T2 · Published 11 October 2026 · By The Ham Desk

Standing Waves, Coax and Matching: Making the Power Arrive

An antenna is only as good as the line that feeds it. This unit covers what standing wave ratio actually measures, why coax loss is a different problem at 2 metres than at 40, and what the boxes called baluns and tuners really do.

Band 2 m
Study time 17
Level Core
questions 10

Antennas and Feed Lines — waveform overlay and radiation pattern

Pre-flight briefing

What SWR is, and what it is not

When the impedance of the antenna does not match the characteristic impedance of the feed line, part of the power reflects back toward the transmitter and the two travelling waves interfere, producing a standing wave on the line. The standing wave ratio is the ratio of the maximum to the minimum voltage along that line: 1:1 means everything is matched, 2:1 means about 11 per cent of the forward power is reflected, and 3:1 about 25 per cent.

Two things follow that beginners often get wrong. First, SWR is not the same as loss - a mismatched line with low loss simply reflects the power back and it appears at the transmitter again; with a lossy line, that power is dissipated in the cable instead. Second, a perfect 1:1 SWR does not mean the antenna is radiating well: a dummy load has a perfect SWR and radiates nothing at all.

Coax, connectors and the loss that rises with frequency

Coaxial cable loss is quoted in decibels per hundred feet and depends heavily on both the cable and the frequency. Thin RG-58 may lose 1.5 dB per hundred feet on 10 metres but over 4 dB per hundred feet at 2 metres; thick foam RG-8 loses far less. Because the loss appears on the way out and again on the way back from the antenna, a 3 dB cable loss halves your effective radiated power, and every additional 3 dB halves it again.

Characteristic impedance is a property of the cable's geometry, not its length: 50 ohms is the amateur standard, 75 ohms is used for video and for some receiving applications, and 300 ohm twin-lead is used for balanced antennas. Velocity factor - typically about 0.66 for solid polyethylene coax - is the fraction of the speed of light at which the signal travels inside the cable, and it is what makes a half-wave coaxial stub shorter than the free-space calculation suggests.

Baluns, tuners and gain arithmetic

A balun connects a balanced antenna, such as a dipole or a Yagi driven element, to unbalanced coaxial cable. A 1:1 current balun (a choke) suppresses current on the outside of the shield; a 4:1 balun also transforms impedance, which suits a folded dipole's nominal 300 ohms down to 50. Without one, the shield becomes part of the antenna, the pattern distorts, and RF appears in the shack.

An antenna tuner sits between transmitter and feed line and transforms whatever impedance the line presents into something near 50 ohms. It protects the final amplifier from a high-SWR load, but it does not reduce loss already present in the cable and does not change the antenna's own radiation efficiency.

Gain figures need care. Antenna gain is a ratio measured against a reference: dBd compares against a half-wave dipole, and dBi compares against an isotropic radiator. Since a dipole has 2.15 dBi of gain, a figure quoted as 5 dBd is 7.15 dBi - the same antenna, described two ways.

Terms you must be able to define
Term	What the examiner wants to hear
SWR	Standing wave ratio: the ratio of maximum to minimum voltage on a mismatched feed line; 1:1 is a perfect match.
Velocity factor	The fraction of the speed of light at which a signal travels inside a cable.
Balun	A device that couples a balanced antenna to unbalanced coax, sometimes also transforming impedance.
dBd / dBi	Gain referenced to a dipole or to an isotropic radiator; dBd + 2.15 = dBi.
Characteristic impedance	The impedance determined by a cable's geometry, independent of its length.

Question deck 10 questions

T4B01 Core

What does a standing wave ratio of 2:1 indicate?

T4B02 Core

What causes power to be reflected back down a feed line?

T4B03 Core

Which coaxial cable would lose more signal on the 2 metre band?

T4B04 Core

What is the characteristic impedance of a coaxial cable?

T4B05 Core

What is the purpose of a balun between a dipole and a coaxial feed line?

T4B06 Core

What is the velocity factor of a coaxial cable?

T4B07 Stretch

An antenna is advertised as having 5 dBd of gain. What does this mean compared with an isotropic radiator?

T4B08 Stretch

Why does a high SWR matter more on a lossy feed line than on a low-loss one?

T4B09 Core

What does an antenna tuner protect the transmitter from?

T4B10 Stretch

A dummy load presents a perfect 1:1 SWR. What does this tell you about its radiation?

Reference sheet

The explanations for all ten questions sit on this sheet. Pick an answer on the left and the matching card lights up.

T4B01 Explanation 1

Key B — About 11 per cent of the forward power is being reflected at the mismatch

SWR 2:1 corresponds to a reflection coefficient of one third in voltage terms and a reflected power of about 11 per cent. A 3:1 SWR reflects about 25 per cent. The rest of the power still reaches the antenna.

Rule Fundamentals - standing waves

T4B02 Explanation 2

Key A — An impedance mismatch between the antenna and the feed line's characteristic impedance

Whenever the load impedance differs from the line's characteristic impedance, part of the wave is reflected. Length is irrelevant to whether a mismatch exists - a 50 ohm line is 50 ohms whether it is three metres or thirty.

Rule Fundamentals - impedance matching

T4B03 Explanation 3

Key D — Thin RG-58

Loss falls as the cable gets fatter: RG-58 loses several dB per hundred feet at 144 MHz while RG-8 loses considerably less. On 2 metre and 70 centimetre runs of any length, the cable choice is as important as the antenna.

Rule Fundamentals - transmission line loss

T4B04 Explanation 4

Key C — The impedance determined by its physical dimensions, independent of length

Characteristic impedance comes from the ratio of inner and outer conductor diameters and the dielectric between them, which is why 50 and 75 ohm cable are both standard and neither can be changed by cutting the cable to a different length.

Rule Fundamentals - transmission lines

T4B05 Explanation 5

Key B — It couples a balanced antenna to an unbalanced line and can prevent current from flowing on the shield

A dipole is a balanced load and coax is an unbalanced line, so without a balun the shield carries current, radiating as part of the antenna. A 1:1 current balun blocks that path; a 4:1 type also steps impedance, which is what a folded dipole needs.

Rule 47 CFR §97.13(c) - RF in the station

T4B06 Explanation 6

Key A — The speed at which a signal travels in the cable as a fraction of the speed of light

A signal travels more slowly in cable than in free space - about 0.66 times the speed of light in solid polyethylene coax - and this fraction is why a half-wave stub or phasing line is physically shorter than the free-space formula predicts.

Rule Fundamentals - velocity factor

T4B07 Explanation 7

Key D — 7.15 dBi, because a dipole itself has 2.15 dBi of gain

Gain referenced to a dipole (dBd) is always 2.15 dB lower than the same gain referenced to an isotropic radiator (dBi), because a dipole already has 2.15 dBi. Advertisers who switch references without saying so can make a modest antenna look 2 dB better than it is.

Rule Fundamentals - antenna gain references

T4B08 Explanation 8

Key C — Reflected power makes extra trips through the cable and each trip dissipates more of it as heat

With a low-loss line, reflected power simply returns to the transmitter and can be re-reflected toward the antenna. With a lossy line, every additional pass through the cable dissipates more of it, which is why ladder line tolerates a 5:1 SWR comfortably and thin coax at 2 metres does not.

Rule Fundamentals - matched loss versus SWR

T4B09 Explanation 9

Key B — The high impedance presented by a mismatched feed line, which would otherwise cause the final amplifier to fold back

A modern transceiver reduces output when it sees a bad match, so the tuner exists to present it with something near 50 ohms. What the tuner cannot do is recover power already lost in the cable, which is why a matched antenna is still better than a tuner.

Rule Fundamentals - matching

T4B10 Explanation 10

Key B — Nothing: it absorbs the power as heat and radiates almost none of it

A dummy load is a 50 ohm resistor in a shielded can, so a low SWR only says the transmitter is happy - not that energy is leaving the antenna. The same reasoning applies to a badly installed antenna with a low SWR: matched, but possibly lossy.

Rule Fundamentals - why SWR is not efficiency

How to use this unit

Work the ten questions in one sitting with the reference sheet covered, then uncover it and read every explanation - including the ones you got right. The wording of the real exam is different, so learn the reasoning rather than the answer letters.