G9A

Feed line impedance, loss and SWR

Work out SWR from load and line impedance, and explain how line loss and tuners change what you measure.

11 pool questions1 on the exam

Syllabus: Feed lines: characteristic impedance and attenuation; standing wave ratio (SWR) calculation, measurement, and effects; antenna feed point matching

Characteristic impedance comes from the line's shape

A line's characteristic impedance (Z₀) is set by its geometry: the spacing between conductors and their radius. Wider spacing raises it, fatter conductors lower it. Length and frequency don't change it.

Cross-section (left) sets Z₀. Slide the length: nothing changes.

Spacing1 inConductor radius0.04 inLine length100 ft

Z₀ (ideal, air spaced)386 Ω

Line	Nominal Z₀
Coax	50 Ω (or 75 Ω)
Window line (ladder line with windows in its web)	450 Ω

RememberSpacing and radius set Z₀. Window line is 450 Ω.

SWR is the bigger impedance divided by the smaller

Power reflects wherever the load impedance differs from the line's Z₀. For a resistive load, SWR = larger ÷ smaller, so it is never below 1:1. Only a perfect match gives no standing waves.

SWR = larger ÷ smaller. Always at least 1:1, never 1:4.

Load resistance200 Ω

SWR4 : 1

Power reflected36%

Load on 50 Ω line	Working	SWR
200 Ω	200 ÷ 50	4:1
10 Ω	50 ÷ 10	5:1
50 Ω	equal	1:1

RememberLarger ÷ smaller. Match the load to Z₀ for no standing waves.

Feed line loss rises with frequency, in dB per 100 feet

Cable loss is quoted in decibels per 100 feet. Coax loses more as frequency rises, so a long run that is fine on 80 m can be costly on 70 cm.

Schematic: no scale on the loss axis. The order and the upward slope are what matter.

Frequency30.9 MHz

Loss is quoted in dB per 100 ft, then scaled by the real length. Every 3 dB halves the power.

Cable loss rating1.5 dB per 100 ftCable length150 ft

Total loss2.25 dB

RememberdB per 100 ft. Higher frequency, more attenuation.

High SWR adds loss, but loss and tuners hide it

Reflected power travels the line again, so in a lossy line a high SWR increases loss. Loss also absorbs the reflection on its way back, so the SWR at the radio end reads lower than at the antenna.

A matching network at the transmitter can make the radio see 1:1, but the mismatch is still at the antenna, so the line still carries the same SWR.

Loss makes the input SWR look better than the antenna's. A tuner makes the radio happy but cannot change the SWR on the line.

SWR at the antenna5 : 1Line loss3 dB

SWR measured at line input2 : 1

RememberTuners and loss change what the radio sees, not the antenna mismatch.

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Compendium articles on the ideas in this lesson, beyond what the exam asks.

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Practise the 11 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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