T1 · Published 13 September 2026 · By The Ham Desk

Dipoles, Feed Lines and What SWR Is Really Telling You

An antenna is a matching device between a transmitter and free space. This unit covers the two numbers that decide whether the match is good - the length of the radiating element and the impedance it presents to the feed line - and the one measurement that tells you how far off you are.

Band 2 m
Study time 15
Level Core
questions 10

Antennas and Feed Lines — waveform overlay and radiation pattern

Pre-flight briefing

Length first

A half-wave dipole in feet is 468 divided by the frequency in megahertz, not 492 divided by frequency. The shortcut number is smaller because it already allows for the end effect, the slight increase in electrical length caused by the wire's thickness and its surroundings. On 146 MHz that is 3.2 feet, roughly 98 cm, fed in the centre with 50 ohm coax.

A quarter-wave vertical is the same idea folded upright: 234 divided by frequency in feet, with the earth or a set of radials making up the missing half. That is why a 2 metre quarter-wave whip is about 50 cm long and why it needs a ground plane or a car body under it.

Feed line losses and velocity factor

Coax loses signal in proportion to length and frequency. Thicker cable wins: RG-8 and RG-213 use a larger centre conductor and far less dielectric loss than thin RG-58, and at 2 metres the difference over a 30 metre run is several decibels, which is most of your signal. Velocity factor explains the other oddity of coax: radio waves travel more slowly inside the dielectric, so a quarter-wave stub of solid polyethylene coax is about 0.66 of the free-space length, and about 0.8 for the foam types.

SWR in plain language

Standing wave ratio compares the power the transmitter sends toward the antenna with the power reflected back. A 1:1 match reflects nothing. At 2:1 about 11 per cent of the power comes back; at 3:1 about 25 per cent; at 5:1 more than 44 per cent never leaves the shack. Reflected power does not vanish - it travels back down the line, and on a long lossy line it is dissipated twice.

Most solid-state transmitters fold back their output as SWR rises to protect the final amplifier, so a badly matched antenna does not just waste power, it can reduce it. A balun at the feed point converts the balanced dipole to the unbalanced coax, and keeping the antenna clear of metal and roughly a half wavelength above ground keeps the impedance near the 73 ohms a resonant dipole presents.

Terms you must be able to define
Term	What the examiner wants to hear
468/f	Half-wave dipole length in feet; 234/f is the quarter-wave vertical version.
End effect	The apparent lengthening of an antenna by its own thickness, already included in the 468 constant.
SWR	Standing wave ratio: how much of the forward power is reflected, expressed as a ratio.
Velocity factor	The fraction of the speed of light at which a signal travels inside a feed line; about 0.66 in solid polyethylene coax.
Balun	A transformer that connects a balanced antenna such as a dipole to unbalanced coaxial feed line.

Question deck 10 questions

T4A01 Core

What is the approximate length of a half-wave dipole cut for 146 MHz?

T4A02 Core

Why is the constant in the dipole formula 468 rather than 492?

T4A03 Core

What is the nominal feed point impedance of a resonant half-wave dipole in free space?

T4A04 Core

Which coaxial cable will have the lowest loss over a 30 metre run at 146 MHz?

T4A05 Recall

What does an SWR of 1:1 indicate?

T4A06 Stretch

Approximately how much of the forward power is reflected when the SWR is 2:1?

T4A07 Core

Why can a very high SWR reduce your effective radiated power?

T4A08 Core

A quarter-wave matching stub is cut from solid polyethylene coax. How long should it be compared with a free-space quarter wavelength?

T4A09 Core

What is the purpose of a balun at the feed point of a dipole?

T4A10 Stretch

What is the approximate length of a quarter-wave vertical for 146 MHz?

Reference sheet

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

T4A01 Explanation 1

Key C — 3.2 feet

468 / 146 = 3.2 feet, or about 98 cm. The 2.05 foot figure is the wavelength in metres misread as feet, and 6.4 feet would resonate near 73 MHz.

Rule Fundamentals - antenna length formula

T4A02 Explanation 2

Key B — It compensates for end effect and converts the result to feet

The physical half wavelength in feet is close to 492/f. Practical antennas resonate slightly short of that because of end effect, and the shortened constant builds in the correction.

Rule Fundamentals - antenna practice

T4A03 Explanation 3

Key A — About 73 ohms

A resonant centre-fed dipole presents about 73 ohms, which is close enough to 50 ohm coax that the mismatch costs little. Folded dipoles and some verticals land near 300 ohms or lower, depending on height above ground.

Rule Fundamentals - antenna impedance

T4A04 Explanation 4

Key D — RG-213, because its larger diameter has less dielectric and conductor loss

Loss falls as the cable gets physically larger. RG-58's thin centre conductor and small dielectric diameter make it the lossiest of the three at VHF, which is why short jumpers use it and long runs do not.

Rule Fundamentals - transmission line loss

T4A05 Explanation 5

Key C — The antenna system is matched and no power is reflected

SWR measures mismatch only. A 1:1 reading says nothing about feed line loss, radiated power or grounding - a dummy load gives a perfect 1:1 while radiating almost nothing.

Rule Fundamentals - standing wave ratio

T4A06 Explanation 6

Key B — About 11 per cent

Reflected power is the square of the reflection coefficient, and for 2:1 that coefficient is 1/3, giving about 11 per cent. The 4 per cent figure belongs to 1.5:1 and 25 per cent to 3:1.

Rule Fundamentals - reflection coefficient

T4A07 Explanation 7

Key A — Many transmitters fold back output to protect the final stage, and reflected power is lost in the feed line

Two mechanisms work together: protective power fold-back in the transmitter, and the double loss suffered by reflected power travelling back and forth along a lossy line.

Rule 47 CFR §97.307 - spurious emission and transmitter performance

T4A08 Explanation 8

Key D — About 0.66 of it, because of the cable's velocity factor

Signals travel more slowly in the dielectric, so physical lengths inside coax are shorter by the velocity factor - about 0.66 for solid polyethylene and about 0.8 for foam dielectric.

Rule Fundamentals - velocity factor

T4A09 Explanation 9

Key C — To couple a balanced antenna to unbalanced coaxial feed line

A dipole is a balanced load and coax is an unbalanced line; without a balun, current flows on the outside of the coax shield, causing pattern distortion and RF in the shack. A balun can also transform impedance, but its defining job is the balanced-to-unbalanced transition.

Rule 47 CFR §97.13(c) - RF exposure and station practice

T4A10 Explanation 10

Key C — About 1.6 feet (49 cm)

234 / 146 = 1.6 feet, about 49 cm - half the dipole figure, as expected. The vertical needs a ground plane or radials to take the place of the missing half of the dipole.

Rule Fundamentals - quarter-wave vertical

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.