Correct answer: 150 metres

Wavelength is related to frequency by:

\[ \lambda(\mathrm{m}) \approx \frac{300}{f(\mathrm{MHz})} \]

A frequency of 1000 kHz is:

\[ f = 1\ \mathrm{MHz} \]

So the wavelength is:

\[ \lambda = \frac{300}{1} = 300\ \mathrm{m} \]

A half-wave antenna has a physical length of approximately:

\[ \frac{\lambda}{2} = \frac{300}{2} = 150\ \mathrm{m} \]

In practice, the actual antenna may be slightly shorter due to end effects and velocity factor, but 150 metres is the correct approximate value.

• 300 metres corresponds to a full wavelength, not a half wavelength.
• 600 metres would be two wavelengths.
• 30 metres corresponds to a much higher frequency band.

Therefore, the approximate physical length of a half-wave antenna at 1000 kHz is 150 metres.

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Correct answer: D — 12 metres

Wavelength and frequency are inversely related: as frequency increases, wavelength decreases. The standard formula uses the speed of light (approximately 300,000,000 m/s or 3 × 10⁸ m/s).

\[ \lambda = \frac{c}{f} \]

Given:

• c = 300,000,000 m/s
• f = 25,000,000 Hz (25 MHz)

\[ \lambda = \frac{300{,}000{,}000}{25{,}000{,}000} = 12\ \mathrm{metres} \]

A quick mental shortcut: divide 300 by the frequency in MHz to get the wavelength in metres. 300 ÷ 25 = 12 m.

• A. 15 metres — this corresponds to 20 MHz (300 ÷ 20 = 15), not 25 MHz.
• B. 32 metres — this corresponds to approximately 9.4 MHz, not 25 MHz.
• C. 4 metres — this corresponds to 75 MHz (300 ÷ 75 = 4), not 25 MHz.

Therefore, the wavelength of a 25 MHz signal is 12 metres.

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Correct answer: C — perpendicular to each other

In any electromagnetic wave, the electric field (E-field) and the magnetic field (H-field) are always oriented at right angles (90°) to each other, and both are perpendicular to the direction of wave propagation. This relationship is a fundamental property of electromagnetic radiation and holds true regardless of antenna type, frequency, or time of day. Around a transmitting antenna, the near-field and far-field both exhibit this orthogonal relationship between the two field components.

• A. parallel to each other — Incorrect; parallel E and H fields would not form a propagating electromagnetic wave. The fields must be orthogonal to carry energy outward.
• B. determined by the type of antenna used — Incorrect; the perpendicular relationship between E and H fields is a universal property of electromagnetic waves, not dependent on antenna design.
• D. variable with the time of day — Incorrect; the geometric relationship between E and H fields is constant and does not change with time of day or any environmental condition.

Therefore, the electric and magnetic fields around an antenna are always perpendicular to each other, as required by the fundamental nature of electromagnetic wave propagation.

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Correct answer: B — electric field

Radio wave polarisation is defined by the orientation of the electric field component of the electromagnetic wave. For example, a vertical antenna produces a vertically polarised wave because its electric field oscillates in the vertical plane. The magnetic field is always perpendicular to the electric field, and it is the electric field's orientation that is used as the reference by convention.

• A. magnetic field — The magnetic field is perpendicular to the electric field and travels with the wave, but polarisation is not defined by it.
• C. inductive field — An inductive (near) field exists close to an antenna but is not a propagating field component and has no role in defining polarisation.
• D. capacitive field — Similarly, a capacitive (near) field exists in the reactive near-field region around an antenna but does not define polarisation.

Therefore, radio wave polarisation is always defined by the orientation of the electric field component of the radiated electromagnetic wave.

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Correct answer: maximum current

A half-wave dipole antenna has a standing wave of current and voltage along its length. At the centre of the dipole:

• the current is maximum
• the voltage is minimum

This point provides a convenient feed impedance (typically around \(50\text{–}75\ \Omega\)), which matches common transmission lines and allows efficient power transfer.

At the ends of the dipole, the situation is reversed, voltage is maximum and current is near zero, which is why end-feeding is uncommon for a simple half-wave dipole.

• maximum voltage occurs at the ends of the antenna, not at the feedpoint.
• maximum resistance is not a correct description of the normal feedpoint impedance.
• resonance describes the operating condition of the antenna, not the physical feed location.

Therefore, a half-wave dipole antenna is normally fed at the point of maximum current.

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Correct answer: height of the antenna

The radiation angle of a horizontal half-wave dipole is strongly influenced by its height above ground (in wavelengths).

• Low height (e.g. < 0.5 λ):

  • produces high-angle radiation (good for NVIS / shorter distances)

• Greater height:

  • produces lower-angle radiation (better for long-distance DX)

Therefore, if high-angle radiation is desired, the key factor is the height of the antenna.

• Wire size has little effect on radiation angle.
• Time of year affects propagation, not antenna pattern.
• Mode of propagation does not control the antenna’s radiation angle.

Therefore, the important factor is the height of the antenna.

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Correct answer: B — quarterwave grounded vertical

A quarter-wave grounded vertical antenna radiates in all compass directions (azimuth) equally, making it omnidirectional in the horizontal plane. It works by using the ground (or a ground plane of radials) as an electrical mirror, producing a radiation pattern equivalent to a half-wave dipole stood on end. Because the antenna is vertical and symmetrical around its axis, signal strength is the same in every compass bearing.

• A. dipole with a reflector only — Adding a reflector behind a dipole creates a directional pattern, concentrating radiation in the forward direction and reducing it to the rear. This is not omnidirectional.
• C. dipole with director only — A director placed in front of a dipole similarly produces a directional (forward-focused) pattern, not equal radiation in all directions.
• D. half-wave horizontal dipole — A horizontal dipole has a figure-eight (bidirectional) radiation pattern broadside to the element, with nulls off the ends. It is not omnidirectional in the horizontal plane.

Therefore, the quarter-wave grounded vertical is the correct choice for an antenna that transmits equally well in all compass directions.

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Correct answer: vertically polarised wave

A groundplane antenna is essentially a vertical radiator mounted above a set of radial conductors that act as a reflective ground.

The electric field produced by a vertical radiator is oriented perpendicular to the earth’s surface, resulting in vertical polarisation of the transmitted radio wave.

• A horizontally polarised wave would require a horizontal radiator.
• Elliptical polarisation requires two orthogonal fields with a phase difference.
• Axial polarisation is not a standard description for this type of antenna.

Therefore, a groundplane antenna emits a vertically polarised wave.

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Correct answer: 300 ohm

A folded dipole consists of two parallel conductors connected at both ends. Compared to a simple half-wave dipole (which has a feedpoint impedance of about \(75\ \Omega\)), the folded dipole increases the impedance by approximately the square of the number of conductors.

For a two-wire folded dipole:

\[ Z_{\text{in}} \approx 4 \times 75\ \Omega = 300\ \Omega \]

• 150 \(\Omega\) is too low for a folded dipole.
• 200 \(\Omega\) is not a typical feedpoint impedance.
• 100 \(\Omega\) is closer to some other antenna types.

Therefore, the feedpoint impedance of a folded dipole is approximately 300 ohm.

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The feedpoint impedance of a half-wave dipole, installed about one wavelength or higher above ground (i.e. in "free space"), is 72 ohm. When the ends are lowered (i.e. into an "inverted V"), the impedance drops to around 50 ohms. The ends of the antenna should be insulated as they are high-voltage low-current points. The connections of the feedline to the antenna should be soldered because the centre of the dipole is a high-current low-voltage point.

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Correct answer: decrease the resonant frequency

An antenna’s resonant frequency depends on its effective electrical length.

Adding a series inductance introduces additional inductive reactance, which electrically lengthens the antenna without increasing its physical length.

This lowers the frequency at which the antenna resonates.

• Increasing the resonant frequency would require reducing the electrical length.
• There is a definite effect on resonance.
• The effect is not negligible.

Therefore, adding series inductance decreases the resonant frequency.

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Correct answer: D — match unbalanced and balanced transmission lines

A balun (contraction of balanced–unbalanced) is a device that interfaces an unbalanced transmission line (such as coaxial cable, where the outer shield is at ground potential) with a balanced load (such as a dipole antenna, where neither conductor is at ground). Without a balun, RF current can flow back down the outside of the coax braid, causing pattern distortion, increased interference, and feed-line radiation.

Baluns may be purely a current/choke type (suppressing common-mode currents) or a transformer type (also providing impedance transformation), but in either case their defining role is the balanced-to-unbalanced transition.

• A – balance harmonic radiation: Harmonic suppression is the job of low-pass filters or trap circuits, not a balun.
• B – reduce unbalanced standing waves: Standing wave ratio is determined by the impedance match between the transmitter, feed line, and antenna — not by the presence of a balun. A balun does not inherently correct an SWR mismatch.
• C – protect the antenna system from lightning strikes: Lightning protection is provided by gas-discharge arrestors, spark gaps, or grounding straps — not by a balun.

Therefore, the purpose of a balun in a transmitting antenna system is to match unbalanced and balanced transmission lines, ensuring correct current distribution on the antenna and preventing feed-line radiation.

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Correct answer: D — duplicates the characteristics of an antenna without radiating signals

A dummy antenna (also called a dummy load) is a non-radiating resistive load that presents the same impedance to a transmitter as a real antenna would — typically 50 Ω. It allows a transmitter to be tested, tuned, or adjusted at full power without actually transmitting a signal over the air. The RF energy is safely dissipated as heat in the resistive element rather than being radiated.

• A is incorrect — a dummy antenna is not used to attenuate a signal generator; it is a load for a transmitter, not an attenuator in a signal path.
• B is incorrect — selectivity relates to a receiver's ability to separate signals by frequency; a dummy antenna plays no role in improving selectivity.
• C is incorrect — a dummy antenna operates at RF impedances (e.g. 50 Ω) and has nothing to do with matching an audio frequency (AF) generator to a receiver.

Therefore, a dummy antenna duplicates the electrical characteristics of a real antenna — providing the correct impedance load — while preventing any signal from being radiated during transmitter testing or adjustment.

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Correct answer: A — 20 metres

A half-wave dipole has a physical length of approximately half the wavelength of the operating frequency. The full wavelength at any frequency is found from the wave equation, and the half-wave length is then halved again (with a small shortening factor of about 5% applied in practice, but the approximation is sufficient here).

\[ \lambda = \frac{c}{f} \]

\[ \lambda = \frac{300{,}000\ \mathrm{km/s}}{7{,}100\ \mathrm{kHz}} = \frac{300\ \mathrm{m\cdot MHz}}{7.1\ \mathrm{MHz}} \approx 42.3\ \mathrm{m} \]

Half of that:

\[ \frac{42.3}{2} \approx 21.1\ \mathrm{m} \]

After applying the standard ~5% practical shortening factor, this comes to approximately 20 metres, consistent with operation on the 40-metre amateur band.

• B. 40 metres — This is the full wavelength at 7100 kHz, not the half-wave length.
• C. 80 metres — This is the full wavelength at roughly 3.5 MHz (the 80-metre band), not at 7100 kHz.
• D. 160 metres — This corresponds to the full wavelength near 1.8 MHz (the 160-metre band), far off the mark.

Therefore, a half-wave antenna resonant at 7100 kHz is approximately 20 metres long.

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Correct answer: 3600 kHz

The antenna has 20 metres of wire on each side, so the total length is:

\[ L = 20 + 20 = 40\ \mathrm{m} \]

A simple half-wave dipole resonates when its total length is approximately half a wavelength:

\[ L \approx \frac{\lambda}{2} \]

So the wavelength is:

\[ \lambda \approx 2L = 80\ \mathrm{m} \]

Frequency and wavelength are related by:

\[ f(\mathrm{MHz}) \approx \frac{300}{\lambda(\mathrm{m})} \]

Substituting:

\[ f \approx \frac{300}{80} = 3.75\ \mathrm{MHz} \approx 3750\ \mathrm{kHz} \]

In practice, real antennas resonate slightly lower due to end effects and conductor diameter, so a value close to 3600 kHz is the best match.

• 3900 kHz is higher than the expected resonant frequency for an 80 m half-wave antenna.
• 7050 kHz corresponds to the 40 m band and would require an antenna roughly half this length.
• 7200 kHz is even higher in frequency and further from the expected resonance.

Therefore, the antenna will be resonant at approximately 3600 kHz.

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Correct answer: 15 metre

A half wave antenna cut for 7 MHz has a fixed physical length equal to \(\lambda/2\) at that frequency. The same antenna will also be resonant at odd multiples of half wavelengths.

At 21 MHz (the 15 metre band), the wavelength is one third of the 7 MHz wavelength:

\[ \lambda_{21} = \frac{\lambda_{7}}{3} \]

The antenna length remains \(\lambda_{7}/2\), which becomes:

\[ \frac{\lambda_{7}}{2} = \frac{3\lambda_{21}}{2} = \frac{3\lambda}{2} \]

A \(3\lambda/2\) dipole is a resonant length, so the antenna can be used on the 15 metre band without changing its physical length.

• 20 metre band (14 MHz) makes the antenna \(1\lambda\) (a full wavelength), which has a very high feedpoint impedance and normally cannot be used directly without matching changes.
• 10 metre band (28 MHz) makes the antenna \(2\lambda\), also poorly matched for a simple centre feed.
• 80 metre band (3.5 MHz) would require an antenna roughly twice as long.

Therefore, the same antenna length can be used without change on the 15 metre band.

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Correct answer: bandwidth

An antenna’s bandwidth is the range of frequencies over which it operates effectively, typically defined as the range where its performance (such as SWR or radiation efficiency) remains within acceptable limits.

This determines how wide a frequency range the antenna can cover without significant mismatch or loss.

• Front-to-back ratio describes directional performance.
• Impedance affects matching to the feedline, not the usable frequency range itself.
• Polarisation describes the orientation of the radiated electric field.

Therefore, the property that broadly defines the effective frequency range of an antenna is its bandwidth.

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Correct answer: A — shortening the radiating element

An antenna resonates when its physical length corresponds to a specific fraction of the operating wavelength (typically a half-wave or quarter-wave). Because wavelength and frequency are inversely related, a shorter antenna resonates at a higher frequency — just as a shorter guitar string produces a higher pitch.

\[ f = \frac{c}{\lambda} \]

where c is the speed of light (approximately 300,000,000 m/s) and λ is the wavelength. Reducing the physical length reduces the resonant wavelength, which raises the resonant frequency.

• B. Lengthening the radiating element — a longer element increases the resonant wavelength, which lowers the resonant frequency, the opposite effect.
• C. Increasing the height of the radiating element — moving the antenna higher changes its environment and ground interaction but does not directly change the resonant frequency of the element itself.
• D. Lowering the radiating element — similarly, reducing height affects the antenna's surroundings but does not shorten the element and therefore does not raise its resonant frequency.

Therefore, shortening the radiating element is the correct way to increase an antenna's resonant frequency.

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Correct answer: limit the electrical length of the antenna

End insulators are used to separate the active radiating portion of the antenna from its supporting ropes or structures.

Without an insulator, the supporting material (especially if damp or conductive) could become part of the antenna, effectively increasing its electrical length and altering its resonant frequency.

The insulator ensures that only the intended wire length contributes to radiation.

• Increasing the effective antenna length is not the purpose; it prevents unintended extension.
• Appearance is not an electrical function.
• Some losses still occur in any antenna system; insulators do not prevent all radiation loss.

Therefore, end insulators are used to limit the electrical length of the antenna.

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Correct answer: A — lengthen the antenna

A resonant antenna's natural frequency is determined by its physical length. A half-wave dipole (or any resonant antenna) resonates when its length corresponds to a specific fraction of the wavelength of the operating frequency. Because wavelength and frequency are inversely related, making the antenna longer increases the wavelength it naturally resonates at, which means the resonant frequency decreases.

\[ f = \frac{c}{\lambda} \]

Where:

• f = frequency (Hz)
• c = speed of light (≈ 300,000,000 m/s)
• λ = wavelength (m)

A longer antenna corresponds to a longer wavelength, and by the formula above, a longer wavelength means a lower resonant frequency.

• B. Centre feed the antenna with TV ribbon — The feed method and feedline type affect impedance matching and SWR, but do not change the antenna's resonant frequency.
• C. Shorten the antenna — Shortening raises the resonant frequency (shorter wavelength), which is the opposite of the desired effect.
• D. Ground one end — Grounding one end changes the antenna's radiation pattern and may affect impedance, but it does not directly lower the resonant frequency in the same predictable way as changing physical length.

Therefore, to lower an antenna's resonant frequency, the operator should lengthen the antenna so it resonates at a longer wavelength and correspondingly lower frequency.

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Correct answer: dipole

A half-wave antenna consists of two equal conductive elements, each approximately:

\[ \frac{\lambda}{4} \]

forming a total length of:

\[ \frac{\lambda}{2} \]

This type of antenna is called a dipole.

• A Yagi is a directional antenna made from multiple elements.
• A beam is a general term for directional antennas.
• “Bi-polar” is not the correct term.

Therefore, a half-wave antenna is called a dipole.

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Correct answer: its length

The resonant frequency of a dipole antenna is primarily determined by its physical length.

A half-wave dipole has a length approximately:

\[ L \approx \frac{\lambda}{2} \]

Since:

\[ f \approx \frac{300}{\lambda} \]

the antenna length directly sets the frequency at which it resonates.

• Height above ground affects radiation pattern and impedance, not primary resonance.
• Transmitter power does not affect resonance.
• Feedline length does not determine antenna resonance.

Therefore, the resonant frequency is mainly determined by its length.

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Correct answer: B — quarter wave vertical

A quarter wave vertical is the classic mobile HF antenna. At 28 MHz, a quarter wavelength is approximately 2.5 metres — a practical length for roof mounting. The car's metal roof acts as a ground plane, completing the antenna's electrical structure (effectively mirroring the antenna to create the equivalent of a half-wave dipole). This gives a low radiation angle and omnidirectional coverage, ideal for mobile operation.

\[ \lambda = \frac{300}{f(\text{MHz})} \]

\[ \frac{\lambda}{4} = \frac{300}{28 \times 4} \approx 2.68\ \mathrm{m} \]

• A. Vertical long wire — a long wire antenna requires a substantial length (many wavelengths) and an appropriate feedpoint/termination; it is not a practical roof-mount mobile antenna.
• C. Horizontal dipole — a horizontal dipole requires supports at both ends well above the car body; mounting it flat on a roof would be impractical and would suffer severe detuning from the nearby metal surface.
• D. Full wave centre fed horizontal — at 28 MHz a full wave is about 10.7 metres; stringing a horizontal wire of that length on a car roof is entirely impractical for mobile use.

Therefore, a quarter wave vertical with the car roof as ground plane is the correct and standard choice for a 28 MHz mobile antenna.

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Correct answer: quarter wave ground plane

A vertical antenna that uses a flat conductive surface (real or simulated) at its base relies on that surface as a ground plane.

This configuration:

• forms the “missing half” of a dipole
• provides a return path for currents
• improves radiation efficiency

The antenna itself is typically:

\[ \frac{\lambda}{4} \]

in length.

• A vertical dipole does not require a ground plane.
• Rhombic and long-wire antennas are different designs.

Therefore, the antenna is a quarter wave ground plane.

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Correct answer: C — receives signals from all points around it equally well

A vertical antenna is omnidirectional in the horizontal plane — it radiates and receives equally in all compass directions. This makes it particularly useful for mobile operation, local nets, and situations where the direction of the other station is unknown or variable. The radiation pattern forms a doughnut shape around the antenna, with maximum signal broadside (horizontally) and a null directly off the tip.

• A. requires few insulators — While a vertical may use fewer insulators than some wire antenna designs, this is not its defining characteristic. A quarter-wave vertical still requires at least base insulation and a good ground or radial system.
• B. is very sensitive to signals coming from horizontal aerials — Antenna polarisation affects cross-polarisation loss, but this is not the main characteristic of a vertical. A vertical is most sensitive to vertically polarised signals, not horizontally polarised ones.
• D. is easy to feed with TV ribbon feeder — TV ribbon (300 Ω balanced twin feeder) is not the typical feed for a vertical antenna. Verticals are most commonly fed with coaxial cable (unbalanced), and impedance matching is still required.

Therefore, the defining characteristic of a vertical antenna is its omnidirectional radiation pattern in the horizontal plane, receiving signals equally well from all directions around it.

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Correct answer: B — voltage is high and current is low

A half-wave dipole is a resonant antenna exactly half a wavelength long. At resonance, standing waves form along the element. The current distribution follows a sine-wave pattern, reaching a maximum (antinode) at the centre of the dipole and falling to zero at the open ends. Voltage behaves oppositely — it is at a minimum (node) at the feed point centre and rises to a maximum at the tips. This is the same behaviour seen at the open end of any resonant transmission line: high voltage, low current.

The relationship is complementary: wherever current is at a peak, voltage is at a null, and wherever voltage is at a peak, current is at a null.

• A (voltage and current both high): Incorrect — current must be zero at the free ends of the conductor; there is nowhere for it to flow.
• C (voltage and current both low): Incorrect — while current is indeed low (zero) at the ends, voltage is at its peak there, not low.
• D (voltage low and current high): Incorrect — this describes the centre feed point of the dipole, not the ends.

Therefore, at the ends of a half-wave dipole the voltage is at its maximum and the current is at its minimum (effectively zero).

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Correct answer: cubical quad

The cubical quad is a commonly used directional antenna on HF bands.

It consists of one or more wire loops supported by a frame and is practical to construct at HF wavelengths.

• Parabolic dishes are generally used at microwave frequencies.
• A 13-element Yagi would be physically very large at HF and is more typical at VHF/UHF.
• A helical Yagi is not a common HF antenna type.

Therefore, an antenna commonly used on HF is the cubical quad.

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Correct answer: C — it concentrates the radiation in one direction

A Yagi antenna achieves gain over a dipole not by generating extra power, but by focusing the available transmit power into a narrower beam in one direction. This directivity is produced by the interaction between the driven element and the parasitic elements — the reflector (slightly longer, placed behind the driven element) and one or more directors (slightly shorter, placed in front). Energy that would otherwise radiate in all directions is instead reinforced in the forward direction and suppressed elsewhere, producing a net gain compared to a dipole radiating equally to the front and rear.

• A — it radiates more power than a dipole: Incorrect. A Yagi cannot radiate more power than is fed into it; it is a passive antenna and obeys conservation of energy. Gain comes from redistribution, not amplification.
• B — more powerful transmitters can use it: Incorrect. Transmitter power is independent of antenna type; any antenna can be connected to a powerful transmitter. This has nothing to do with antenna gain.
• D — it can be used for more than one band: Incorrect. Yagis are inherently narrow-band antennas, typically cut for a single frequency band. Multi-band operation is not a defining characteristic and does not explain gain.

Therefore, a Yagi's power gain over a dipole results from concentrating radiated energy in one direction rather than creating additional power.

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Correct answer: in the direction of the director end of the boom

In a Yagi antenna:

• the reflector is placed behind the driven element
• the director(s) are placed in front

The reflector pushes energy forward, while the directors focus and guide it in that same direction.

This results in:

• maximum radiation toward the director end

• reduced radiation toward the reflector (good front-to-back ratio)

• It is not strongest toward the reflector.

• Radiation is not at right angles to the boom.

• The feeder direction is unrelated.

Therefore, maximum radiation is in the direction of the director end of the boom.

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Correct answer: parasitic elements

In a Yagi-Uda antenna, only one element (the driven element) is directly connected to the feedline.

The other elements:

• the reflector
• the director(s)

are not electrically connected to the transmitter or receiver. Instead, they are excited by the electromagnetic field radiated by the driven element.

These elements re-radiate energy due to induced currents, and their lengths are chosen to:

• reflect energy toward the front (reflector)
• focus energy in the forward direction (directors)

This improves:

• forward gain
• directivity
• front-to-back ratio

Because they are not directly driven but interact with the radiated field, they are known as parasitic elements.

• Oscillators generate signals.
• Tuning stubs are used for impedance matching.
• Matching units adjust feedline-to-antenna impedance.

Therefore, the reflector and directors in a Yagi antenna are called parasitic elements.

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Correct answer: hypothetical point source

An isotropic antenna is a theoretical reference antenna that radiates equally in all directions in three-dimensional space.

It is considered a point source with no preferred direction of radiation, producing a perfectly spherical radiation pattern. This makes it useful as a standard reference when comparing antenna gain.

• A half-wave dipole has a directional radiation pattern and is not isotropic.
• An infinitely long wire does not radiate uniformly in all directions.
• A dummy load is designed not to radiate at all.

Therefore, an isotropic antenna is a hypothetical point source.

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Correct answer: most of the energy is radiated at a low angle

A 5/8 wavelength vertical antenna is commonly used on VHF because its radiation pattern concentrates more energy at low elevation angles compared with a quarter-wave antenna.

Low-angle radiation is desirable for VHF base and mobile operation because it:

• improves communication range along the Earth’s surface
• enhances coverage to distant stations rather than straight upward
• is well suited to line-of-sight and slightly beyond line-of-sight paths

Although a 5/8 λ antenna has a more complex feed impedance and usually requires matching, its radiation efficiency at low angles makes it very popular for VHF work.

• easy to match the antenna to the transmitter is incorrect, a 5/8 λ antenna typically requires a matching network.
• a convenient length on VHF is not the technical reason for its use.
• the angle of radiation is high is the opposite of the desired effect.

Therefore, many VHF base and mobile antennas are 5/8 wavelength long because most of the energy is radiated at a low angle.

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Correct answer: B — angle of radiation

When working stations at great distances (DX), the most critical antenna characteristic is its angle of radiation — the angle above the horizon at which the antenna launches most of its energy into the sky. For long-distance HF propagation, a low angle of radiation is needed so the signal reaches the ionosphere at a shallow angle, allowing it to refract back to earth at a distant point. High-angle radiation tends to return to earth much closer to the transmitter (or skip overhead entirely for very long paths).

• A. Sunspot activity — Sunspot activity affects ionospheric propagation conditions globally, but it is not a characteristic of the antenna itself; you cannot change sunspot activity by choosing a different antenna.
• C. Impedance — Impedance matching is important for efficient power transfer, but a well-matched antenna with a high radiation angle will still underperform on DX compared to a low-angle antenna.
• D. Bandwidth — Bandwidth determines the range of frequencies an antenna covers efficiently, but it has little bearing on how far a signal will travel once the antenna is resonant on the desired frequency.

Therefore, for working distant stations, selecting an antenna with a low angle of radiation is far more important than any other listed factor.

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Correct answer: the ionosphere can change the polarisation of the signal from moment to moment

On HF, most long-distance communication occurs via skywave propagation through the ionosphere.

As radio waves pass through the ionised layers, their polarisation can be altered due to:

• refraction
• multiple reflections
• changing ionospheric conditions

This means the received signal’s polarisation may vary continuously, regardless of the transmitting antenna’s original polarisation.

• Ground and sky waves do not continually shift polarisation in this manner.
• The earth’s magnetic field is not the primary cause.
• Receiver selectivity does not affect signal polarisation.

Therefore, on HF the ionosphere can change the signal’s polarisation from moment to moment.

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