Antenna Fundamentals and Parameters Questions and Answers

Q: Why does a wet radome degrade a satellite link disproportionately?

A continuous film of water absorbs strongly and reflects part of the wave, adding several decibels of loss precisely when rain attenuation on the path is already high. Hydrophobic coatings and heaters are used to break the film up. The focal length is a geometric property of the reflector, no short circuit occurs, and physical temperature is not the mechanism. The two losses coinciding is what makes the outage statistics worse than either alone would suggest.

Q: What is the main reason antenna specifications are quoted in decibels?

A link may span twelve orders of magnitude, so working logarithmically turns unwieldy products into sums and keeps every figure within a couple of significant digits. Accuracy is unaffected by the notation, regulations use decibels because the industry does rather than the reverse, and a decibel figure is meaningless without its reference, which is why dBi, dBd, dBm and dBW all exist as distinct units. Stating the reference is more important in decibels, not less.

Q: What does dBm express?

A dBm figure is power in decibels above one milliwatt, so zero dBm is one milliwatt and thirty dBm is one watt. Referring to a watt gives dBW, which is thirty decibels lower numerically, gain against a dipole is dBd, and field strength against a microvolt per metre is dBuV/m. Mixing these references is the most frequent arithmetic error in radio engineering, and writing the reference explicitly in every line of a budget is the cure.

101.

Why does a wet radome degrade a satellite link disproportionately?

A.A water film is lossy and reflective at microwave frequencies
B.Water changes the focal length of the reflector
C.Water shorts out the feed to the reflector surface
D.Water raises the physical temperature of the antenna

Answer: A

A continuous film of water absorbs strongly and reflects part of the wave, adding several decibels of loss precisely when rain attenuation on the path is already high. Hydrophobic coatings and heaters are used to break the film up. The focal length is a geometric property of the reflector, no short circuit occurs, and physical temperature is not the mechanism. The two losses coinciding is what makes the outage statistics worse than either alone would suggest.

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102.

What is the main reason antenna specifications are quoted in decibels?

A.Decibels remove the need to state a reference
B.Large ratios become manageable numbers that add rather than multiply
C.Decibels are more accurate than ratios
D.Regulators require decibel notation by law

Answer: B

A link may span twelve orders of magnitude, so working logarithmically turns unwieldy products into sums and keeps every figure within a couple of significant digits. Accuracy is unaffected by the notation, regulations use decibels because the industry does rather than the reverse, and a decibel figure is meaningless without its reference, which is why dBi, dBd, dBm and dBW all exist as distinct units. Stating the reference is more important in decibels, not less.

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103.

What does dBm express?

A.Gain referred to a matched dipole
B.Field strength referred to one microvolt per metre
C.Power referred to one milliwatt
D.Power referred to one watt

Answer: C

A dBm figure is power in decibels above one milliwatt, so zero dBm is one milliwatt and thirty dBm is one watt. Referring to a watt gives dBW, which is thirty decibels lower numerically, gain against a dipole is dBd, and field strength against a microvolt per metre is dBuV/m. Mixing these references is the most frequent arithmetic error in radio engineering, and writing the reference explicitly in every line of a budget is the cure.

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104.

A transmitter delivers 20 W. What is this in dBm?

A.13 dBm
B.20 dBm
C.33 dBm
D.43 dBm

Answer: D

One watt is 30 dBm, and twenty is thirteen decibels above one:

\[ 30 + 10\log_{10} 20 \approx 30 + 13 = 43 \]

so 43 dBm. Building the answer from the familiar anchor of 30 dBm for a watt, and adding ten decibels per factor of ten and three per factor of two, avoids reaching for a calculator. Twenty watts is a common transmitter rating and 43 dBm is worth recognising on sight.

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