G2 · Published 16 October 2026 · By The Ham Desk

Measuring, Modelling and Matching Real Antennas

A design on paper becomes an antenna when you measure it. This unit covers the instruments that tell you what you actually built, the common multiband designs and what they really present to the transmitter, and the ways a nearby structure changes everything.

Band 40 m
Study time 17
Level Core
questions 10

Waveform and antenna diagram for HF Antenna Systems — HF Antenna Systems — waveform overlay and radiation pattern

Pre-flight briefing

Instruments, and where you put them

An SWR meter samples forward and reflected power and displays their ratio, and where it is inserted changes what it tells you. Placed at the transmitter it reports what the transmitter sees, which is what matters for protecting the final amplifier; placed at the antenna it reports the antenna's actual match. The two readings differ whenever the feed line has loss, because the reflected wave is attenuated on its way back and the transmitter sees a better match than the antenna really has. A 3 dB cable loss can make a 3:1 antenna look like 1.5:1 at the radio.

An antenna analyser does more: it sweeps a range of frequencies and reports impedance, SWR, and the resonant frequency where reactance crosses zero. That is how you trim an element to length - cut for the resonant point, not for the lowest SWR reading, because a low SWR at the wrong frequency usually means loss rather than resonance.

A field strength meter confirms that power is actually being radiated, and an SWR analyser plus a dummy load together let you separate transmitter problems from antenna problems.

Multiband designs and their real impedances

A folded dipole is two parallel elements joined at the ends, and the folding raises the feed point impedance to roughly 300 ohms - four times that of a simple dipole - which is why a 4:1 balun is normally used with coax. It is wider in bandwidth than a single-wire dipole of the same length, which makes it useful on 40 and 80 metres where bandwidth matters.

An off-centre-fed dipole is fed at roughly one third of its length rather than in the middle, which presents around 200 to 300 ohms and, with a suitable transformer, works on several harmonically related bands. The trade is that the pattern is no longer symmetrical and the feed point may need a choke to keep RF off the coax shield.

A terminated folded dipole is deliberately lossy: resistive loading absorbs power at the low end of each band, which produces very wide, very smooth coverage at the cost of several dB of efficiency. It is the antenna of choice when a wideband, low-interference receive antenna matters more than radiated power.

What the surroundings do

An antenna does not exist in free space. A metal mast, tower, guy wires, gutters and even wet foliage all couple to the radiating element and change both its resonant frequency and its radiation pattern; a dipole mounted alongside a metal mast behaves like a different antenna entirely, often with a distorted pattern and a shifted resonance. That is why antenna modelling software asks for the height above ground and for the presence of nearby conductors.

Modelling programs solve the antenna's currents numerically and produce an impedance and a pattern before anything is built, which is how a design is tuned for a specific height. The results are only as good as the model: an antenna modelled in free space will not match one installed at 10 metres over average soil.

Finally, antennas are reciprocal: the pattern used to transmit is identical to the pattern used to receive. That means every gain and directivity statement applies both ways, and improving the antenna improves both your signal and your ability to hear the other station.

Terms you must be able to define
Term	What the examiner wants to hear
Antenna analyser	An instrument that sweeps frequency and reports impedance, SWR and resonance.
Folded dipole	A two-wire dipole whose folding raises the feed point impedance to about 300 ohms.
Off-centre-fed dipole	A multiband dipole fed about one third along its length, presenting 200 to 300 ohms.
Terminated folded dipole	A deliberately lossy broadband antenna traded for bandwidth.
Reciprocity	The principle that an antenna's transmit and receive patterns are identical.

Question deck 10 questions

G4B01 Core

Why can an SWR meter at the transmitter read better than the antenna's actual match?

G4B02 Core

What does an antenna analyser measure that an SWR meter does not?

G4B03 Core

What is the approximate feed point impedance of a folded dipole?

G4B04 Core

What is the principal advantage of a folded dipole over a simple dipole?

G4B05 Core

How is an off-centre-fed dipole constructed?

G4B06 Stretch

What is the trade-off in using a terminated folded dipole?

G4B07 Core

What effect does a metal mast alongside a dipole have?

G4B08 Core

What does the principle of reciprocity tell you about an antenna?

G4B09 Stretch

Why should an antenna be trimmed for resonance rather than for the lowest SWR reading?

G4B10 Stretch

Why can a field strength meter be useful after installing an antenna?

Reference sheet

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

G4B01 Explanation 1

Key C — Feed line loss attenuates the reflected wave on its way back, so the transmitter sees a better match than the antenna has

Reflected power makes two trips through the cable, so a lossy line absorbs some of it and the ratio measured at the shack improves. This is why an SWR reading at the transmitter is not proof that the antenna is well matched - at 2 metres with thin coax, the difference can be large.

Rule Fundamentals - SWR measurement

G4B02 Explanation 2

Key B — Impedance and the resonant frequency across a range of frequencies

An analyser sweeps and reports complex impedance, so you can see where reactance crosses zero - the true resonance - rather than only where SWR dips. Trimming an element to resonance, rather than to the lowest SWR, is how an antenna is adjusted correctly.

Rule Fundamentals - antenna measurement

G4B03 Explanation 3

Key A — About 300 ohms

Folding the element raises the impedance by roughly a factor of four over the 73 ohms of a simple dipole, giving about 300 ohms - which is why a 4:1 balun is the standard way to feed one from 50 ohm coax.

Rule Fundamentals - folded dipole

G4B04 Explanation 4

Key D — Wider bandwidth at the same resonant frequency

The folded construction broadens the impedance-versus-frequency curve, so the antenna stays matched over more of the band without a tuner. Gain is essentially the same as a simple dipole - folding changes impedance, not directivity.

Rule Fundamentals - folded dipole bandwidth

G4B05 Explanation 5

Key C — It is fed at about one third of its length, presenting 200 to 300 ohms and working on several harmonically related bands

Moving the feed point away from the centre raises the impedance at the fundamental and at each harmonic to a range a transformer can handle, which is what makes one wire work on 80, 40, 20 and 10 metres. The cost is an asymmetric pattern and a need to choke the feed line.

Rule Fundamentals - multiband dipoles

G4B06 Explanation 6

Key B — Very wide, smooth coverage in exchange for several dB of lost efficiency

Resistive loading absorbs power deliberately, which flattens the response across a very wide frequency range. The antenna is a good choice for receiving and for wideband coverage, and a poor choice when radiated power matters most.

Rule Fundamentals - terminated antennas

G4B07 Explanation 7

Key A — It changes the resonant frequency and distorts the radiation pattern

A nearby conductor couples to the radiating element and re-radiates, which shifts resonance and fills in nulls in the pattern. This is why the same antenna design performs differently on different masts, and why modelling software asks about nearby metalwork.

Rule Fundamentals - antenna surroundings

G4B08 Explanation 8

Key D — The radiation pattern used for transmitting is the same as the pattern used for receiving

Reciprocity means every statement about an antenna's gain, directivity or pattern applies to both directions of the link, so one good antenna improves both what you send and what you hear. It also means a pattern measured on receive is a valid description of the transmit pattern.

Rule Fundamentals - reciprocity

G4B09 Explanation 9

Key C — A low SWR at the wrong frequency often indicates loss rather than resonance

Resistive loss makes an antenna look better matched than it is, so a broad, shallow SWR curve can mean a lossy installation rather than a good one. Adjusting to the frequency where reactance crosses zero puts the antenna where it actually radiates efficiently.

Rule Fundamentals - resonance versus match

G4B10 Explanation 10

Key C — It confirms that power is actually being radiated rather than absorbed in a lossy feed line or matching network

A low SWR combined with no radiated field is the classic signature of a lossy system - a wet coax connector, a resistive balun, or a termination absorbing the power. A field strength meter resolves the ambiguity that an SWR reading alone cannot.

Rule Fundamentals - verifying radiation

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.