Antenna Fundamentals and Parameters Questions and Answers

Answer: A

Halving the power is a loss of three decibels, so:

\[ G = 20 - 3 = 17 \]

giving 17 dBi. Efficiency can only reduce gain below directivity, never raise it, which immediately rules out the largest option. Doing the arithmetic in decibels rather than in ratios is quicker here, and remembering that a factor of two is three decibels covers a surprising proportion of the calculations that arise in practice.

Answer: B

A cable with high loss attenuates the reflected wave twice, so a badly matched or even disconnected antenna can present a flattering standing wave ratio at the transmitter end. Measuring at the antenna, or correcting for known cable loss, settles it quickly. Polarisation mismatch is a real possibility and worth checking next, meter error is less likely, and excessive gain is not a fault that presents this way. The general lesson is that a match measured at the wrong end tells you little.

Answer: C

Boresight is the direction of the peak of the main beam, the axis a directional antenna is pointed along. It should coincide with the mechanical axis but often does not, and the difference is specified as boresight error, which matters greatly in tracking and radar systems. Nulls, mounting geometry and polarisation orientation are separate matters, though a mechanical reference is normally provided so that the electrical boresight can be set from a survey.

Answer: D

Steering by phase rather than by true time delay means the beam points where the phase gradient matches the required delay, and since that delay is frequency dependent the beam direction moves as frequency changes. Wideband arrays therefore use true time delay steering. Element resonance, mutual coupling and feed loss all vary with frequency too, but none of them produces the systematic squint that the phase versus delay distinction does.

Answer: A

Elements close together illuminate one another, so the current on each depends on its neighbours and the driving point impedance changes with the array environment. It is why an element measured in isolation behaves differently once in an array, and why array design must solve for all elements together. Cable crosstalk is a separate and usually smaller effect, and shared feed impedance or common resonance are design choices rather than a coupling mechanism.

Answer: B

The voltage at each terminal now includes contributions induced by every other element, so the ratio of voltage to current, which is the driving point impedance, shifts and may become quite different from the isolated value. In a closely spaced array it can even go negative in the real part for a parasitic element. Feeder length, array gain and feeding arrangement are all separate variables. This is the practical reason arrays are matched as built rather than element by element.

Answer: C

At certain scan angles, a surface wave supported by the array structure couples strongly and the active reflection coefficient approaches unity, so almost no power is radiated. It is a resonance of the whole periodic structure rather than a failure of any element. Grating lobes are a distinct problem caused by element spacing exceeding about half a wavelength, and physical obstruction and feed cancellation are not the mechanism. Both effects limit the usable scan volume.

Answer: D

When the elements are spaced too widely, a second direction exists in which the contributions add in phase, producing a full sized lobe away from the intended beam. Keeping the spacing at or below half a wavelength avoids them for all scan angles, though wider spacing is tolerable if the scan range is limited. Amplitude taper affects side lobes rather than grating lobes, and feed loss and small phase errors degrade performance without producing a second beam.

Answer: A

Random errors scatter power out of the main beam, so gain falls and the scattered energy appears as a raised side lobe floor, the relationship being given by the Ruze equation for surface errors on a reflector. A systematic linear phase gradient is what steers a beam; random errors do not. Polarisation and bandwidth are affected by other mechanisms. Surface tolerance is therefore specified as a fraction of a wavelength, which is what makes millimetre wave reflectors so demanding to build.

Answer: B

A given physical bump produces a path error that is a larger fraction of a cycle at shorter wavelengths, so the same dish is excellent at one gigahertz and useless at one hundred. The usual criterion is a root mean square error below about one sixteenth of a wavelength. Thermal expansion is a cause of error rather than the reason for the units, ease of statement is not a physical argument, and feed illumination is a separate design variable.