Radiation from a Dipole Questions and Answers

Answer: A

Because the current tapers from a maximum at the feed to zero at each end, the average is half the peak and the effective length is half the physical length. For a half wave dipole the sinusoidal distribution gives an effective length of lambda over pi, about 0.32 wavelengths against a physical 0.5. The quantity matters because it converts an incident field strength into a terminal voltage, which is the basis of every field strength measurement that is ever made.

Answer: B

The effective length is half the physical length for a short dipole, so:

\[ V = 10 \times 0.5 = 5 \]

millivolts, that is 5 mV open circuit. Loading the antenna with a matched receiver halves it again at the terminals. Effective length is a purely geometric factor at this stage and does not involve loss, which enters later through the efficiency. This calculation is the first line of any field strength measurement.

Answer: C

Effective length converts a field strength into a voltage while effective aperture converts a power density into a power, so they express the same physical collecting ability in different variables and are connected through the antenna and load impedances. Neither is the square of the other in general. Both apply on receive, and by reciprocity both have transmit counterparts. Choosing between them is a matter of whether the problem is posed in fields or in powers.

Answer: D

Any dielectric shell refracts and partly reflects the wave, so a small loss and some pattern distortion are unavoidable, though careful choice of thickness relative to the wavelength minimises the reflection. Nothing is truly transparent at radio frequencies. Polarisation is preserved unless the radome is deliberately made anisotropic, and no arrangement reverses the pattern. The design question is always how much degradation is acceptable for the protection gained.

Answer: A

The ends are voltage antinodes, so a lossy insulator or a wet support there dissipates power and may flash over. Long high quality insulators or end capacitance hats are used to manage it. Current is minimal at the ends so heating from current is not the concern, radiation resistance is a terminal property affected only slightly, and polarisation depends on orientation. Knowing where the voltage and current maxima fall predicts most of the mechanical design of a transmitting antenna.

Answer: B

The first two are correct and are the standard facts about the antenna. The third is the reverse of the truth: the axial direction is a null, and the maximum is broadside. Every linear current distribution shares that axial null, because the far field of a current element is proportional to the sine of the angle from the wire. Getting the orientation right is essential before any coverage prediction is attempted.

Answer: C

The dipole's 73 ohms halves to 36.5 for the monopole over a ground plane and quadruples to about 292 for the folded form, while the short dipole falls to a couple of ohms by the square law. Three of the four numbers derive from the first by a simple factor, which is why the dipole value is the one to memorise. The pattern of relationships is more useful than the individual figures.

Answer: D

The folded dipole's four to one transformation puts it near 292 ohms, well above the plain dipole's 73, the monopole's 36.5 and the short dipole's two. High radiation resistance is desirable because it dominates the fixed loss resistance and so gives good efficiency, and it also eases matching to open wire feeder. The ordering follows directly from the geometry in each case rather than from any material property.

Answer: A

The terminal voltage now includes contributions induced by every other element, so the ratio of voltage to current changes and may differ greatly from the isolated value. In a close spaced array a parasitic element can even present a negative resistance component to a neighbour. The feeder is unchanged, radiation resistance is not shared in any simple sense, and the array factor concerns the pattern rather than the impedance. Arrays are therefore matched as built.

Answer: B

At half wavelength spacing the mutual impedance is around minus 13 plus j29 ohms, the negative real part reflecting that the neighbour's field opposes the driving voltage at that separation. Values are tabulated against spacing and vary strongly with it, passing through zero at certain separations. Recognising that mutual impedance can be negative and complex is the key point; the exact figure matters less than the fact that it must always be included in the design.