Aperture Antennas Questions and Answers

Answer: C

Between the reactive near field and the far field lies the Fresnel or radiating near field, where power flows outward but the pattern has not settled into its final shape. Near field scanners work in it and transform the result. The far field is Fraunhofer, the storage zone is reactive, and the focal region is a separate idea belonging to focused apertures. Naming the three regions and knowing what changes between them prevents most confusion about where a measurement is valid.

Answer: D

The aperture gain relation makes gain proportional to effective area and inversely proportional to the square of the wavelength, so both a larger dish and a higher frequency raise it quadratically. The other options get the powers or the direction of the wavelength dependence wrong. Because the relation ties an electrical quantity to a physical one, it is the bridge between the mechanical design of a structure and the link budget it has to satisfy, and it is used more often than any other formula in this field.

Answer: A

Doubling the diameter quadruples the area and so raises the gain by six decibels, while halving the beamwidth in both planes. The three decibel answer would apply to doubling the area rather than the diameter, which is a very common slip. Nothing compensates, and the twelve decibel figure double counts the two planes that are already contained in the area. Being clear whether a change is applied to a linear dimension or to an area prevents most errors of this kind.

Answer: B

The criterion contains the square of the aperture, so doubling the diameter quadruples the distance at which the far field begins. This is why measuring a large antenna outdoors becomes impossible so quickly and why near field scanning was developed. The wavelength does enter, but it is unchanged here, and a narrower beam does not bring the far field closer. Quadratic scaling is what turns a manageable problem into an impossible one over a modest range of sizes.

Answer: C

The dominant rectangular waveguide mode has a half sine amplitude distribution across the broad wall, and the flare carries that distribution to the aperture, giving a natural taper in one plane and a near uniform distribution in the other. That asymmetry is why a simple pyramidal horn has different beamwidths and side lobes in the two planes. Wall absorption is negligible, the probe is centred, and diffraction acts beyond the aperture rather than within it.

Answer: D

One plane carries the tapered half sine distribution of the guide mode and the other a nearly uniform one, so the beamwidths and side lobe levels differ even when the aperture is square. Corrugated and dual mode horns exist precisely to equalise them. Physical length affects the phase error rather than the amplitude asymmetry, polarisation is fixed by the mode, and measurement distance is the same for both planes. Tracing a pattern asymmetry back to an aperture asymmetry is the standard line of reasoning.

Answer: A

Because the wave in a horn spreads spherically, the path from the apex to the aperture edge is longer than to its centre, and that difference grows with the flare angle, producing a quadratic phase error that broadens the beam and fills the nulls. An optimum horn balances that error against the aperture size, which is why gain does not rise indefinitely with flare. The other options misattribute the effect to amplitude or to matching alone.

Answer: B

An optimum horn is proportioned so that the gain from the enlarged aperture just balances the loss from the quadratic phase error, usually at a permitted edge phase error of about a quarter or half a wavelength. Making it longer for the same mouth would reduce the phase error but is often impractical. Perfect matching, equal beamwidths and zero phase error are desirable properties achieved by other means. Optimising a product of two competing effects is a pattern seen throughout aperture design.

Answer: C

A pyramidal horn's geometry is simple enough that its gain can be calculated to within a few tenths of a decibel from its measured dimensions, which makes it a primary standard for gain measurement by substitution. Its gain is modest rather than the highest available and does vary with frequency in a known way, and it is not especially cheap. An antenna whose performance can be predicted from measurable mechanical quantities is exactly what a standard needs to be.

Answer: D

Measuring the transmission between each of the three possible pairs gives three equations from which all three antenna gains follow, with no calibrated standard needed. It is the route by which a primary gain standard is established in the first place. Pattern measurement, selection testing and range validation are all different procedures. A measurement scheme that bootstraps absolute values out of relative ones is an elegant and widely reused idea.