Antenna Fundamentals and Parameters Questions and Answers

Q: How does free space path loss vary with distance?

Power density falls as the inverse square of range, so path loss rises as the square of distance, twenty decibels per decade or six decibels for each doubling. The fourth power law belongs to a two ray ground reflection model at long range over a plane earth, which is a different situation with a different physical cause. Path loss also rises as the square of frequency at constant gain, which is a consequence of the shrinking effective aperture rather than of anything happening in the space between.

Q: What is effective isotropic radiated power?

EIRP is the product of the power delivered to the antenna and its gain relative to an isotropic radiator, so it states the power an isotropic source would need to produce the same field in the main beam. It is the quantity regulators limit, because it describes what a receiver in the beam actually experiences. Total radiated power is a different figure, smaller by the gain, and the other two options describe no standard quantity.

Q: Why is antenna gain often quoted in dBi rather than dBd?

An isotropic radiator is a mathematical reference that is the same everywhere, whereas a dipole reference depends on the dipole's environment and its own efficiency. The two differ by 2.15 decibels, the gain of a half wave dipole over isotropic, so dBi figures are the larger rather than the smaller. No isotropic antenna can be built at all, which is precisely why it is used as an abstraction, and regulations are written in either but usually in dBi.

Q: A dipole has a gain of 2.15 dBi. What is its gain in dBd?

The dBd scale is referred to a half wave dipole, so a dipole has by definition a gain of 0 dBd, and the 2.15 decibel difference is exactly the dipole's own gain over isotropic. To convert, subtract 2.15 from a dBi figure to get dBd, or add it going the other way. Manufacturers have been known to quote whichever gives the larger number, so the reference should always be checked before two antennas are compared.

Q: What is the reciprocity theorem as applied to antennas?

Reciprocity means an antenna's radiation pattern, gain, impedance and polarisation are identical whether it is transmitting or receiving, which is why patterns can be measured in whichever direction is convenient. The exception is any antenna containing a non-reciprocal component such as a ferrite isolator, and power handling obviously differs. The other options misuse the word reciprocal in its arithmetic sense, which is not what the theorem is about.

Q: What is the effective length of a receiving antenna?

Effective length is the induced open circuit voltage divided by the incident electric field, so it has units of metres and provides the link between a field strength in volts per metre and a receiver input voltage. For a short dipole it is half the physical length because the current tapers to zero at the ends. Physical length, resonant length and current distribution are related to it but none is its definition, and the distinction matters in field strength measurement.

Q: What is the antenna factor used in electromagnetic compatibility measurement?

Antenna factor converts the voltage a measuring receiver reports into the field strength that produced it, and is quoted in decibels per metre against frequency on the calibration certificate of every measurement antenna. It is essentially the reciprocal of effective length with the receiver impedance folded in. Cable loss is added separately, and the relation between radiated and conducted emission is a property of the equipment under test.

Q: What contributes to the noise temperature of a receiving antenna?

Antenna noise temperature sums the brightness temperature of everything the pattern sees, sky through the main beam and warm ground through the side lobes and spillover, together with the contribution of ohmic loss in the structure. This is why a satellite dish pointed low at the horizon performs worse than one pointed high, and why side lobe control matters in radio astronomy. Receiver noise is added separately in the system noise temperature.

Q: Why does pointing a dish antenna closer to the horizon raise its noise temperature?

At low elevation the main beam looks through much more atmosphere, which is warm and lossy, and the side lobes increasingly see ground at about 290 kelvin instead of cold sky. Both raise the noise temperature and reduce the ratio of gain to system temperature. Gain itself does not change with pointing, polarisation rotation is a separate ionospheric effect, and the physical temperature of the feed is unchanged. This is why satellite earth stations specify a minimum elevation angle.

31.

How does free space path loss vary with distance?

A.It is independent of distance beyond the far field
B.It rises as the square of the distance
C.It rises linearly with distance
D.It rises as the fourth power of distance

Answer: B

Power density falls as the inverse square of range, so path loss rises as the square of distance, twenty decibels per decade or six decibels for each doubling. The fourth power law belongs to a two ray ground reflection model at long range over a plane earth, which is a different situation with a different physical cause. Path loss also rises as the square of frequency at constant gain, which is a consequence of the shrinking effective aperture rather than of anything happening in the space between.

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32.

Free space path loss doubles in distance from 1 km to 2 km. By how much does it increase?

A.12 dB
B.20 dB, which is one decade
C.6 dB
D.3 dB

Answer: C

The inverse square law gives a factor of four in power for a doubling of distance:

\[ 10\log_{10} 4 \approx 6 \]

so the loss rises by 6 dB. Three decibels would be a factor of two and twenty decibels a factor of a hundred, corresponding to a tenfold change in range. Being able to convert quickly between ratios and decibels is the single most useful arithmetic skill in link budgeting, and the six per doubling rule is worth committing to memory.

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33.

What is effective isotropic radiated power?

A.The power actually radiated by the antenna in all directions
B.The power an isotropic antenna would accept at its terminals
C.The power density measured at one kilometre from the antenna
D.Transmitter power multiplied by the antenna gain

Answer: D

EIRP is the product of the power delivered to the antenna and its gain relative to an isotropic radiator, so it states the power an isotropic source would need to produce the same field in the main beam. It is the quantity regulators limit, because it describes what a receiver in the beam actually experiences. Total radiated power is a different figure, smaller by the gain, and the other two options describe no standard quantity.

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34.

Why is antenna gain often quoted in dBi rather than dBd?

A.The isotropic reference is unambiguous and universal
B.The dipole reference gives larger numbers
C.Isotropic antennas are easier to build for comparison
D.Regulators require the dipole reference in all cases

Answer: A

An isotropic radiator is a mathematical reference that is the same everywhere, whereas a dipole reference depends on the dipole's environment and its own efficiency. The two differ by 2.15 decibels, the gain of a half wave dipole over isotropic, so dBi figures are the larger rather than the smaller. No isotropic antenna can be built at all, which is precisely why it is used as an abstraction, and regulations are written in either but usually in dBi.

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35.

A dipole has a gain of 2.15 dBi. What is its gain in dBd?

A.Minus 2.15 dBd
B.0 dBd
C.2.15 dBd
D.4.3 dBd

Answer: B

The dBd scale is referred to a half wave dipole, so a dipole has by definition a gain of 0 dBd, and the 2.15 decibel difference is exactly the dipole's own gain over isotropic. To convert, subtract 2.15 from a dBi figure to get dBd, or add it going the other way. Manufacturers have been known to quote whichever gives the larger number, so the reference should always be checked before two antennas are compared.

Discuss in Forum

36.

What is the reciprocity theorem as applied to antennas?

A.The impedance seen at the feed is the reciprocal of the load
B.Power flows equally in both directions along the feeder
C.The pattern and impedance are the same for transmit and receive
D.The gain on receive is the reciprocal of the gain on transmit

Answer: C

Reciprocity means an antenna's radiation pattern, gain, impedance and polarisation are identical whether it is transmitting or receiving, which is why patterns can be measured in whichever direction is convenient. The exception is any antenna containing a non-reciprocal component such as a ferrite isolator, and power handling obviously differs. The other options misuse the word reciprocal in its arithmetic sense, which is not what the theorem is about.

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37.

What is the effective length of a receiving antenna?

A.The physical length of the radiating conductor
B.The length at which the antenna becomes resonant
C.The distance from the feed point to the current maximum
D.The ratio of open circuit voltage to incident field strength

Answer: D

Effective length is the induced open circuit voltage divided by the incident electric field, so it has units of metres and provides the link between a field strength in volts per metre and a receiver input voltage. For a short dipole it is half the physical length because the current tapers to zero at the ends. Physical length, resonant length and current distribution are related to it but none is its definition, and the distinction matters in field strength measurement.

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38.

What is the antenna factor used in electromagnetic compatibility measurement?

A.The ratio of incident field strength to receiver input voltage
B.The ratio of antenna gain to its physical volume
C.The correction applied for cable loss in a test setup
D.The ratio of radiated to conducted emission from a device

Answer: A

Antenna factor converts the voltage a measuring receiver reports into the field strength that produced it, and is quoted in decibels per metre against frequency on the calibration certificate of every measurement antenna. It is essentially the reciprocal of effective length with the receiver impedance folded in. Cable loss is added separately, and the relation between radiated and conducted emission is a property of the equipment under test.

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39.

What contributes to the noise temperature of a receiving antenna?

A.The transmitted power reflected back from a mismatched load
B.The sky, the ground and any losses in the antenna itself
C.Only the thermal noise of the receiver front end
D.Only the physical temperature of the antenna structure

Answer: B

Antenna noise temperature sums the brightness temperature of everything the pattern sees, sky through the main beam and warm ground through the side lobes and spillover, together with the contribution of ohmic loss in the structure. This is why a satellite dish pointed low at the horizon performs worse than one pointed high, and why side lobe control matters in radio astronomy. Receiver noise is added separately in the system noise temperature.

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40.

Why does pointing a dish antenna closer to the horizon raise its noise temperature?

A.The polarisation rotates and introduces a mismatch
B.The feed becomes physically warmer at low elevation
C.More of the pattern sees warm ground and a longer atmospheric path
D.The gain falls as the elevation angle is reduced

Answer: C

At low elevation the main beam looks through much more atmosphere, which is warm and lossy, and the side lobes increasingly see ground at about 290 kelvin instead of cold sky. Both raise the noise temperature and reduce the ratio of gain to system temperature. Gain itself does not change with pointing, polarisation rotation is a separate ionospheric effect, and the physical temperature of the feed is unchanged. This is why satellite earth stations specify a minimum elevation angle.

Discuss in Forum

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