Login or Register for FREE!
Login or Register for FREE!
Subel. L08
79 questions
Subelement L08

Antennas.

Section L08

In a Yagi 3 element directional antenna, the ____________ is primarily for mechanical support purposes.

  • driven element
  • director
  • Correct Answer
    boom
  • reflector

The 'boom' supports the elements of the Yagi.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

In a Yagi 3 element directional antenna, the ________ is the longest radiating element.

  • Correct Answer
    reflector
  • director
  • driven element
  • boom

The 'boom' supports the elements of the Yagi. Element dimensions on a Yagi; the 'Driven' = a half-wave dipole, 95% of a half-wavelength in free space = (300 / MHz / 2) * 95%. The 'Reflector', in back of the 'driven' = 5% longer than the 'driven'. The 'Director', in front of the 'driven, = 5% shorter than the 'driven'.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

In a Yagi 3 element directional antenna, the ______________ is the shortest radiating element.

  • reflector
  • driven element
  • Correct Answer
    director
  • boom

The 'boom' supports the elements of the Yagi. Element dimensions on a Yagi; the 'Driven' = a half-wave dipole, 95% of a half-wavelength in free space = (300 / MHz / 2) * 95%. The 'Reflector', in back of the 'driven' = 5% longer than the 'driven'. The 'Director', in front of the 'driven, = 5% shorter than the 'driven'.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

In a Yagi 3 element directional antenna, the ______________is not the longest nor the shortest radiating element.

  • director
  • reflector
  • Correct Answer
    driven element
  • boom

The 'boom' supports the elements of the Yagi. Element dimensions on a Yagi; the 'Driven' = a half-wave dipole, 95% of a half-wavelength in free space = (300 / MHz / 2) * 95%. The 'Reflector' (in back of the 'driven') = 5% longer than the 'driven'. The 'Director' (in front of the 'driven) = 5% shorter than the 'driven'.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What does horizontal wave polarization mean?

  • The electric lines of force of a radio wave are perpendicular to the Earth's surface
  • The magnetic lines of force of a radio wave are parallel to the Earth's surface
  • Correct Answer
    The electric lines of force of a radio wave are parallel to the Earth's surface
  • The electric and magnetic lines of force of a radio wave are perpendicular to the Earth's surface

An electromagnetic wave comprises an electrical field and a magnetic field. Wave Polarization describes the position of the ELECTRIC field with respect to the Earth's surface. On a dipole antenna or on the 'driven' element of a Yagi, the electric field is developed between the tips of the radiating element.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What does vertical wave polarization mean?

  • The electric lines of force of a radio wave are parallel to the Earth's surface
  • Correct Answer
    The electric lines of force of a radio wave are perpendicular to the Earth's surface
  • The magnetic lines of force of a radio wave are perpendicular to the Earth's surface
  • The electric and magnetic lines of force of a radio wave are parallel to the Earth's surface

An electromagnetic wave comprises an electrical field and a magnetic field. Wave Polarization describes the position of the ELECTRIC field with respect to the Earth's surface. On a dipole antenna or on the 'driven' element of a Yagi, the electric field is developed between the tips of the radiating element.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What electromagnetic wave polarization does a Yagi antenna have when its elements are parallel to the Earth's surface?

  • Circular
  • Correct Answer
    Horizontal
  • Helical
  • Vertical

An electromagnetic wave comprises an electrical field and a magnetic field. Wave Polarization describes the position of the ELECTRIC field with respect to the Earth's surface. On a dipole antenna or on the 'driven' element of a Yagi, the electric field is developed between the tips of the radiating element.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What electromagnetic wave polarization does a half-wavelength antenna have when it is perpendicular to the Earth's surface?

  • Horizontal
  • Parabolical
  • Correct Answer
    Vertical
  • Circular

An electromagnetic wave comprises an electrical field and a magnetic field. Wave Polarization describes the position of the ELECTRIC field with respect to the Earth's surface. On a dipole antenna or on the 'driven' element of a Yagi, the electric field is developed between the tips of the radiating element.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Polarization of an antenna is determined by:

  • the magnetic field
  • Correct Answer
    the orientation of the electric field relative to the Earth's surface
  • the height of the antenna
  • the type of antenna

An electromagnetic wave comprises an electrical field and a magnetic field. Wave Polarization describes the position of the ELECTRIC field with respect to the Earth's surface. On a dipole antenna or on the 'driven' element of a Yagi, the electric field is developed between the tips of the radiating element.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

An isotropic antenna is:

  • a dummy load
  • a half-wave reference dipole
  • Correct Answer
    a hypothetical point source
  • an infinitely long piece of wire

'Isotropic' means "equal radiation in all directions". An 'isotropic antenna', also called 'isotropic radiator' is an HYPOTHETICAL point source. Plotting the pattern in all planes around the source would yield a 'sphere' as a pattern. The 'isotropic antenna' is used as a reference to compare the gain of real antennas.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is the antenna radiation pattern for an isotropic radiator?

  • A parabola
  • A cardioid
  • A unidirectional cardioid
  • Correct Answer
    A sphere

'Isotropic' means "equal radiation in all directions". An 'isotropic antenna', also called 'isotropic radiator' is an HYPOTHETICAL point source. Plotting the pattern in all planes around the source would yield a 'sphere' as a pattern. The 'isotropic antenna' is used as a reference to compare the gain of real antennas.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

VHF signals from a mobile station using a vertical whip antenna will normally be best received using a:

  • Correct Answer
    vertical ground-plane antenna
  • random length of wire
  • horizontal ground-plane antenna
  • horizontal dipole antenna

key words: VHF, VERTICAL. On 'line of sight' propagation (common at Very High Frequencies) and with Ground Wave propagation (common at the low end of High Frequencies), a significant loss is incurred if the antennas on both extremities do NOT have the same polarization.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

A dipole antenna will emit a vertically polarized wave if it is:

  • fed with the correct type of RF
  • too near to the ground
  • parallel with the ground
  • Correct Answer
    mounted vertically

An electromagnetic wave comprises an electrical field and a magnetic field. Wave Polarization describes the position of the ELECTRIC field with respect to the Earth's surface. On a dipole antenna or on the 'driven' element of a Yagi, the electric field is developed between the tips of the radiating element.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

If an electromagnetic wave leaves an antenna vertically polarized, it will arrive at the receiving antenna, by ground wave:

  • horizontally polarized
  • polarized in any plane
  • Correct Answer
    vertically polarized
  • polarized at right angles to original

key words: GROUND WAVE. On 'line of sight' propagation (common at Very High Frequencies) and with Ground Wave propagation (common at the low end of High Frequencies), a significant loss is incurred if the antennas on both extremities do NOT have the same polarization.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Compared with a horizontal antenna, a vertical antenna will receive a vertically polarized radio wave:

  • at weaker strength
  • without any comparative difference
  • if the antenna changes the polarization
  • Correct Answer
    at greater strength

On 'line of sight' propagation (common at Very High Frequencies) and with Ground Wave propagation (common at the low end of High Frequencies), a significant loss is incurred if the antennas on both extremities do NOT have the same polarization.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

If an antenna is made longer, what happens to its resonant frequency?

  • It stays the same
  • It disappears
  • Correct Answer
    It decreases
  • It increases

Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. Wavelength and frequency have an inverse relationship. Antennas on the 80 metre HF (3.5 to 4.0 MHz) band are much longer than antennas on the 2 metre VHF band (144 to 148 MHz).

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

If an antenna is made shorter, what happens to its resonant frequency?

  • Correct Answer
    It increases
  • It stays the same
  • It disappears
  • It decreases

Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. Wavelength and frequency have an inverse relationship. Antennas on the 2 metre VHF band (144 to 148 MHz) are much shorter than antennas on the 80 metre HF band (3.5 to 4.0 MHz).

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The wavelength for a frequency of 25 MHz is:

  • 4 metres (13.1 ft)
  • 32 metres (105 ft)
  • Correct Answer
    12 metres (39.4 ft)
  • 15 metres (49.2 ft)

Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. In this example, 300 / 25 = 12 metres.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The velocity of propagation of radio frequency energy in free space is:

  • Correct Answer
    300 000 kilometres per second
  • 3000 kilometres per second
  • 150 kilometres per second
  • 186 000 kilometres per second

Radio waves in free space travel at the speed of light: 300 000 kilometres per second.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Adding a series inductance to an antenna would:

  • increase the resonant frequency
  • have little effect
  • have no change on the resonant frequency
  • Correct Answer
    decrease the resonant frequency

A series inductance in an antenna is termed a "loading coil". It makes the antenna appear LONGER electrically than its physical size. Making the antenna longer brings down the resonant frequency.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The resonant frequency of an antenna may be increased by:

  • lengthening the radiating element
  • Correct Answer
    shortening the radiating element
  • lowering the radiating element
  • increasing the height of the radiating element

Wavelength and frequency have an inverse relationship. Increasing the resonant frequency (shorter wavelength) can be achieved by shortening the radiating element.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The speed of a radio wave:

  • Correct Answer
    is the same as the speed of light
  • is infinite in space
  • is always less than half speed of light
  • varies directly with frequency

Radio waves in free space travel at the speed of light: 300 000 kilometres per second.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

At the end of suspended antenna wire, insulators are used. These act to:

  • Correct Answer
    limit the electrical length of the antenna
  • increase the effective antenna length
  • allow the antenna to be more easily held vertically
  • prevent any loss of radio waves by the antenna

Insulators mark the end of the antenna. Thus, wet support ropes or metallic support wires do not become part of the antenna.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

To lower the resonant frequency of an antenna, the operator should:

  • ground one end
  • centre feed it with TV ribbon transmission line
  • Correct Answer
    lengthen it
  • shorten it

Wavelength and frequency have an inverse relationship. Decreasing the resonant frequency (longer wavelength) can be achieved by lengthening the radiating element.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

One solution to multiband operation with a shortened radiator is the "trap dipole" or trap vertical. These "traps" are actually:

  • large wire-wound resistors
  • coils wrapped around a ferrite rod
  • hollow metal cans
  • Correct Answer
    a coil and capacitor in parallel

"Antenna traps" are parallel resonant circuits which exhibit high impedance at resonance. Electrically speaking, they cut-off the antenna at the trap position when operated at the resonant frequency of the trap.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The wavelength corresponding to a frequency of 2 MHz is:

  • Correct Answer
    150 m (492 ft)
  • 360 m (1181 ft)
  • 1500 m (4921 ft)
  • 30 m (98 ft)

Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. In this example, 300 / 2 = 150 metres.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is a parasitic beam antenna?

  • An antenna where the driven element obtains its radio energy by induction or radiation from director elements
  • An antenna where all elements are driven by direct connection to the transmission line
  • An antenna where wave traps are used to magnetically couple the elements
  • Correct Answer
    An antenna where some elements obtain their radio energy by induction or radiation from a driven element

The term 'parasite' means "feeding off something else". For instance, in a Yagi, there is only one 'driven' element where the transmission line attaches. The 'reflector' and 'director' capture energy off the 'driven' and re-radiate it.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

How can the bandwidth of a parasitic beam antenna be increased?

  • Correct Answer
    Use larger diameter elements
  • Use traps on the elements
  • Use tapered-diameter elements
  • Use closer element spacing

'Antenna bandwidth' is the range of frequencies over which an antenna is usable. Larger-diameter elements means "thicker" elements. With "fatter" elements, resonance isn't as sharp. Antenna 'bandwidth' is increased.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

If a parasitic element slightly shorter than a horizontal dipole antenna is placed parallel to the dipole 0.1 wavelength from it and at the same height, what effect will this have on the antenna's radiation pattern?

  • Correct Answer
    A major lobe will develop in the horizontal plane, from the dipole toward the parasitic element
  • A major lobe will develop in the horizontal plane, parallel to the two elements
  • A major lobe will develop in the vertical plane, away from the ground
  • The radiation pattern will not be affected

key words: PARASITIC, SHORTER. A 'slightly shorter parasitic' element is the description of a 'Director'. A dipole and a 'director' in front of it make up a two-element Yagi. Radiation will be enhanced toward the 'director' at the expense of the back.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

If a parasitic element slightly longer than a horizontal dipole antenna is placed parallel to the dipole 0.1 wavelength from it and at the same height, what effect will this have on the antenna's radiation pattern?

  • A major lobe will develop in the horizontal plane, parallel to the two elements
  • A major lobe will develop in the vertical plane, away from the ground
  • The radiation pattern will not be affected
  • Correct Answer
    A major lobe will develop in the horizontal plane, from the parasitic element toward the dipole

key words: PARASITIC, LONGER. A 'slightly longer parasitic' element is the description of a 'reflector'. A dipole and a 'reflector' behind it make up a two-element Yagi. Radiation will be enhanced away from the 'reflector', towards the radiating element (the dipole, the 'driven').

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The property of an antenna, which defines the range of frequencies to which it will respond, is called its:

  • Correct Answer
    bandwidth
  • front-to-back ratio
  • impedance
  • polarization

'Antenna Bandwidth' is the range of frequencies over which Standing Wave Ratio (SWR) is acceptable.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Approximately how much gain does a half-wave dipole have over an isotropic radiator?

  • 6.0 dB
  • Correct Answer
    2.1 dB
  • 1.5 dB
  • 3.0 dB

An 'isotropic radiator' radiates equally well in ALL directions ( radiation pattern is a 'sphere'). A dipole in free space has a radiation pattern similar to a donut ( maximum radiation broadside from the antenna, none towards the ends ). This concentration of radiation produce a gain of 2.1 dB over an isotropic antenna.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is meant by antenna gain?

  • The numerical ratio of the signal in the forward direction to the signal in the back direction
  • The numerical ratio of the amount of power radiated by an antenna compared to the transmitter output power
  • The power amplifier gain minus the transmission line losses
  • Correct Answer
    The numerical ratio relating the radiated signal strength of an antenna to that of another antenna

Antenna Gain is a ratio, expressed in decibel, of the radiation of a given antenna against some reference antenna. For example, the expression 'dBi' means decibel over an isotropic radiator.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is meant by antenna bandwidth?

  • Antenna length divided by the number of elements
  • The angle between the half-power radiation points
  • The angle formed between two imaginary lines drawn through the ends of the elements
  • Correct Answer
    The frequency range over which the antenna may be expected to perform well

'Antenna Bandwidth' is the range of frequencies over which Standing Wave Ratio (SWR) is acceptable.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

In free space, what is the radiation characteristic of a half-wave dipole?

  • Omnidirectional
  • Maximum radiation at 45 degrees to the plane of the antenna
  • Correct Answer
    Minimum radiation from the ends, maximum broadside
  • Maximum radiation from the ends, minimum broadside

A dipole in free space has a radiation pattern similar to a donut ( maximum radiation broadside from the antenna, none towards the ends ). This concentration of radiation produce a gain of 2.1 dB over an isotropic antenna.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The gain of an antenna, especially on VHF and above, is quoted in dBi. The "i" in this expression stands for:

  • Correct Answer
    isotropic
  • ideal
  • ionosphere
  • interpolated

Antenna Gain is a ratio, expressed in decibel, of the radiation of a given antenna against some reference antenna. For example, the expression 'dBi' means decibel over an isotropic radiator.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The front-to-back ratio of a beam antenna is:

  • Correct Answer
    the ratio of the maximum forward power in the major lobe to the maximum backward power radiation
  • the forward power of the major lobe to the power in the backward direction both being measured at the 3 dB points
  • undefined
  • the ratio of the forward power at the 3 dB points to the power radiated in the backward direction

'Beam antenna' is another name for a Yagi. 'Front to back' is a ratio in decibels of the power radiated in the most favoured direction (front) to the power radiated towards the back of the antenna.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

How do you calculate the length in metres (feet) of a quarter-wavelength vertical antenna?

  • Divide 150 (491) by the antenna's operating frequency in MHz
  • Correct Answer
    Divide 71.5 (234) by the antenna's operating frequency in MHz
  • Divide 468 (1532) by the antenna's operating frequency in MHz
  • Divide 300 (982) by the antenna's operating frequency in MHz

key words: QUARTER-wavelength. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. Answer: 95 % of one quarter wavelength in free space = '300 / 4 * 0.95' divided by frequency in megahertz = 71.3 divided by frequency in megahertz.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

If you made a quarter-wavelength vertical antenna for 21.125 MHz, how long would it be?

  • 6.76 metres (22.2 ft)
  • Correct Answer
    3.36 metres (11.0 ft)
  • 3.6 metres (11.8 ft)
  • 7.2 metres (23.6 ft)

key words: QUARTER-wavelength. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. Answer: 95 % of one quarter wavelength in free space = '300 / 4 * 0.95' divided by frequency in megahertz = 71.3 divided by frequency in megahertz. In this example, '300 / 21.125 MHz / 4 * 0.95' = 3.37 metres.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

If you made a half-wavelength vertical antenna for 223 MHz, how long would it be?

  • Correct Answer
    64 cm (25.2 in)
  • 128 cm (50.4 in)
  • 105 cm (41.3 in)
  • 134.6 cm (53 in)

key words: HALF-wavelength. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. Answer: 95 % of one half wavelength in free space = '300 / 2 * 0.95' divided by frequency in megahertz = 143 divided by frequency in megahertz. In this example, '300 / 223 MHz / 2 * 0.95' = 0.64 metres.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Why is a 5/8-wavelength vertical antenna better than a 1/4-wavelength vertical antenna for VHF or UHF mobile operations?

  • A 5/8-wavelength antenna is easier to install on a car
  • A 5/8-wavelength antenna can handle more power
  • Correct Answer
    A 5/8-wavelength antenna has more gain
  • A 5/8-wavelength antenna has less corona loss

The 'five eights' wavelength antenna focuses energy somewhat better towards the horizon (lower radiation angle) than a regular quarter-wave antenna.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

If a magnetic-base whip antenna is placed on the roof of a car, in what direction does it send out radio energy?

  • Most of it goes in one direction
  • Correct Answer
    It goes out equally well in all horizontal directions
  • Most of it is aimed high into the sky
  • Most of it goes equally in two opposite directions

An upright antenna element radiates equally well all around it in the horizontal plane.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is an advantage of downward sloping radials on a ground plane antenna?

  • It lowers the radiation angle
  • Correct Answer
    It brings the feed point impedance closer to 50 ohms
  • It increases the radiation angle
  • It brings the feed point impedance closer to 300 ohms

Radials are the three or four rods simulating ground at the base of an elevated vertical antenna (ground plane antenna). Sloping radials (lower than 90 degrees) BRING up the impedance from about 30 ohms to 50 ohms for a better direct match to coaxial cable.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What happens to the feed point impedance of a ground-plane antenna when its radials are changed from horizontal to downward-sloping?

  • It approaches zero
  • Correct Answer
    It increases
  • It decreases
  • It stays the same

Radials are the three or four rods simulating ground at the base of an elevated vertical antenna (ground plane antenna). Sloping radials (lower than 90 degrees) BRING up the impedance from about 30 ohms to 50 ohms for a better direct match to coaxial cable.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Which of the following transmission lines will give the best match to the base of a quarter-wave ground-plane antenna?

  • 300 ohms coaxial cable
  • Correct Answer
    50 ohms coaxial cable
  • 300 ohms balanced transmission line
  • 75 ohms balanced transmission line

A quarter-wave ground plane antenna exhibits a feedpoint impedance fairly close to 50 ohms.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The main characteristic of a vertical antenna is that it will:

  • be very sensitive to signals coming from horizontal antennas
  • require few insulators
  • be easy to feed with TV ribbon transmission line
  • Correct Answer
    receive signals equally well from all compass points around it

An upright antenna element radiates equally well all around it in the horizontal plane. It is termed 'omni-directional'.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Why is a loading coil often used with an HF mobile vertical antenna?

  • To filter out electrical noise
  • Correct Answer
    To tune out capacitive reactance
  • To lower the losses
  • To lower the Q

Short answer: a coil (inductor) has a behaviour totally opposite to capacitors; 'cancelling reactive capacitance' makes sense. A short antenna (e.g., 2.5 m) operated on HF frequencies (wavelengths of 10 to 80 metres) looks like an antenna operated well below its natural resonant frequency. If you think of an ideal antenna as a resonant circuit where capacitive and inductive reactances cancel each other, you'll note that CAPACITIVE reactance ( XC = 1 over '2 * PI * f * C' ) grows below the resonant frequency. A "loading coil" cancels out that capacitive reactance.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is the main reason why so many VHF base and mobile antennas are 5/8 of a wavelength?

  • The angle of radiation is high giving excellent local coverage
  • It is easy to match the antenna to the transmitter
  • It's a convenient length on VHF
  • Correct Answer
    The angle of radiation is low

The 'five eights' wavelength antenna focuses energy somewhat better towards the horizon (lower radiation angle) than a regular quarter-wave antenna.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

How many directly driven elements do most Yagi antennas have?

  • Two
  • Three
  • None
  • Correct Answer
    One

Generally speaking, a parasitic beam antenna has one 'driven' element where the transmission line attaches.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Approximately how long is the driven element of a Yagi antenna for 14.0 MHz?

  • 5.21 metres (17 feet)
  • 10.67 metres (35 feet)
  • 20.12 metres (66 feet)
  • Correct Answer
    10.21 metres (33.5 feet)

key word: DRIVEN. Same approximate length as a HALF-WAVE dipole. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. Answer: 95 % of one half wavelength in free space = '(300 / 2) * 0.95' divided by frequency in megahertz = 143 divided by frequency in megahertz. In this example, '(300 / 14 MHz / 2) * 0.95' = 10.18 metres.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Approximately how long is the director element of a Yagi antenna for 21.1 MHz?

  • 5.18 metres (17 feet)
  • 3.2 metres (10.5 feet)
  • 12.8 metres (42 feet)
  • Correct Answer
    6.4 metres (21 feet)

key word: DIRECTOR. About 5% SHORTER than the 'driven' which is itself the approximate length of a HALF-WAVE dipole. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. The 'driven' would be 95 % of one half wavelength in free space = '(300 / 2) * 0.95' divided by frequency in megahertz. The DIRECTOR is another 95% of the length of the 'driven'. In this example, the director becomes (300 / 21.1 MHz / 2) * 0.95 * 0.95 = 6.42 metres.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Approximately how long is the reflector element of a Yagi antenna for 28.1 MHz?

  • 2.66 metres (8.75 feet)
  • Correct Answer
    5.33 metres (17.5 feet)
  • 4.88 metres (16 feet)
  • 10.67 metres (35 feet)

key word: REFLECTOR. About 5% LONGER than the 'driven' which is itself the approximate length of a HALF-WAVE dipole. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. The 'driven' would be 95 % of one half wavelength in free space = '(300 / 2) * 0.95' divided by frequency in megahertz. The REFLECTOR is 1.05 times the length of the 'driven'. In this example, the reflector becomes (300 / 28.1 MHz / 2) * 0.95 * 1.05 = 5.32 metres.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is one effect of increasing the boom length and adding directors to a Yagi antenna?

  • SWR increases
  • Weight decreases
  • Wind load decreases
  • Correct Answer
    Gain increases

More directors is the primary means of augmenting gain. [ Weight and 'wind load' certainly increase then. ]

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What are some advantages of a Yagi with wide element spacing?

  • Correct Answer
    High gain, less critical tuning and wider bandwidth
  • High gain, lower loss and a low SWR
  • High front-to-back ratio and lower input resistance
  • Shorter boom length, lower weight and wind resistance

'Lower loss', 'lower input resistance' and 'shorter boom length' are all misleading clues.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Why is a Yagi antenna often used for radiocommunications on the 20-metre band?

  • It is smaller, less expensive and easier to erect than a dipole or vertical antenna
  • It provides the highest possible angle of radiation for the HF bands
  • Correct Answer
    It helps reduce interference from other stations off to the side or behind
  • It provides excellent omnidirectional coverage in the horizontal plane

20 metres is an amateur band with global reach. It is open during day time even during solar cycle lows. The directive antenna pattern of a Yagi permits reducing interference by focusing energy in one direction only.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What does "antenna front-to-back ratio" mean in reference to a Yagi antenna?

  • The number of directors versus the number of reflectors
  • Correct Answer
    The power radiated in the major radiation lobe compared to the power radiated in exactly the opposite direction
  • The relative position of the driven element with respect to the reflectors and directors
  • The power radiated in the major radiation lobe compared to the power radiated 90 degrees away from that direction

'Front to back' is a ratio in decibels of the power radiated in the most favoured direction (front) to the power radiated towards the back of the antenna.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is a good way to get maximum performance from a Yagi antenna?

  • Use a reactance bridge to measure the antenna performance from each direction around the antenna
  • Avoid using towers higher than 9 metres (30 feet) above the ground
  • Correct Answer
    Optimize the lengths and spacing of the elements
  • Use RG-58 transmission line

All dimensions in Yagis must be optimized: the lengths and positions of each elements influence final performance. [ Center frequency, feedpoint impedance, forward gain, antenna bandwidth and front-to-back ratio all change with changing physical dimensions. ]

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The spacing between the elements on a three-element Yagi antenna, representing the best overall choice, is _____ of a wavelength.

  • 0.10
  • 0.50
  • 0.75
  • Correct Answer
    0.20

Two tenths of a wavelength is reputed to be an optimum choice on a 3-element beam.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

If the forward gain of a six-element Yagi is about 10 dBi, what would the gain of two of these antennas be if they were "stacked"?

  • 20 dBi
  • 10 dBi
  • Correct Answer
    13 dBi
  • 7 dBi

This is a trick question. Two identical antennas side by side doubles the radiated power. An increase of 2 in power is a gain of +3 dB. The gain of the array becomes 10 dBi + 3 dB = 13 dBi.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

If you made a half-wavelength dipole antenna for 28.150 MHz, how long would it be?

  • 10.5 metres (34.37 ft)
  • 28.55 metres (93.45 ft)
  • 10.16 metres (33.26 ft)
  • Correct Answer
    5.08 metres (16.62 ft)

key words: half-wavelength DIPOLE. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. A 'dipole' is approximately 95 % of one half wavelength in free space = '(300 / 2) * 0.95' divided by frequency in megahertz. In this example, the dipole must be (300 / 28.15 MHz / 2) * 0.95 = 5.06 metres. The frequency is in the 10 metre band of 28.0 to 29.7 MHz, a dipole there must be necessarily 5 metres long.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is one disadvantage of a random wire antenna?

  • You must use an inverted T matching network for multi-band operation
  • Correct Answer
    You may experience RF feedback in your station
  • It usually produces vertically polarized radiation
  • It must be longer than 1 wavelength

Because the 'random wire' and 'long wire' antennas frequently originate right at the back of the antenna tuner in your station, stray RF (radio frequency) can be a problem.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is the low angle radiation pattern of an ideal half-wavelength dipole HF antenna in free space installed parallel to the Earth?

  • It is a circle (equal radiation in all directions)
  • It is two smaller lobes on one side of the antenna, and one larger lobe on the other side
  • It is a figure-eight, off both ends of the antenna
  • Correct Answer
    It is a figure-eight, perpendicular to the antenna

Picture an horizontal dipole viewed from above. If you plotted radiation all around it, the plot would look like a "number eight": peak radiation at 90 degrees (broadside) from the antenna, negligible radiation from the ends.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The impedances in ohms at the feed point of the dipole and folded dipole in free space are, respectively:

  • 52 and 100
  • 52 and 200
  • Correct Answer
    73 and 300
  • 73 and 150

Feedpoint impedance of a dipole in free space: 73 ohms. Feedpoint impedance of a Folded Dipole: 300 ohms.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

A horizontal dipole transmitting antenna, installed at an ideal height so that the ends are pointing North/South, radiates:

  • Correct Answer
    mostly to the East and West
  • mostly to the South and North
  • mostly to the South
  • equally in all directions

Picture an horizontal dipole viewed from above, if you plotted radiation all around it, the plot would look like a "number eight": peak radiation at 90 degrees (broadside) from the antenna, negligible radiation from the ends.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

How does the bandwidth of a folded dipole antenna compare with that of a simple dipole antenna?

  • Correct Answer
    It is greater
  • It is essentially the same
  • It is less than 50%
  • It is 0.707 times the bandwidth

'Antenna Bandwidth' is the range of frequencies over which Standing Wave Ratio (SWR) is acceptable. The Folded Dipole can be operated over a wider range of frequencies than a regular dipole.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is a disadvantage of using an antenna equipped with traps?

  • It is too sharply directional at lower frequencies
  • It must be neutralized
  • It can only be used for one band
  • Correct Answer
    It may radiate harmonics more readily

An antenna with traps is a multi-band antenna (i.e., resonant at more than one frequency). If the transmitter leaks harmonic energy (multiples of the operating frequency), this harmonic energy may be more readily radiated by a multi-band antenna. For example, traps are inserted in an antenna for 80 metres to permit operation on 40 metres; if your transmitter puts out 'harmonics' while you operate on 80 m ( say, 3.5 MHz ), the second harmonic falls in the 40 m band. The antenna is also resonant at that frequency and would freely radiate the harmonics.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is an advantage of using a trap antenna?

  • It has high directivity at the higher frequencies
  • It has high gain
  • It minimizes harmonic radiation
  • Correct Answer
    It may be used for multi-band operation

The only reason why antenna traps (parallel resonant circuits) are useful is to permit operation on more than one band from the same physical antenna. Through their high impedance at resonance, traps shorten the antenna by making the antenna sections beyond them inaccessible.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

If you were to cut a half wave dipole for 3.75 MHz, what would be its approximate length?

  • 32 meters (105 ft)
  • 45 meters (145 ft)
  • 75 meters (245 ft)
  • Correct Answer
    38 meters (125 ft)

Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. The dipole is approximately 95 % of one half wavelength in free space = '(300 / 2) * 0.95' divided by frequency in megahertz. In this example, the dipole must be cut to (300 / 3.75 MHz / 2) * 0.95 = 38 metres. [ 3.75 MHz is in the 80 metre band of 3.5 to 4.0 MHz, a DIPOLE there must be below 40 metres long ].

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is a cubical quad antenna?

  • A center-fed wire 1/2-electrical wavelength long
  • A vertical conductor 1/4-electrical wavelength high, fed at the bottom
  • Four straight, parallel elements in line with each other, each approximately 1/2-electrical wavelength long
  • Correct Answer
    Two or more parallel four-sided wire loops, each approximately one-electrical wavelength long

The 'cubical quad' is a parasitic array made of one-wavelength LOOPS (square or diamond-shaped).

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What is a delta loop antenna?

  • An antenna system made of three vertical antennas, arranged in a triangular shape
  • An antenna made from several triangular coils of wire on an insulating form
  • Correct Answer
    An antenna whose elements are each a three sided loop whose total length is approximately one electrical wavelength
  • A large copper ring or wire loop, used in direction finding

A 'delta' is a parasitic array made of one-wavelength LOOPS with a triangular shape.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Approximately how long is each side of a cubical quad antenna driven element for 21.4 MHz?

  • Correct Answer
    3.54 metres (11.7 feet)
  • 0.36 metres (1.17 feet)
  • 14.33 metres (47 feet)
  • 143 metres (469 feet)

key word: CUBICAL QUAD. A four-sided loop. Loop antennas are roughly 1 wavelength long. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. The 'driven' element in a LOOP is 2% longer than a full wavelength in free space = '300 * 1.02' divided by frequency in megahertz. In this example, ONE side of the quad becomes (300 * 1.02) / 21.4 MHz / 4 = 3.57 metres.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Approximately how long is each side of a cubical quad antenna driven element for 14.3 MHz?

  • Correct Answer
    5.36 metres (17.6 feet)
  • 21.43 metres (70.3 feet)
  • 53.34 metres (175 feet)
  • 7.13 metres (23.4 feet)

key word: CUBICAL QUAD. A four-sided loop. Loop antennas are roughly 1 wavelength long. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. The 'driven' element in a LOOP is 2% longer than a full wavelength in free space = '300 * 1.02' divided by frequency in megahertz. In this example, ONE side of the quad becomes (300 * 1.02) / 14.3 MHz / 4 = 5.35 metres.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Approximately how long is each leg of a symmetrical delta loop antenna driven element for 28.7 MHz?

  • 10.67 metres (35 feet)
  • Correct Answer
    3.5 metres (11.5 feet)
  • 2.67 metres (8.75 feet)
  • 7.13 metres (23.4 feet)

key word: DELTA LOOP. A three-sided loop. Loop antennas are roughly 1 wavelength long. Wavelength (lambda) in metres IN FREE SPACE is 300 divided by frequency in megahertz. The 'driven' element in a LOOP is 2% longer than a full wavelength in free space = '300 * 1.02' divided by frequency in megahertz. In this example, ONE side of the DELTA becomes (300 * 1.02) / 28.7 MHz / 3 = 3.55 metres.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Which statement about two-element delta loops and quad antennas is true?

  • They are effective only when constructed using insulated wire
  • They perform poorly above HF
  • Correct Answer
    They compare favourably with a three-element Yagi
  • They perform very well only at HF

Because quads and deltas focus energy in both planes, horizontal and vertical, the two-element quad performs similarly to a three-element Yagi.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Compared to a dipole antenna, what are the directional radiation characteristics of a cubical quad antenna?

  • The quad has less directivity in the horizontal plane but more directivity in the vertical plane
  • The quad has less directivity in both horizontal and vertical planes
  • Correct Answer
    The quad has more directivity in both horizontal and vertical planes
  • The quad has more directivity in the horizontal plane but less directivity in the vertical plane

A quad with its four-sided architecture focuses energy in the vertical (up and down) AND horizontal (left to right) planes.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Moving the feed point of a multi-element quad antenna from a side parallel to the ground to a side perpendicular to the ground will have what effect?

  • Correct Answer
    It will change the antenna polarization from horizontal to vertical
  • It will change the antenna polarization from vertical to horizontal
  • It will significantly decrease the antenna feed point impedance
  • It will significantly increase the antenna feed point impedance

In your head, squish the quad from the top down, it now looks like a Folded Dipole. If the Folded dipole is horizontal, it is polarized horizontally. Flip it 90 degrees and it now has a vertical polarization.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

What does the term "antenna front-to-back ratio" mean in reference to a delta loop antenna?

  • The power radiated in the major radiation lobe compared to the power radiated 90 degrees away from that direction
  • The number of directors versus the number of reflectors
  • Correct Answer
    The power radiated in the major radiation lobe compared to the power radiated in exactly the opposite direction
  • The relative position of the driven element with respect to the reflectors and directors

Same as a Yagi. 'Front to back' is a ratio in decibels of the power radiated in the most favoured direction (front) to the power radiated towards the back of the antenna.

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The cubical "quad" or "quad" antenna consists of two or more square loops of wire. The driven element has an approximate overall length of:

  • Correct Answer
    one wavelength
  • three-quarters of a wavelength
  • two wavelengths
  • one-half wavelength

key words: LOOP, OVERALL length. A loop antenna is a little over 1 wavelength long (1.02 wavelength to be precise).

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

The delta loop antenna consists of two or more triangular structures mounted on a boom. The overall length of the driven element is approximately:

  • Correct Answer
    one wavelength
  • one-quarter of a wavelength
  • two wavelengths
  • one-half of a wavelength

key words: LOOP, OVERALL length. A loop antenna is a little over 1 wavelength long (1.02 wavelength to be precise).

Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.

Tags: none

Go to L07 Go to L09a
HamStudy.org™ is copyright 2024 Signal Stuff™, All rights reserved.
View Privacy Policy    |    Get help with HamStudy.org™