3.5 Angle of Radiation, Antenna Height and Current Distribution

3.5 Angle of Radiation, Antenna Height and Current Distribution

ACMA Exam Focus: Syllabus item 6.8 requires you to recall that a low angle of vertical radiation is desirable for long distance communications. Syllabus item 6.10 requires you to recall the current and voltage distribution on the dipole and quarter-wave ground plane antennas, and the feedpoint impedances of half-wave dipoles, folded dipoles and quarter-wave ground planes. Both are near-certain question sources on the Standard theory paper.

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1. What "angle of radiation" actually means

The angle of radiation (also called the take-off angle or elevation angle) is the angle, measured upward from the horizon, at which an antenna radiates most strongly in the vertical plane. Zero degrees points along the horizon; ninety degrees points straight up at the zenith.

Every antenna has an elevation pattern as well as the horizontal (azimuth) pattern, and the take-off angle is the elevation of the strongest lobe in it. You cannot dial it in on the transceiver: it is fixed by the antenna's geometry, its polarisation and, above all, its height above ground measured in wavelengths.

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2. Why a low angle wins DX

A skywave signal leaves the antenna at some angle, travels up to the F2 layer at roughly 250-400 km, is refracted back down, and returns to earth some distance away. The shallower the launch angle, the longer and flatter that arc becomes, and the further the signal travels per hop.

The reason low angles matter is not simply that fewer hops sound tidier — every hop costs signal. Each ground reflection between hops throws away roughly 2-6 dB over average land (much less, around 0.5 dB, over salt water), and every pass through the D and E regions adds absorption.

Worked hop budget — Melbourne (VK3) to London, short path about 16,900 km:

• At a 3-degree take-off angle: $16{,}900 \div 4{,}000 \approx 4.2$, so about 5 hops, meaning 4 intermediate ground reflections. At 4 dB each that is 16 dB of reflection loss.
• At a 20-degree take-off angle: $16{,}900 \div 1{,}700 \approx 10$ hops, meaning 9 ground reflections, or 36 dB of reflection loss.

The difference is 20 dB — more than three S-points (an S-unit is nominally 6 dB) — before you count the extra ionospheric absorption on every additional pass. In practice the high-angle path simply does not make it. This is the whole content of syllabus 6.8.

The flip side: high angles are useful too

A near-vertical take-off angle is exactly what you want for NVIS (Near Vertical Incidence Skywave). The signal goes almost straight up on 80 m or 40 m, comes almost straight back down, and blankets a circle of a few hundred kilometres with no skip zone. For an emergency net covering inland New South Wales or a state-wide WICEN exercise, a low dipole is the correct antenna, not a compromise.

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3. Height above ground sets the angle

Ground under a horizontal antenna behaves like a mirror: the reflected signal recombines with the direct signal, reinforcing at some angles and cancelling at others. Because that reflection suffers a phase reversal of about 180 degrees, there is always a deep null along the horizon, and the first maximum appears at:

$\theta_1 \approx \arcsin\left(\frac{\lambda}{4h}\right)$

where $h$ is the height above ground and $\lambda$ the wavelength, both in the same units.

Two qualifications the examiner may probe. First, above about one wavelength a second, higher lobe appears; that is not wasted energy, it usefully fills in the mid-range distances. Second, ground conductivity matters: a good reflecting surface such as salt water or damp coastal soil produces a cleaner, lower lobe, while dry rocky or sandy ground absorbs part of the reflected wave and effectively raises the take-off angle.

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4. Worked example: one 15 m mast, three bands

A VK5 amateur has a single 15 m mast and swaps dipoles on it. Wavelength is $\lambda = 300 / f_{\text{MHz}}$ metres.

20 m band, 14.2 MHz: $\lambda = 300 / 14.2 = 21.1$ m, so the antenna is $15 / 21.1 = 0.71\lambda$ high.

$\sin\theta = \frac{21.1}{4 \times 15} = 0.352 \Rightarrow \theta \approx 21^\circ$

That is a genuine DX antenna — fine for VK to JA, VK to W6 and VK to Europe.

40 m band, 7.1 MHz: $\lambda = 42.3$ m, so the mast is only $0.35\lambda$ high.

$\sin\theta = \frac{42.3}{60} = 0.70 \Rightarrow \theta \approx 45^\circ$

Excellent for interstate contacts of 500-800 km; a poor performer toward Europe.

80 m band, 3.6 MHz: $\lambda = 83.3$ m, so the mast is $0.18\lambda$ high. Now $\lambda / 4h = 83.3 / 60 = 1.39$, which is greater than 1, so the formula has no solution — there is no low lobe at all. The antenna radiates essentially straight up: a first-class NVIS antenna for a state-wide net, and a poor DX antenna.

This is the everyday Australian reality. A full wavelength on 80 m is about 83 m of height, which no suburban block can support, and that is precisely why serious VK low-band DX operators turn to verticals over radial fields, or set up near salt water.

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5. Verticals: inherently low-angle radiators

A quarter-wave vertical is vertically polarised and, over a decent ground system, produces a main lobe typically between 15 and 25 degrees — and under 10 degrees over salt water. That makes it the natural choice for low-band DX where you cannot get a horizontal antenna high enough.

The catch is that a quarter-wave vertical is only half an antenna. The missing half is supplied by the ground or by a radial system acting as the electrical image. If that return path is lossy, the missing power is dissipated as $I^2R$ heat in the soil rather than radiated. Remedies:

• Ground-mounted: lay many radials — 16 as a minimum, 32 or more preferred. They need not be resonant; more is better.
• Elevated ground plane: as few as two to four resonant quarter-wave radials work well because they are clear of the lossy soil.
• Salt water: a near-perfect ground plane, which is why coastal and island operations sound so strong.

The trade-off is noise. A vertical hears low-angle DX well but also picks up more man-made electrical noise than a horizontal antenna, so many stations use a vertical to transmit and a quieter horizontal antenna to receive.

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6. Current and voltage distribution

A resonant antenna carries a standing wave. On a centre-fed half-wave dipole:

• Current is maximum at the centre and falls to a minimum (essentially zero) at each end.
• Voltage is maximum at each end and minimum at the centre.

On a quarter-wave ground plane, which is simply half a dipole working against its image in the radial system, the same pattern applies to the vertical element: current maximum at the feedpoint (base) falling to a minimum at the top, with voltage maximum at the top and minimum at the base.

Because impedance is the ratio of voltage to current, $Z = V / I$, the distribution immediately tells you the impedance at any point:

• Centre of a dipole: high current, low voltage, so a low impedance current feed.
• Ends of a dipole: high voltage, low current, so a very high impedance voltage feed.

Safety note: the ends of a dipole and the top of a vertical are the high-voltage points and can inflict an RF burn, so they must be kept out of reach.

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7. Feedpoint impedances to memorise

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8. Common exam traps

• The syllabus wording is low angle for long distance. High gain alone does not make a DX antenna if all that gain points at 45 degrees.
• Height is what matters, and it is measured in wavelengths, not metres. A 12 m high antenna is superb on 10 m and hopeless on 80 m.
• Current is maximum at the dipole centre — not voltage. Reversing these is the single most common error on this item.
• Keep the three numbers separate: 36 ohms ground plane, 73 ohms dipole, 300 ohms folded dipole.