Your Answer : (Not Answered)

Correct Answer : B

Trapping in the duct carries the signal along the surface with little spreading loss, so targets are detected at ranges far beyond line of sight, though the coverage above the duct is correspondingly reduced. Both effects matter operationally. Absorption, backscatter and depolarisation are separate phenomena. A mechanism that gives more range in one region at the cost of a hole in another is a trade rather than a pure benefit, and it must be understood as such.

Your Answer : (Not Answered)

Correct Answer : D

A meteor entering the atmosphere leaves a column of ionisation at around a hundred kilometres that reflects signals for a fraction of a second to several seconds, and a burst mode system exploits these fleeting openings for messages over about two thousand kilometres. The solid particle is far too small to reflect radio waves. Dust and cometary material are not the mechanism. A communication system built around events lasting less than a second is a striking example of matching a protocol to a propagation mode.

Your Answer : (Not Answered)

Correct Answer : B

Ionising radiation is strongest high up but there is little gas to ionise, while lower down there is plenty of gas but the radiation has been absorbed, so the production rate peaks at an intermediate height. Different wavelengths and different constituents peak at different heights, which gives the several layers. Gravitational separation does occur but is not the layer mechanism, and the layers are physically real. A product of two opposing trends peaking in between is the classic Chapman argument.

Your Answer : (Not Answered)

Correct Answer : A

Medium wave broadcasting depends on the surface wave for its stable daytime service area, which is why masts are vertical and ground systems elaborate. Satellite links are free space paths, microwave links are space wave, and long distance high frequency working is sky wave. Identifying the mechanism a service depends on tells you at once what will affect it: ground conductivity for the first, rain for the second, terrain for the third and the ionosphere for the fourth.

Your Answer : (Not Answered)

Correct Answer : C

The space wave is the vector sum of the direct path and the specularly reflected path, and it dominates whenever both antennas are elevated and the frequency is high. The surface wave is a separate guided component, ionospheric return is the sky wave, and diffraction is a further mechanism beyond the horizon. Line of sight microwave and mobile links are space wave propagation. Naming the mechanism correctly determines which model and which variables apply.

Your Answer : (Not Answered)

Correct Answer : D

Cross polar radiation grows away from the axis in almost every aperture antenna, because the curvature of the reflector or the geometry of the feed generates it, so a rise off boresight is expected rather than alarming. Specifications therefore state the level over a stated angular region. Alignment errors, range reflections and feed faults produce different and usually less systematic signatures. Knowing what a healthy measurement looks like is a prerequisite for recognising a faulty one.

Your Answer : (Not Answered)

Correct Answer : B

A flat slab of lossy material would present an abrupt change of impedance and reflect strongly, whereas a pyramid grades the effective impedance from that of air to that of the absorber over several wavelengths. Performance therefore degrades below the frequency at which the pyramids are a wavelength or so long. Surface area, scattering and strength are incidental. Gradual transitions to avoid reflection is the same idea that shapes a horn and a tapered transmission line.

Your Answer : (Not Answered)

Correct Answer : A

Pyramidal or wedge absorber presents a gradual transition from air to lossy material, so a wave entering it is attenuated rather than reflected, typically by forty decibels or more. The shielding against outside signals, and the containment of radiation, are provided by the metal walls behind the absorber rather than by the absorber itself. Climate control is a separate service. Separating the shielding function from the anechoic function clarifies what each part of the chamber does.

Your Answer : (Not Answered)

Correct Answer : D

Tissue is a lossy dielectric close to the radiating structure, so it shifts the resonance and absorbs a substantial fraction of the power, and losses of several decibels are common. Designs are evaluated with standardised hand and head phantoms for this reason. A hand is a poor ground plane and a good absorber, the plastic case provides no shielding at all, and polarisation is set by the current direction. Anything within a fraction of a wavelength becomes part of the antenna, and a hand is no exception.

Your Answer : (Not Answered)

Correct Answer : B

The first is correct and the second is exactly wrong. The centre is a current maximum but a voltage null, and it is the fringing field at the voltage maxima on the two open ends that escapes and radiates. Modelling those two regions as slots half a wavelength apart predicts the broadside pattern correctly. Confusing where the current is largest with where the radiation comes from is a common error and it leads to quite wrong conclusions about feed placement.

Your Answer : (Not Answered)

Correct Answer : D

The half wave standing wave puts a voltage maximum and current null at each open end and the reverse at the centre, so the ratio of voltage to current, which is the resistance, falls from hundreds of ohms to zero. Feeding anywhere between selects an intermediate value. Substrate thickness and ground plane spacing are uniform, and the centre does not radiate much because the fields there do not fringe. Locating the voltage and current maxima explains the impedance behaviour of every resonant structure.

Your Answer : (Not Answered)

Correct Answer : A

The subreflector stands directly in front of the vertex, so the central region of the main reflector is shadowed and contributes nothing, which is the blockage loss counted in the efficiency budget. Illumination does reach the centre on transmit but is intercepted, surface accuracy is generally best near the vertex, and the equal path property of the paraboloid holds everywhere. A loss that is already being paid can sometimes be turned to convenience, and the vertex hole is a good example.

Your Answer : (Not Answered)

Correct Answer : C

The common azimuth over elevation arrangement turns the whole structure about a vertical axis and tilts the dish about a horizontal one carried on the turntable. It is mechanically simple and gives full sky coverage except for a keyhole at the zenith, where the azimuth rate would become infinite. Polar mounts follow the sky without that problem but are harder to build large. Feed steering and sideways tilting describe other arrangements used in special cases.

Your Answer : (Not Answered)

Correct Answer : C

Near the peak of a sum beam the output barely changes with angle, whereas the difference channel passes through zero and reverses phase, so a small angular error produces a large and signed output. Dividing by the sum channel removes the dependence on signal amplitude, which the difference channel alone does not have. Its gain on boresight is zero rather than high, and its lobes are wider than the sum beam. Measuring against a null rather than a peak is a recurring theme wherever precision is needed.

Your Answer : (Not Answered)

Correct Answer : C

The field from a neighbour falls with distance, so at a quarter wavelength the induced current is a large fraction of the driven one and the impedance shifts markedly. At a full wavelength it is much weaker. Feed length depends on the layout, resonant frequency is a property of the element, and grating lobes appear at wide rather than close spacing. Coupling is the reason an array cannot be designed one element at a time.

Your Answer : (Not Answered)

Correct Answer : D

Because scan loss and grating lobes limit a face to about sixty degrees either side of its normal, three or four faces are used to cover the full circle. A single face cannot reach ninety degrees usefully since the projected aperture goes to zero, and two faces would leave large gaps at the seams. Six would be wasteful. The economics of face count against scan capability is a central decision in radar system design.

Your Answer : (Not Answered)

Correct Answer : D

A grating lobe exists mathematically but only becomes visible when its direction falls within real space, and steering the beam pushes it towards the horizon. An array that never scans far can therefore use spacing up to nearly a full wavelength and use fewer elements for a given aperture. Coupling does not depend on the beam direction in that way, spacing certainly enters the array factor, and the element pattern attenuates rather than suppresses. Checking the units of each quantity catches a wrong formula before the arithmetic begins.

Your Answer : (Not Answered)

Correct 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.

Your Answer : (Not Answered)

Correct Answer : D

A counterpoise is a set of conductors raised above the earth to provide the return path for a monopole, avoiding the loss of a poor ground. Even a few elevated radials can outperform many buried ones. Mechanical counterweights, parasitic elements and capacitive top hats are all different devices, the last being a way of raising the current in a short whip rather than providing a return path. Managing the return current is half the design of any vertical antenna.

Your Answer : (Not Answered)

Correct Answer : A

An infinite biconical structure is defined entirely by angles, so it presents the same impedance at every frequency, and a truncated practical version is broadband over many octaves. Its pattern resembles a fat dipole, broadside with axial nulls, rather than a narrow beam. It radiates linear polarisation and needs a feed like anything else. The idea that a structure specified only by angles is frequency independent also underlies the log periodic and spiral antennas.