E2 · Published 28 October 2026 · By The Ham Desk

Designing and Modelling Directional Antennas

The last antenna unit is about making a pattern do what you want. It covers what actually controls a Yagi's gain, how ground and neighbouring conductors change the numbers you designed for, and the modelling and measurement ideas that let you predict performance before you climb the tower.

Band 40 m
Study time 19
Level Stretch
questions 10

Waveform and antenna diagram for Antenna Design and Transmission Lines — Antenna Design and Transmission Lines — waveform overlay and radiation pattern

Pre-flight briefing

What controls the pattern

For a Yagi the single most important dimension is boom length. Gain rises roughly with the length of the boom in wavelengths, and it does so whether that length is filled with many closely spaced elements or fewer widely spaced ones. Adding a fourth or fifth element to a short boom buys noticeably less than lengthening the same boom, because beyond a point the extra directors are working in the field of the ones already there rather than adding new aperture.

Element spacing is the compromise that follows. Widening the spacing between directors lowers the feedpoint impedance, narrows the bandwidth, and eventually produces side lobes and a ragged pattern instead of more forward gain. Optimum designs put directors 0.15 to 0.25 wavelength apart and the reflector a little further back, with the driven element adjusted so that a reasonable impedance appears at the feed point.

The driven element is the element that gets the power; the others are parasitic, meaning they re-radiate what they pick up with a phase shift that steers energy forward. A log-periodic antenna gives up the sharp resonance of a Yagi in exchange for coverage of a wide frequency range, and it pays for that bandwidth with lower gain for the same boom length - a trade that is often worth making on a single antenna serving several bands.

The world is not free space

A vertical antenna's pattern and feedpoint impedance depend on the ground beneath it. Real ground reflects imperfectly, and the angle at which the reflected wave leaves determines whether the antenna favours low-angle radiation for long distance or high-angle radiation for regional work. The ground system also supplies the missing half of the antenna: with a poor radial field, loss resistance replaces radiation resistance and the antenna becomes a heater rather than a radiator.

Neighbouring conductors change the design as well. When two elements sit near each other, the current in one induces a voltage in the other, and the effect - mutual coupling - shifts both impedances. This is why a Yagi's elements must be tuned in place rather than on a test bench, and why a metal boom changes the resonant length of every element it passes through. The usual correction is to lengthen each element slightly to compensate for the boom's effect, with the amount depending on the boom diameter relative to the element diameter.

The same coupling explains why stacked antennas must be spaced at a sensible fraction of a wavelength and fed in phase through equal lengths of line. Two antennas too close together interact strongly, the pattern develops unwanted lobes, and the combined impedance may not match what the design predicted.

Modelling, measuring and receiving antennas

Antenna modelling software solves the currents on a wire structure and reports the pattern, gain and feedpoint impedance. Its value is that it lets a design be compared and adjusted before anything is built, and its limitation is that the answer is only as good as the model: an antenna modelled over perfect ground will not match one standing over a suburban lot, and a model that ignores the tower, the feed line or the roof will be optimistic.

The Smith chart remains the standard picture for matching work. It plots the complex impedance plane inside a circle, so that any combination of resistance and reactance becomes a point, and a length of transmission line becomes a simple rotation around the centre. That makes it easy to see how much line is needed to rotate an impedance to a place a matching network can reach.

Receiving antennas sometimes optimise the opposite of what transmitting antennas do. A terminated Beverage antenna is a long wire run close to the ground, pointed at the region of interest; it is inefficient on transmit but quiet on receive, with a smooth unidirectional pattern that favours signals arriving at low angles over locally generated noise. A small receiving loop has deep nulls broadside to its plane, so rotating it can place a null on a local noise source and bring a weak signal out of the interference.

Terms you must be able to define
Term	What the examiner wants to hear
Boom length	The dimension that most directly sets a Yagi's forward gain, measured in wavelengths.
Parasitic element	A Yagi element that is not fed and re-radiates with a phase shift that steers the pattern.
Mutual coupling	The interaction between nearby elements that shifts their impedances from the isolated values.
Boom correction	Lengthening Yagi elements to compensate for the effect of the metal boom passing through them.
Terminated Beverage	A long low receiving wire with a terminating resistor, valued for its quiet low-angle pattern.

Question deck 10 questions

E4B01 Core

Which dimension most directly determines a Yagi antenna's forward gain?

E4B02 Core

What happens when director spacing on a Yagi is made much wider than the optimum?

E4B03 Core

What is the principal trade-off of a log-periodic antenna compared with a Yagi of the same boom length?

E4B04 Stretch

Why must Yagi elements be adjusted after they are mounted on the boom?

E4B05 Stretch

What is boom correction in Yagi construction?

E4B06 Core

How does the ground beneath a vertical antenna affect its performance?

E4B07 Stretch

What is the Smith chart used for in antenna work?

E4B08 Stretch

What is the advantage of a terminated Beverage antenna on the low bands?

E4B09 Core

What is the practical use of the nulls in a small receiving loop's pattern?

E4B10 Stretch

Why can an antenna model over perfect ground be misleading?

Reference sheet

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

E4B01 Explanation 1

Key A — The boom length in wavelengths

Gain scales with the aperture the antenna presents, and for a Yagi that aperture is set by boom length. Adding elements helps only because it lets a longer boom be filled usefully; past the optimum spacing, extra elements add little.

Rule Fundamentals - Yagi design

E4B02 Explanation 2

Key D — Feedpoint impedance falls, bandwidth narrows and side lobes appear without a gain increase

Spacing beyond the optimum trades bandwidth and pattern quality for nothing. The design point is a compromise around 0.15 to 0.25 wavelength between directors, with the reflector slightly further back.

Rule Fundamentals - Yagi spacing

E4B03 Explanation 3

Key C — Much wider frequency coverage in exchange for lower gain

A log-periodic keeps its pattern and impedance roughly constant across a wide range by scaling successive elements, but it uses its boom less efficiently than a Yagi. It is the standard choice when one antenna must cover several bands without a tuner or traps.

Rule Fundamentals - log-periodic antennas

E4B04 Explanation 4

Key B — Mutual coupling between elements and the effect of the metal boom shift the resonant lengths from their isolated values

When conductors are close together each one's current induces a voltage in the others, and the boom itself is a conductor passing through every element. Both effects move the resonant frequency, so a design that is perfect on a test bench is off-frequency on the tower.

Rule Fundamentals - mutual coupling

E4B05 Explanation 5

Key A — Lengthening each element slightly to compensate for the metal boom passing through it

A metal boom adds capacitance and shortens the electrical length of each element mounted on it, raising the resonant frequency. The usual fix is to make each element a little longer, with the amount depending on the ratio of boom diameter to element diameter.

Rule Fundamentals - boom correction

E4B06 Explanation 6

Key D — It sets the take-off angle through reflection and, if the ground system is poor, adds loss resistance that reduces efficiency

Real ground reflects imperfectly, and the ground system supplies the image that makes a quarter-wave vertical work. With too few radials, loss resistance replaces radiation resistance and power goes into heating the soil rather than into the sky.

Rule Fundamentals - ground effects

E4B07 Explanation 7

Key C — Plotting complex impedance so that a length of line appears as a rotation and a matching solution can be read off

The chart maps the entire impedance plane inside a circle, so resistance and reactance become a single point. Moving along a lossless line rotates that point around the centre by an amount set by the electrical length, which makes matching a graphical rather than an algebraic exercise.

Rule Fundamentals - Smith chart

E4B08 Explanation 8

Key B — It has a quiet, smooth low-angle receiving pattern that favours distant signals over local noise

A long wire run close to the ground and terminated in its characteristic impedance is deliberately inefficient, and that inefficiency is the point: it responds to the low-angle wave from a distant station while picking up much less of the high-angle noise and static around the receiving site.

Rule Fundamentals - receiving antennas

E4B09 Explanation 9

Key A — Rotating the loop places a null on a local noise source to improve reception of a weak signal

A small loop has deep nulls broadside to its plane. Since most local interference arrives from a fixed direction, orienting the loop so that source falls in a null can lift a weak DX signal out of the noise, a technique that goes back to the earliest days of radio direction finding.

Rule Fundamentals - loop antennas

E4B10 Explanation 10

Key A — Real ground reflects imperfectly, and the model ignores nearby conductors such as the tower, roof or feed line

Modelling is valuable for comparing designs, but its output is only as good as its inputs. Real soil has finite conductivity and permittivity, and the structures around an installed antenna reradiate and absorb energy that the model knows nothing about.

Rule Fundamentals - antenna modelling

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.