Antenna Fundamentals and Parameters Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

Answer: D

The ratio of gain to system noise temperature, quoted in decibels per kelvin, states how well a station can receive weak signals, since carrier to noise ratio is proportional to it. Improving it by raising gain or lowering noise temperature is equally effective, which is why a cooled low noise amplifier can substitute for a larger dish. EIRP is the corresponding transmit figure of merit, and the other two describe polarisation purity and pattern rejection.