Antenna Fundamentals and Parameters Questions and Answers

Answer: A

A dipole is balanced while coaxial cable is not, so without a balun current flows on the outside of the coaxial braid, which radiates, distorts the pattern and makes the measured impedance depend on the feeder length. Some baluns also transform impedance, a four to one type for instance, but that is an additional function rather than the definition. Lightning protection and polarisation are unrelated. Feeder radiation is one of the commonest causes of a pattern that does not match prediction.

Answer: B

If the outside of the braid is radiating, it forms part of the antenna, so altering the feeder length alters the whole structure and with it the impedance seen at the transmitter. That dependence on cable length is the classic diagnostic and it is cured by a balun or by ferrite chokes. Gain change, a perfect match and polarisation reversal are not signatures of common mode current, and a suspiciously perfect match usually points to a lossy cable instead.

Answer: C

The folded arrangement carries the same total current shared between two conductors, and the voltage is doubled, which raises the impedance by a factor of four to about 300 ohms. That matches ribbon feeder directly and, with a four to one balun, matches fifty ohm coaxial cable. It also gives a wider bandwidth than a plain dipole, which is why it is the driven element of most Yagi arrays. The factor is exactly four for two equal conductors and rises further if their diameters differ.

Answer: D

A perfectly conducting plane produces an image of the monopole beneath it, so the fields above the plane are those of a full dipole. Because only half the space is illuminated, the radiation resistance is half that of the dipole, about 36 ohms, and the directivity is doubled. The plane carries induced current but does not itself resonate or radiate independently. The image principle is one of the most useful shortcuts in antenna analysis.

Answer: A

Radiating into a hemisphere rather than a sphere halves the resistance of the equivalent dipole, giving about 36 ohms. The convenient consequence is a reasonable match to fifty ohm cable without any transformer. Real ground is not perfect, so loss in the earth adds to this and a radial system is laid to reduce it. The half wave dipole figure of 73 ohms is the one being halved, so quoting it here is the natural trap.

Answer: B

Return current flows through the earth, which is a poor conductor, so a system of buried or elevated radials carries that current in copper instead and can raise efficiency from a few per cent to most of the way. Broadcast installations use a hundred and twenty quarter wave radials for that reason. Height, bandwidth and polarisation are governed by the element itself. It is a good illustration of loss resistance being just as important as radiation resistance.

Answer: C

Below resonance a dipole or monopole looks strongly capacitive, so a series loading coil is added to cancel the reactance and bring the antenna to resonance. That is why mobile whips carry a visible coil. The coil has loss of its own, which combines with a small radiation resistance to give poor efficiency and a very narrow bandwidth. A reactance is by definition not resistive, and only at resonance does it vanish.

Answer: D

The first is correct: a series inductance cancels the capacitive reactance and produces resonance at a frequency far below the natural one. The second is the reverse of the truth, since loading raises the stored to dissipated energy ratio and so narrows the bandwidth sharply. The fundamental limit relating size, bandwidth and efficiency is the Chu Harrington limit, and it says plainly that a small antenna cannot be both broadband and efficient however cleverly it is matched.

Answer: A

The limit states that an antenna confined within a sphere of a given radius has a minimum quality factor, so bandwidth and efficiency cannot both be large when the sphere is small compared with a wavelength. It explains why mobile telephone and buried antennas are always a compromise. Maximum gain from an aperture is a separate result, power handling is a breakdown question, and noise temperature depends on what the pattern sees.

Answer: B

Aperture efficiency is the ratio of effective to physical area, and it accounts for illumination taper, spillover past the reflector, blockage by the feed and surface errors. A typical prime focus dish reaches fifty to sixty per cent and a well designed Cassegrain rather more. Radiated over accepted power is radiation efficiency, gain over directivity is the same thing, and the last option describes beam efficiency, which matters in radiometry.