Radiation from a Dipole Questions and Answers

Answer: C

Equal phase means the contributions add broadside, where the path lengths are equal, and cancel along the array axis, where the half wavelength spacing gives a hundred and eighty degree path difference. This is the broadside array. Feeding the same pair in antiphase reverses the situation and gives an end fire pattern. No linear current distribution radiates along a dipole axis, so the last option is impossible in any case.

Answer: D

The quarter wave path difference and the ninety degree feed phase reinforce in one axial direction and cancel in the other, giving a unidirectional end fire pattern with a front to back ratio limited only by the accuracy of the amplitudes and phases. This is the basis of the cardioid pattern used in direction finding and of the two element phased array. Reversing the phase reverses the direction of the beam, which is how such arrays are switched.

Answer: A

The array factor is the pattern that a set of isotropic sources at the same positions and phases would produce, and the total pattern is that factor multiplied by the element pattern. This pattern multiplication principle makes array design tractable: the geometry and the element are designed almost independently. Current ratio and mutual impedance are inputs to the calculation rather than the factor itself, and gain does not simply multiply by element count.

Answer: B

For an array of identical, identically oriented elements, the resultant pattern is the product of the single element pattern and the array factor. It is the reason a Yagi's pattern can be understood as a dipole pattern shaped by the parasitic array geometry. Gains do not simply add, beamwidth narrows in a way that depends on the aperture rather than the count alone, and impedance is altered by mutual coupling rather than by multiplication.

Answer: C

The driven element illuminates the parasite, inducing a current whose magnitude and phase depend on the parasite's length and spacing, and that current re-radiates. Making it slightly longer than resonance produces a reflector and slightly shorter a director. It has no electrical connection to the feeder, does not oscillate on its own, and the mirror analogy is loose at best since the element is far smaller than the wave it redirects.

Answer: D

A parasite longer than resonance is inductive, so its re-radiated field lags and the combination reinforces on the far side, making it a reflector; a shorter, capacitive parasite acts as a director. One reflector is usually enough because the field behind the driven element is already weak, while several directors continue to add gain. Nothing is connected to the feed but the driven element, and all elements lie parallel.

Answer: A

End effects and the reduced characteristic impedance of a fat element mean the resonant length falls further below a half wavelength as the diameter grows, from about five per cent for a thin wire to considerably more for a tubular element. Radiation resistance changes only slightly, current still vanishes at the ends whatever the diameter, and mechanical sag is a construction matter. The shortening factor is tabulated against length to diameter ratio for this reason.

Answer: B

The dielectric slows the wave along the conductor a little, so an insulated wire resonates at a slightly shorter physical length than a bare one, typically by two or three per cent for ordinary plastic covering. The effect is small but enough to matter when an antenna is cut to a calculated length and then found low in frequency. Nothing in the geometry doubles, and the claim of no effect ignores a real if modest shift that every practitioner meets.

Answer: C

Charge accumulating at the tips adds a small capacitance that makes the element behave as though it were slightly longer, so it must be cut physically shorter to resonate. Nearby insulators, supports and end caps all increase it. Current is at a minimum rather than a maximum at the ends, so loss there is small; pattern lobes come from length in wavelengths; and breakdown is a genuine risk at high power but is a separate matter from the shortening.

Answer: D

Where a standing wave of current has a node, the standing wave of voltage has an antinode, so the ends carry the highest potential and are the places most likely to arc at high power. The feed point at the centre is the opposite, a current maximum and a voltage minimum. Radiation resistance is not defined per unit length, and insulation responds to the voltage rather than causing it. This is why end insulators must be rated generously in transmitting antennas.