaning
1:1	perfect match, nothing reflected
4:1	mismatch

Instrument	Tells you
Directional wattmeter	forward and reflected power, so the SWR
Antenna analyzer	whether the antenna is resonant at your operating frequency

Remember1:1 is perfect. The higher the ratio, the worse the mismatch.

Lost power becomes heat

Power lost in a feed line is converted into heat. It isn't radiated and it doesn't distort the signal.

Lost power doesn't vanish: it warms the cable. Sizes are illustrative.

Foam dielectric coax has less loss per foot than solid.

Solid-state transmitters cut their output as SWR rises to protect the output transistors.

RememberLoss = heat. High SWR makes the radio back off to protect itself.

Water is what kills coax

Foam is mostly air, so it loses less per foot. Water in the cable is what kills it.

Moisture contamination causes coax to fail. That's why the outer jacket must resist UV light: sun damage cracks the jacket and lets water in.

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