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Subelement E9
ANTENNAS AND TRANSMISSION LINES
Section E9A
Isotropic and gain antennas: definitions; uses; radiation patterns; Basic antenna parameters: radiation resistance and reactance, gain, beamwidth, efficiency
Which of the following describes an isotropic antenna?
  • A grounded antenna used to measure earth conductivity
  • A horizontally polarized antenna used to compare Yagi antennas
  • A theoretical antenna used as a reference for antenna gain
  • A spacecraft antenna used to direct signals toward the earth

An isotropic radiator is a theoretical point source of electromagnetic waves which radiates the same intensity of radiation in all directions. It has no preferred direction of radiation. It radiates uniformly in all directions over a sphere centred on the source.

Isotropic radiators are used as reference radiators with which other sources are compared.

Source: Wikipedia - Isotropic Radiator

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How much gain does a 1/2-wavelength dipole in free space have compared to an isotropic antenna?
  • 1.55 dB
  • 2.15 dB
  • 3.05 dB
  • 4.30 dB

A reference dipole antenna is defined to have 2.15dBi gain.

A useful conversion between dBd and dBi is as follows:

dBi = dBd + 2.15

dBd = dBi - 2.15

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Which of the following antennas has no gain in any direction?
  • Quarter-wave vertical
  • Yagi
  • Half-wave dipole
  • Isotropic antenna

Isotropic antennas are ideal (theoretical) antennas that have equal power in all directions. They are used as references for antenna gain.

The word "isotropic" means "uniform in all orientations/directions".

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Why would one need to know the feed point impedance of an antenna?
  • To match impedances in order to minimize standing wave ratio on the transmission line
  • To measure the near-field radiation density from a transmitting antenna
  • To calculate the front-to-side ratio of the antenna
  • To calculate the front-to-back ratio of the antenna
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Which of the following factors may affect the feed point impedance of an antenna?
  • Transmission-line length
  • Antenna height, conductor length/diameter ratio and location of nearby conductive objects
  • Constant feed point impedance
  • Sunspot activity and time of day

Feed point impedance is the sum of radiation resistance, ohmic loss (pure resistance), and reactance caused by non-resonance.

Antenna height Radiation resistance is proportional to height; it decreases as antenna is closer to the ground.

conductor length/diameter ratio Radiation resistance is inversely proportional to thickness; thicker conductor gives less radiation resistance.

location of nearby conductive objects buildings, other antenna.

summarized from AARL's extra class manual.

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What is included in the total resistance of an antenna system?
  • Radiation resistance plus space impedance
  • Radiation resistance plus transmission resistance
  • Transmission-line resistance plus radiation resistance
  • Radiation resistance plus ohmic resistance

Feed point impedance is the sum of radiation resistance, ohmic loss which is pure resistance and reactance caused by non-resonance which is not included in the possible answers.

  • K4AGO

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What is a folded dipole antenna?
  • A dipole one-quarter wavelength long
  • A type of ground-plane antenna
  • A dipole constructed from one wavelength of wire forming a very thin loop
  • A dipole configured to provide forward gain

The key is to remember that this antenna is one wavelength long "folded" in to a thin loop one-half wavelength long.

For more information see wikipedia: Folded Dipole Antenna

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What is meant by antenna gain?
  • The ratio relating the radiated signal strength of an antenna in the direction of maximum radiation to that of a reference antenna
  • The ratio of the signal in the forward direction to that in the opposite direction
  • The ratio of the amount of power radiated by an antenna compared to the transmitter output power
  • The final amplifier gain minus the transmission-line losses, including any phasing lines present
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What is meant by antenna bandwidth?
  • Antenna length divided by the number of elements
  • The frequency range over which an antenna satisfies a performance requirement
  • The angle between the half-power radiation points
  • The angle formed between two imaginary lines drawn through the element ends

Bandwidth is a measurement of frequencies, as in the width of a range of frequencies.

An antenna's bandwidth is the range of frequencies it works best on.

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How is antenna efficiency calculated?
  • (radiation resistance / transmission resistance) x 100%
  • (radiation resistance / total resistance) x 100%
  • (total resistance / radiation resistance) x 100%
  • (effective radiated power / transmitter output) x 100%
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Which of the following choices is a way to improve the efficiency of a ground-mounted quarter-wave vertical antenna?
  • Install a good radial system
  • Isolate the coax shield from ground
  • Shorten the radiating element
  • Reduce the diameter of the radiating element
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Which of the following factors determines ground losses for a ground-mounted vertical antenna operating in the 3-30 MHz range?
  • The standing-wave ratio
  • Distance from the transmitter
  • Soil conductivity
  • Take-off angle

Hint: GROUND losses is in the SOIL. The only answer talking about the ground. m

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How much gain does an antenna have compared to a 1/2-wavelength dipole when it has 6 dB gain over an isotropic antenna?
  • 3.85 dB
  • 6.0 dB
  • 8.15 dB
  • 2.79 dB

A 1/2-wave dipole antenna has approximately \(2.15 dB\) of gain over an isotropic antenna. So \(6 dB - 2.15 dB = 3.85 dB\).

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How much gain does an antenna have compared to a 1/2-wavelength dipole when it has 12 dB gain over an isotropic antenna?
  • 6.17 dB
  • 9.85 dB
  • 12.5 dB
  • 14.15 dB

Given: It's well-known that a half-wave dipole has \(2.15 dB\) gain over an ideal isotropic radiator.

Let \(H =\) the gain of an ideal isotropic radiator

Let \(W =\) the gain of a half-wave dipole

Therefore \(W = H + 2.15 dB\), or solving for \(H\), \(H = W - 2.15 dB\)

Now, let \(X =\) the gain of the antenna in question

If that antenna in question exhibits \(12 dB\) over an isotropic antenna, then

\(X = H + 12 dB\)

Substituting, we get

\(X = W - 2.15 dB + 12 dB\) \(= W + 9.85 dB\)

Therefore, the antenna in question (\(X\)) exhibits a gain of \(9.85 dB\) over that (\(W\)) of a half-wave dipole.

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What is meant by the radiation resistance of an antenna?
  • The combined losses of the antenna elements and feed line
  • The specific impedance of the antenna
  • The value of a resistance that would dissipate the same amount of power as that radiated from an antenna
  • The resistance in the atmosphere that an antenna must overcome to be able to radiate a signal

The electrical resistance of an antenna is composed of its ohmic resistance plus its radiation resistance. The energy lost due to radiation resistance is the energy that is converted to electromagnetic radiation.

It isn't the combined losses of antenna elements and feed line is wrong because radiation resistance is not related to feed line losses.

It isn't the specific impedance of the antenna because the radiation resistance is only a portion of the antenna's impedance.

It isn't the resistance in the atmosphere that an antenna must overcome because the radiation resistance is not a property of the atmosphere. It is determined by the geometry of the antenna.

The correct answer is therefore the value of a resistance that would dissipate the same amount of power as that radiated from an antenna

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