Difficulty: Medium
A rectangular air guide has broad dimension 3 cm. What is its TE10 cutoff?
A 2.5 GHz
B 10 GHz
C 5 GHz
D 20 GHz
Answer: C. 5 GHz. For TE10, fc = c/(2a) = 3 × 10^8/(0.06) = 5 GHz.
At f = 2fc, a wave has free-space wavelength 5 cm. What is guide wavelength?
A 2.50 cm
B 5.77 cm
C 4.33 cm
D 10.0 cm
Answer: B. 5.77 cm. λg = λ0/sqrt(1-(fc/f)^2) = 5/sqrt(0.75) cm.
At twice cutoff, phase velocity in an ideal waveguide is approximately what?
A 0.500c
B 1.155c
C 0.866c
D 2.000c
Answer: B. 1.155c. vp = c/sqrt(1-(fc/f)^2); setting f = 2fc gives 1.155c.
For the same ideal guide at twice cutoff, group velocity is approximately what?
A 0.866c
B 0.500c
C 1.155c
D 2.000c
Answer: A. 0.866c. vg = c sqrt(1-(fc/f)^2), giving 0.866c at twice cutoff.
A cavity resonates at 10 GHz with 100 MHz bandwidth. What is its loaded Q?
A 10
B 100
C 50
D 1000
Answer: B. 100. Loaded Q is resonant frequency divided by bandwidth.
A load has reflection-coefficient magnitude 0.1. What is return loss?
A 10 dB
B 1 dB
C 20 dB
D 40 dB
Answer: C. 20 dB. Return loss is -20 log10(|Γ|), which equals 20 dB.
What VSWR results from a reflection-coefficient magnitude of 0.5?
A 1.5:1
B 2:1
C 5:1
D 3:1
Answer: D. 3:1. VSWR = (1+|Γ|)/(1-|Γ|) = 1.5/0.5 = 3.
If |Γ| equals 0.2, what fraction of incident power is reflected?
A 2%
B 4%
C 20%
D 40%
Answer: B. 4%. Reflected power fraction is |Γ|² = 0.04.
A coupler receives 20 dBm and its coupled port reads -10 dBm. What is coupling factor?
A 10 dB
B 20 dB
C 30 dB
D 40 dB
Answer: C. 30 dB. Coupling factor is input power minus coupled-port power.
A coupler’s coupled port reads -15 dBm and isolated port -45 dBm. What is directivity?
A 30 dB
B 15 dB
C 45 dB
D 60 dB
Answer: A. 30 dB. Directivity is the coupled-port to isolated-port power ratio in dB.
An RF signal is 10.0 GHz and the local oscillator is 9.3 GHz. What is the difference IF?
A 70 MHz
B 9.3 GHz
C 19.3 GHz
D 700 MHz
Answer: D. 700 MHz. A mixer produces sum and difference frequencies; 10.0 – 9.3 = 0.7 GHz.
With a 10 GHz RF signal and a 9 GHz LO, which mixer product is the sum frequency?
A 1 GHz
B 19 GHz
C 9 GHz
D 10 GHz
Answer: B. 19 GHz. The sum product is the sum of RF and LO frequencies.
A matched two-port has S21 = -6 dB. About what fraction of input power is transmitted?
A 6%
B 50%
C 25%
D 75%
Answer: C. 25%. -6 dB is close to a power ratio of 0.25.
A two-port’s S11 is -20 dB. About what fraction of incident power is reflected?
A 0.1%
B 10%
C 20%
D 1%
Answer: D. 1%. |S11| is 0.1 in voltage, so reflected power is 0.1² = 1%.
Why must unused VNA ports be terminated during a reflection measurement?
A Prevent extra reflections
B Increase source frequency
C Create a standing wave
D Boost device gain
Answer: A. Prevent extra reflections. A mismatched unused port can send reflections back into the device under test.
Moving the VNA calibration plane to the DUT terminals removes what?
A Device noise
B Carrier frequency
C Fixture effects
D Antenna polarization
Answer: C. Fixture effects. Calibration at the reference plane accounts for intervening cables and adapters.
A resonator has Q = 200 at 4 GHz. Approximate its 3 dB bandwidth?
A 20 MHz
B 2 MHz
C 200 MHz
D 800 MHz
Answer: A. 20 MHz. Bandwidth is f0/Q = 4 GHz/200 = 20 MHz.
An ideal 1:4 power divider splits input equally. Each output is lower by about what?
A 3 dB
B 9 dB
C 12 dB
D 6 dB
Answer: D. 6 dB. Each branch receives one quarter power, giving 10 log10(4) ≈ 6 dB loss.
What power is represented by +10 dBm?
A 1 mW
B 100 mW
C 10 mW
D 10 W
Answer: C. 10 mW. Each 10 dB step is a factor of ten; 0 dBm is 1 mW.
In a klystron, bunching occurs after electrons are initially what?
A Velocity modulated
B Polarization rotated
C Frequency doubled
D Magnetically trapped
Answer: A. Velocity modulated. An RF gap changes electron velocities, leading faster electrons to catch slower ones.
Why can a TWT provide wide microwave bandwidth?
A One narrow cavity only
B Distributed beam-wave interaction
C No electron beam
D Only passive resonance
Answer: B. Distributed beam-wave interaction. Interaction along a slow-wave structure supports amplification over a broad band.
A quarter-wave transformer is used to match which kind of load?
A Resistive mismatch
B Purely radiating source
C DC battery
D Open optical fibre
Answer: A. Resistive mismatch. A λ/4 section transforms one real impedance to another at its design frequency.
At 10 GHz, a substrate has effective permittivity 4. What is a quarter guided wavelength?
A 1.5 mm
B 7.5 mm
C 15 mm
D 3.75 mm
Answer: D. 3.75 mm. λg ≈ λ0/sqrt(εeff) = 30/2 = 15 mm; one quarter is 3.75 mm.
An ideal mixer driven by two tones creates which basic products?
A Sum and difference
B Only the average
C Only the input tones
D DC only
Answer: A. Sum and difference. Multiplication of sinusoids produces sum- and difference-frequency components.
A receiver uses a 10.5 GHz LO for a 10 GHz signal and 500 MHz IF. What is the image frequency?
A 9 GHz
B 10.5 GHz
C 11.5 GHz
D 11 GHz
Answer: D. 11 GHz. The other RF frequency 500 MHz from the LO is 11 GHz, and also converts to the same IF.