Short dipole far-field magnitude varies with angle approximately as?
A |cos θ|
B Constant
C |sin θ|
D sin² θ only
Answer: C. |sin θ|. The short-dipole far-field magnitude is proportional to sin θ measured from its axis.
Half-wave dipole directivity is closest to?
A 1.64
B 1.00
C 2.50
D 4.00
Answer: A. 1.64. A thin half-wave dipole has directivity about 1.64, or 2.15 dBi.
Resonant half-wave dipole radiation resistance is near?
A 18 Ω
B 36.5 Ω
C 300 Ω
D 73 Ω
Answer: D. 73 Ω. The free-space half-wave dipole radiation resistance is approximately 73 ohms.
Quarter-wave monopole over an ideal ground has radiation resistance near?
A 73 Ω
B 36.5 Ω
C 18 Ω
D 150 Ω
Answer: B. 36.5 Ω. The ideal monopole radiates into a hemisphere and has about half the dipole's 73-ohm resistance.
With directivity 5 and radiation efficiency 0.8, linear gain is?
A 2
B 5.8
C 4
D 6.25
Answer: C. 4. Gain equals directivity times radiation efficiency: 5 × 0.8 = 4.
At λ=0.3 m and gain 10, effective aperture is about?
A 0.225 m²
B 0.0716 m²
C 0.00716 m²
D 0.716 m²
Answer: B. 0.0716 m². Aₑ = λ²G/(4π) = 0.09 × 10/(4π), about 0.0716 square metres.
Maximum power transfer requires the receiver load to equal the complex conjugate of which impedance?
A Free-space impedance
B Antenna impedance
C Cable wavelength
D Array spacing
Answer: B. Antenna impedance. For a receiving antenna, maximum power transfer occurs when the load is the complex conjugate of its source impedance.
Isotropic 1 W radiation at 10 m gives power density near?
A 0.796 mW/m²
B 0.0796 mW/m²
C 7.96 mW/m²
D 79.6 mW/m²
Answer: A. 0.796 mW/m². S = P/(4πR²) = 1/(4π·100), about 0.796 milliwatts per square metre.
Medium refractive index n=1.5 at 300 MHz gives wavelength near?
A 0.333 m
B 0.667 m
C 1.50 m
D 0.100 m
Answer: B. 0.667 m. λ = c/(nf); here c/f is 1 metre, then divide by 1.5.
For Hᵣₘₛ=2 mA/m in free space, Eᵣₘₛ is about?
A 0.0754 V/m
B 0.377 V/m
C 1.51 V/m
D 0.754 V/m
Answer: D. 0.754 V/m. Eᵣₘₛ = η₀Hᵣₘₛ; 377×0.002 is about 0.754 V/m.
With μr=1 and εr=4, a lossless dielectric has intrinsic impedance near?
A 377 Ω
B 94.25 Ω
C 754 Ω
D 188.5 Ω
Answer: D. 188.5 Ω. η = η₀√(μr/εr) = 377/2 = 188.5 ohms.
Aperture area 0.5 m² at λ=0.25 m yields ideal directivity near?
A 100.5
B 25.1
C 50.3
D 201
Answer: A. 100.5. D ≈ 4πA/λ² = 4π·0.5/0.0625, about 100.5.
If Umax=12 W/sr and Prad=12π W, directivity equals?
A 2
B 4
C 1
D 12
Answer: B. 4. D = 4πUmax/Prad = 4π·12/(12π) = 4.
RMS electric field 1 V/m in free space gives average power density near?
A 0.265 mW/m²
B 26.5 mW/m²
C 377 mW/m²
D 2.65 mW/m²
Answer: D. 2.65 mW/m². For RMS fields S=E²/η₀; 1/377 is about 2.65 milliwatts per square metre.
Using RMS feed current, radiation resistance satisfies?
A Irms² / Prad
B Prad × Irms²
C Prad / Irms
D Prad / Irms²
Answer: D. Prad / Irms². Radiated power equals RMS current squared times radiation resistance.
What impedance should a quarter-wave transformer use between 50 Ω and 200 Ω?
A 125 Ω
B 50 Ω
C 100 Ω
D 250 Ω
Answer: C. 100 Ω. The transformer impedance is √(50×200) = 100 ohms.
If Pin=100 W and Prad=75 W, radiation efficiency is?
A 25%
B 50%
C 75%
D 90%
Answer: C. 75%. Radiation efficiency is Prad/Pin, or 75/100 = 75 percent.
With εr=4 and μr=1, phase velocity is?
A 1.5×10⁸ m/s
B 3×10⁸ m/s
C 0.75×10⁸ m/s
D 6×10⁸ m/s
Answer: A. 1.5×10⁸ m/s. v = c/√(εrμr) = c/2.
Doubling a similar aperture's diameter at fixed wavelength multiplies ideal directivity by?
A Two
B Four
C Eight
D Sixteen
Answer: B. Four. Area grows with diameter squared, and ideal aperture directivity is proportional to area.
Element pattern combines with which factor to form the array pattern?
A Array factor
B Noise factor
C Ground factor
D Feed voltage
Answer: A. Array factor. For identical array elements, the total pattern is the element pattern multiplied by the array factor.
Amplitude tapering across an array usually lowers sidelobes but broadens the?
A Null depth
B Carrier frequency
C Main beam
D Element spacing
Answer: C. Main beam. Tapering suppresses sidelobes at the cost of a wider main beam.
Directivity increases when peak radiation intensity rises while total radiated power?
A Doubles too
B Falls to zero
C Stays fixed
D Becomes reactive
Answer: C. Stays fixed. Directivity is proportional to peak radiation intensity divided by average intensity for fixed total radiated power.
Equal-phase broadside arrays use what progressive phase?
A 90°
B 45°
C 180°
D 0°
Answer: D. 0°. Equal-phase excitation produces a broadside maximum for a uniform linear array.
Resonance makes an antenna's input impedance ideally?
A Purely resistive
B Purely capacitive
C Purely inductive
D Infinite
Answer: A. Purely resistive. At resonance, the net reactive part cancels, leaving a real input impedance.
Total radiated power integrates radiation intensity over?
A π steradians
B 2π steradians
C 4π steradians
D One steradian
Answer: C. 4π steradians. Radiated power is the integral of radiation intensity over the full sphere.