Radiation from a Dipole Questions and Answers

Answer: A

A null is a direction of minimum or zero radiation, produced where contributions from different parts of the antenna cancel. Nulls are as useful as maxima: a loop's null gives a bearing, and an array null can be steered onto an interferer. The word describes a direction rather than a frequency or an impedance, and confusing the pattern domain with the circuit domain is a frequent source of muddle.

Answer: B

Wavelength is the wave speed divided by the frequency, so doubling the frequency halves the wavelength and with it every antenna dimension expressed in metres. A dipole for 300 MHz is half the length of one for 150 MHz. The inverse relationship is the reason antenna sizes shrink so dramatically as one moves up the spectrum, and it underlies almost every practical consequence in this topic.

Answer: C

The half wave dipole's directivity of 1.64 converts to 2.15 dBi, and since the antenna is essentially lossless its gain is the same figure. Zero dBi would be an isotropic radiator, three decibels corresponds to a ratio of two, and quoting 1.64 as a decibel value confuses the ratio with its logarithm. That last confusion is common enough to be worth guarding against explicitly.

Answer: D

At resonance the reactance vanishes, so the feed sees a pure resistance and no matching network is needed, which is the whole convenience of the arrangement. Resonance does not change the pattern, and a non-resonant antenna properly matched can be just as efficient, as travelling wave designs demonstrate. Resonant elements are usually longer rather than shorter than the loaded alternatives. Ease of matching is the reason, and it is a practical rather than a physical one.

Answer: A

The name is a contraction of balanced to unbalanced, and the device allows a symmetric antenna such as a dipole to be fed by coaxial cable without current flowing on the outside of the braid. Some types also transform impedance. It has nothing to do with joining antennas, with mechanical mounting or with power supplies. Omitting one is among the commonest causes of a pattern and an impedance that will not match prediction.