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Subel. ZLH
40 questions
Subelement ZLH

From Transmitter to Receiver

Section ZLH27

Antennas

In this diagram the item U corresponds to the

  • Correct Answer
    boom
  • reflector
  • driven element
  • director
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In this diagram the item V corresponds to the

  • boom
  • Correct Answer
    reflector
  • driven element
  • director
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In this diagram the item X corresponds to the

  • boom
  • reflector
  • Correct Answer
    director
  • driven element
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The antenna in this diagram has two equal lengths of wire shown as 'X' forming a dipole between insulators. The optimum operating frequency will be when the

  • length X+X equals the signal wavelength
  • dimensions are changed with one leg doubled in length
  • Correct Answer
    length X+X is a little shorter than one-half of the signal wavelength
  • antenna has one end grounded

Correct answer: length X+X is a little shorter than one-half of the signal wavelength

A centre-fed dipole antenna is normally designed to be approximately half a wavelength long at its operating frequency. In practice, the physical length must be made slightly shorter than \(\lambda/2\) because of end effects and the velocity factor of the wire, which cause the antenna to appear electrically longer than its physical length.

This shortening is typically a few percent, depending on wire diameter and construction.

  • length X+X equals the signal wavelength would make the antenna a full-wave dipole, which has a very high feedpoint impedance and is not normally used in this configuration.
  • dimensions are changed with one leg doubled in length would unbalance the antenna and is not required for normal dipole operation.
  • antenna has one end grounded describes a different antenna type, not a centre-fed dipole.

Therefore, the optimum operating frequency occurs when the total length X + X is slightly shorter than one-half of the signal wavelength.

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The antenna in this diagram can be made to operate on several bands if the following item is installed at the points shown at 'X' in each wire

  • a capacitor
  • an inductor
  • a fuse
  • Correct Answer
    a parallel-tuned trap
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The physical length of the antenna shown in this diagram can be shortened and the electrical length maintained, if one of the following items is added at the points shown at 'X' in each wire

  • Correct Answer
    an inductor
  • a capacitor
  • an insulator
  • a resistor
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The approximate physical length of a half-wave antenna for a frequency of 1000 kHz is

  • 300 metres
  • 600 metres
  • Correct Answer
    150 metres
  • 30 metres

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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The wavelength for a frequency of 25 MHz is

  • 15 metres
  • 32 metres
  • 4 metres
  • Correct Answer
    12 metres
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Magnetic and electric fields about an antenna are

  • parallel to each other
  • determined by the type of antenna used
  • Correct Answer
    perpendicular to each other
  • variable with the time of day
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Radio wave polarisation is defined by the orientation of the radiated

  • magnetic field
  • Correct Answer
    electric field
  • inductive field
  • capacitive field
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A half wave dipole antenna is normally fed at the point of

  • maximum voltage
  • Correct Answer
    maximum current
  • maximum resistance
  • resonance

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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An important factor to consider when high angle radiation is desired from a horizontal half-wave antenna is the

  • size of the antenna wire
  • time of the year
  • Correct Answer
    height of the antenna
  • mode of propagation
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An antenna which transmits equally well in all compass directions is a

  • dipole with a reflector only
  • Correct Answer
    quarterwave grounded vertical
  • dipole with director only
  • half-wave horizontal dipole
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A groundplane antenna emits a

  • horizontally polarised wave
  • elliptically polarised wave
  • axially polarised wave
  • Correct Answer
    vertically polarised wave
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The impedance at the feed point of a folded dipole antenna is approximately

  • Correct Answer
    300 ohm
  • 150 ohm
  • 200 ohm
  • 100 ohm
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The centre impedance of a 'half-wave' dipole in 'free space' is approximately

  • 52 ohm
  • Correct Answer
    73 ohm
  • 100 ohm
  • 150 ohm

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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The effect of adding a series inductance to an antenna is to

  • increase the resonant frequency
  • have no change on the resonant frequency
  • have little effect
  • Correct Answer
    decrease the resonant frequency
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The purpose of a balun in a transmitting antenna system is to

  • balance harmonic radiation
  • reduce unbalanced standing waves
  • protect the antenna system from lightning strikes
  • Correct Answer
    match unbalanced and balanced transmission lines
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A dummy antenna

  • attenuates a signal generator to a desirable level
  • provides more selectivity when a transmitter is being tuned
  • matches an AF generator to the receiver
  • Correct Answer
    duplicates the characteristics of an antenna without radiating signals
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A half-wave antenna resonant at 7100 kHz is approximately this long

  • Correct Answer
    20 metres
  • 40 metres
  • 80 metres
  • 160 metres
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An antenna with 20 metres of wire each side of a centre insulator will be resonant at approximately

  • Correct Answer
    3600 kHz
  • 3900 kHz
  • 7050 kHz
  • 7200 kHz

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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A half wave antenna cut for 7 MHz can be used on this band without change

  • 10 metre
  • Correct Answer
    15 metre
  • 20 metre
  • 80 metre

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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This property of an antenna broadly defines the range of frequencies to which it will be effective

  • Correct Answer
    bandwidth
  • front-to-back ratio
  • impedance
  • polarisation
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The resonant frequency of an antenna may be increased by

  • Correct Answer
    shortening the radiating element
  • lengthening the radiating element
  • increasing the height of the radiating element
  • lowering the radiating element
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Insulators are used at the end of suspended antenna wires to

  • increase the effective antenna length
  • Correct Answer
    limit the electrical length of the antenna
  • make the antenna look more attractive
  • prevent any loss of radio waves by the antenna
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To lower the resonant frequency of an antenna, the operator should

  • Correct Answer
    lengthen the antenna
  • centre feed the antenna with TV ribbon
  • shorten the antenna
  • ground one end
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A half-wave antenna is often called a

  • bi-polar
  • Yagi
  • Correct Answer
    dipole
  • beam
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The resonant frequency of a dipole antenna is mainly determined by

  • its height above the ground
  • Correct Answer
    its length
  • the output power of the transmitter used
  • the length of the transmission line
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A transmitting antenna for 28 MHz for mounting on the roof of a car could be a

  • vertical long wire
  • Correct Answer
    quarter wave vertical
  • horizontal dipole
  • full wave centre fed horizontal
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A vertical antenna which uses a flat conductive surface at its base is the

  • vertical dipole
  • Correct Answer
    quarter wave ground plane
  • rhombic
  • long wire
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The main characteristic of a vertical antenna is that it

  • requires few insulators
  • is very sensitive to signals coming from horizontal aerials
  • Correct Answer
    receives signals from all points around it equally well
  • is easy to feed with TV ribbon feeder
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At the ends of a half-wave dipole the

  • voltage and current are both high
  • Correct Answer
    voltage is high and current is low
  • voltage and current are both low
  • voltage low and current is high
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An antenna type commonly used on HF is the

  • parabolic dish
  • Correct Answer
    cubical quad
  • 13-element Yagi
  • helical Yagi
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A Yagi antenna is said to have a power gain over a dipole antenna for the same frequency band because

  • it radiates more power than a dipole
  • more powerful transmitters can use it
  • Correct Answer
    it concentrates the radiation in one direction
  • it can be used for more than one band
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The maximum radiation from a three element Yagi antenna is

  • in the direction of the reflector end of the boom
  • Correct Answer
    in the direction of the director end of the boom
  • at right angles to the boom
  • parallel to the line of the coaxial feeder
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The reflector and director(s) in a Yagi antenna are called

  • oscillators
  • tuning stubs
  • Correct Answer
    parasitic elements
  • matching units
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An isotropic antenna is a

  • half wave reference dipole
  • infinitely long piece of wire
  • dummy load
  • Correct Answer
    hypothetical point source
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The main reason why many VHF base and mobile antennas in amateur use are 5/8 of a wavelength long is that

  • it is easy to match the antenna to the transmitter
  • it is a convenient length on VHF
  • the angle of radiation is high giving excellent local coverage
  • Correct Answer
    most of the energy is radiated at a low angle
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A more important consideration when selecting an antenna for working stations at great distances is

  • sunspot activity
  • Correct Answer
    angle of radiation
  • impedance
  • bandwidth
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On VHF and UHF bands, polarisation of the receiving antenna is important in relation to the transmitting antenna, but on HF it is relatively unimportant because

  • Correct Answer
    the ionosphere can change the polarisation of the signal from moment to moment
  • the ground wave and the sky wave continually shift the polarisation
  • anomalies in the earth's magnetic field profoundly affect HF polarisation
  • improved selectivity in HF receivers makes changes in polarisation redundant
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