Waveguide Components Questions and Answers

Answer: A

The key marks both statements true with the reason explaining the assertion. A magic tee is a four-port hybrid junction that combines the E-plane and H-plane tees, so it divides power in both planes, and when its ports are matched it has the advantage that nothing is reflected back. The hybrid junction of the magic tee is formed about an imaginary plane that bisects two of its arms, and that symmetry produces the isolation between the sum and difference ports and the power division. The answer is that both are true and (R) is the correct explanation of (A).

Answer: D

The klystron operates on velocity modulation.

In a klystron, the electron beam passes through the gap of an input (buncher) cavity, where the RF field speeds up some electrons and slows down others. Over the drift space the fast electrons catch up with the slow ones, so the beam forms bunches: velocity modulation is turned into current (density) modulation. The bunched beam then gives energy to the RF field in the output (catcher) cavity, producing amplification, or in a reflex klystron, oscillation.

Amplitude, frequency and pulse modulation are ways of impressing information on a carrier, not the mechanism by which a klystron works.

Answer: B

The duplexer in a RADAR system is a microwave switch.

A radar using one antenna for both transmitting and receiving needs a duplexer to connect the antenna to the transmitter during the high-power pulse and to the receiver during the listening time between pulses, while protecting the sensitive receiver from the transmitted pulse. It does this with switching devices such as TR (transmit-receive) and ATR tubes, ferrite circulators or PIN diode switches.

It generates no signal, as an oscillator would, does not amplify, and is not an active filter.

Answer: D

The increasing order is (i), (ii), (iii), (iv): L, S, C, X.

1. L band: 1-2 GHz.
2. S band: 2-4 GHz.
3. C band: 4-8 GHz.
4. X band: 8-12 GHz.

These are the IEEE radar band designations, continuing upwards with Ku, K and Ka. Every other option starts with a higher band or puts X before C.

Answer: D

The increasing order is S23, S33, S12, S13, which is option D: (c), (d), (a), (b).

For the ideal lossless E-plane (series) tee, with port 3 the arm that joins the broad wall, the standard scattering matrix is:

\[ S = \begin{bmatrix} 1/2 & 1/2 & 1/\sqrt{2} \\ 1/2 & 1/2 & -1/\sqrt{2} \\ 1/\sqrt{2} & -1/\sqrt{2} & 0 \end{bmatrix} \]

So S23 = -0.707 (S13 and S23 are 180 degrees out of phase), S33 = 0 (the side arm is matched), S12 = 0.5 and S13 = 0.707, giving -0.707 < 0 < 0.5 < 0.707.

Options A and C put S33 or S12 below S23 and so ignore its negative sign. Option B puts S23 last, which is its order by magnitude, not by value. The question rests on this ideal-junction matrix, which is the form textbooks give; the key agrees with it.

Answer: A

The forward-biased model is an inductor in series with a resistor.

Under forward bias the intrinsic region of a PIN diode is flooded with injected carriers, so it behaves as a small resistance R_f that falls as the bias current rises. In series with it is the package and lead inductance L. That simple series L-R circuit is the forward-bias microwave model, which is why PIN diodes are used as current-controlled resistors in switches and attenuators.

The third option, lead inductance in series with a parallel resistance and capacitance, is the reverse-bias model, in which the depleted I-region acts as a capacitor with a large parallel resistance. The fourth, a capacitor in series with a resistor, is a simplified reverse-bias model, and the second, a resistor across the inductor, is not the standard model for either bias.

Answer: C

The transferred electron device is the Gunn diode.

In gallium arsenide, electrons in a strong field transfer from a low-energy, high-mobility valley of the conduction band to a higher, low-mobility valley. Above a threshold field the average velocity therefore falls as the field rises, a negative differential resistance, and the Gunn diode uses it to generate microwave oscillations.

The klystron, the travelling-wave tube and the magnetron are vacuum tubes. They work by velocity modulation and bunching of an electron beam, or by electrons moving in crossed electric and magnetic fields, not by transferred electrons in a semiconductor.

Answer: D

Faraday rotation is used in (1) and (2), circulators and isolators.

In a magnetised ferrite the plane of polarisation of a wave rotates by an amount that does not depend on the direction of travel, so the effect is non-reciprocal. A Faraday rotation isolator uses this to pass a wave in one direction and absorb it in the other, and a Faraday rotation circulator routes power from each port to the next in one rotational direction only.

The directional coupler and the magic tee are reciprocal passive junctions made of ordinary waveguide. They need no ferrite and do not use Faraday rotation, so any option including (3) or (4) is wrong.

Answer: B

The correct matching is a-ii, b-i, c-iv, d-iii.

• Klystron: electrons are velocity-modulated and bunched, and their kinetic energy is converted into RF energy, giving RF amplification (ii).
• Magnetron: a crossed-field oscillator that runs from a DC anode supply and converts that DC power into microwave power (i).
• Gunn diode: a transferred-electron device used as an oscillator in the range of about 10 GHz up towards 1 THz (iv).
• PIN diode: works by high-level injection of carriers into its intrinsic region, which makes it a current-controlled resistor at microwave frequencies (iii).

Only one code pairs the klystron with RF amplification by kinetic energy, its defining principle.

Answer: B

The matching is a-iv, b-iii, c-ii, d-i. Ferrite devices such as circulators and isolators are the non-reciprocal components of microwave systems. A magnetron is a cross field device, since its electric and magnetic fields are at right angles. A klystron works by velocity modulation that leads to bunching of the electrons. A Gunn diode is used as a microwave oscillator. Option (A) gives the magnetron as bunching. Option (C) gives the ferrite device as a cross field device. Option (D) lists the pairs in plain order, which does not fit. Only (B) is right.