G9A: Feed Lines and SWR – Ham Radio General License Study Guide

G9A covers the transmission lines that carry RF power between a transmitter and an antenna — their physical properties, how they are rated, what causes power loss and reflections, and how standing wave ratio (SWR) is calculated and interpreted. The eleven questions in this group address characteristic impedance, cable types, attenuation characteristics, reflected power, matching requirements, SWR calculation, and the interaction between line loss and SWR readings.

Topics include what determines characteristic impedance of a parallel feed line, the impedance of window line, what causes reflected power at the antenna feed point, how coaxial cable attenuation changes with frequency, the units used for feed line loss, what must be done to prevent standing waves, what a transmitter-end matching network does to SWR on the feed line, SWR calculations for mismatched impedances, and how transmission line loss affects the SWR reading at the transmitter.

Characteristic Impedance and Feed Line Types

The characteristic impedance of a transmission line is a fixed property of the line's physical construction — not its length or the signal frequency. For a parallel conductor (open-wire) feed line, characteristic impedance is determined by two physical factors:

• The distance between the centers of the conductors
• The radius of the conductors

A wider spacing or thinner conductors results in higher impedance. A narrower spacing or thicker conductors results in lower impedance.

Feed Line Attenuation and Loss

Feed line loss is measured and specified in decibels per 100 feet (dB/100 ft). This unit tells you how much signal power is lost for every 100 feet of cable at a given frequency. The specification is always given for a specific frequency because coaxial cable attenuation increases with frequency — a cable rated at 0.5 dB/100 ft at 30 MHz may be 2 dB/100 ft at 450 MHz. This is why coax that works fine on HF may be unacceptably lossy at UHF without upgrading to low-loss cable.

The primary reasons coaxial cable loss increases with frequency:

• The skin effect concentrates current in a thinner layer at the conductor surface, increasing effective resistance
• Dielectric loss in the insulating material increases at higher frequencies

SWR, Reflected Power, and Impedance Matching

Reflected power at an antenna feed point is caused by a difference between the feed line's characteristic impedance and the antenna's feed point impedance. When these two impedances match, all power travels from the transmitter to the antenna and is radiated — no reflection. When they differ, a portion of the power is reflected back toward the transmitter, setting up standing waves on the feed line.

To prevent standing waves, the antenna feed point impedance must be matched to the characteristic impedance of the feed line. This is accomplished through matching networks, baluns, stub matching, or antenna design (such as using a folded dipole to raise the feed impedance).

Effect of High SWR on Loss

High SWR increases loss in a lossy transmission line. When power reflects back and forth on the line, each pass through the lossy cable dissipates additional power as heat. The greater the mismatch (higher SWR), the more current flowing in the line, and the greater the I²R losses.

Matching Network at the Transmitter End

An important concept: a matching network (antenna tuner) placed at the transmitter end of the feed line makes the transmitter see a 1:1 SWR, protecting the transmitter and allowing it to deliver full power. However, it does not change the SWR on the feed line between the tuner and the antenna. If the SWR on the feed line is 5:1, it remains 5:1 after adding a transmitter-end tuner — the extra loss from the mismatch still occurs in the cable.

Effect of Line Loss on SWR Reading

A lossy transmission line attenuates both the forward and reflected waves. The reflected wave is attenuated twice (once going forward to the antenna, once coming back) before it reaches the SWR meter at the transmitter. This means higher transmission line loss results in a lower SWR reading at the transmitter input — the reflected power has been absorbed by the cable before it can be measured. A very lossy cable can show low SWR even with a badly mismatched antenna, giving a misleadingly good reading.

SWR Calculations

SWR is calculated as the ratio of the larger impedance to the smaller:

G9A Practice Questions