Interview questions & answers
Q1. What is the difference between antenna gain and directivity?
Directivity is a purely geometric measure of how concentrated the antenna's radiation is in its peak direction compared to an isotropic radiator, while gain = efficiency × directivity accounts for ohmic losses in the antenna conductors and dielectric. A half-wave dipole has a directivity of 2.15 dBi (relative to isotropic), and if its radiation efficiency is 90%, its gain is 2.15 + 10·log(0.9) = 1.61 dBi. In real antenna datasheets, 'gain' always means realized gain including losses, while 'directivity' is sometimes quoted for theoretical comparison — mistaking one for the other leads to incorrect link budget calculations.
Follow-up: What is the radiation efficiency of an antenna and how does it affect link budget calculations?
Q2. What is the radiation resistance of an antenna?
Radiation resistance Rrad is the equivalent resistance that would dissipate the same power as the antenna radiates when the same current flows through it; it is a way of quantifying radiated power in circuit terms. A half-wave dipole in free space has Rrad ≈ 73 Ω, which is why dipole antennas are easily matched to 75 Ω coaxial cable. For a short dipole of length λ/50, Rrad drops to about 0.8 Ω, meaning most of the generator power is wasted in the antenna's ohmic resistance rather than radiated, making short dipoles very inefficient without matching networks.
Follow-up: How does radiation resistance change as a dipole antenna is shortened from λ/2 to λ/10?
Q3. What is the half-power beamwidth (HPBW) and why does it matter?
Half-power beamwidth is the angular width between the two directions in the radiation pattern where the radiated power density drops to half (-3 dB) of the peak value, and it indicates how precisely the antenna can be aimed or how wide a coverage area it illuminates. A 28 GHz 5G base station antenna with a 10° HPBW in the azimuth plane can serve a 10° sector, while the same physical aperture at 3.5 GHz has a much wider beam because beamwidth is inversely proportional to aperture size in wavelengths. Narrower beamwidth corresponds to higher directivity, which is why high-gain satellite dish antennas require precise pointing.
Follow-up: What is the approximate relationship between aperture size in wavelengths and beamwidth?
Q4. What is the effective aperture of an antenna?
Effective aperture Ae = Gλ²/(4π) is the equivalent collecting area of an antenna in receiving mode — the area that would capture the same power from an incident plane wave if it intercepted all energy falling on Ae. A GPS patch antenna with gain 4 dBi (G ≈ 2.5) at 1.575 GHz (λ = 190 mm) has Ae = 2.5 × 0.19²/(4π) = 7.2 cm², much less than its physical area of about 25 cm². The Friis transmission equation uses Ae to calculate received power, and understanding it explains why large dish antennas in radio telescopes are needed to receive very weak signals.
Follow-up: How does the effective aperture relate to the physical aperture of a parabolic dish antenna?
Q5. What is antenna impedance and how is a dipole matched to a coaxial feed?
Antenna impedance is the complex impedance at the feed point: Z_ant = R_rad + R_loss + jX, where X is the reactive component that determines the resonant condition. A half-wave dipole at resonance has Z_ant ≈ 73 + j0 Ω, closely matching 75 Ω coaxial cable, which is why 75 Ω cable is standard for TV antennas fed by half-wave dipoles. Matching to 50 Ω cable requires a balun or matching network — a simple folded dipole has an impedance of about 300 Ω and is matched to 75 Ω cable via a 4:1 balun, which is the configuration used in classic rabbit-ear TV antennas.
Follow-up: What is a balun and why is it needed when connecting a coaxial cable to a dipole antenna?
Q6. What is the Friis transmission equation and how is it used?
The Friis equation P_r = P_t · G_t · G_r · (λ/(4πd))² gives the received power P_r in terms of transmitted power, transmitter and receiver gains, wavelength, and distance d, where (λ/4πd)² is the free-space path loss. For a 2.4 GHz Wi-Fi link at 10 m with P_t = 100 mW, G_t = G_r = 2 dBi (1.58 linear), and λ = 0.125 m: P_r = 0.1 × 1.58 × 1.58 × (0.125/125.7)² = −46 dBm. This equation is the foundation of every wireless link budget calculation, and every RF engineer uses it daily to estimate coverage range and required transmit power.
Follow-up: How does path loss vary with frequency at the same physical distance?
Q7. What is a ground plane and what effect does it have on antenna radiation?
A ground plane is a conducting surface beneath the antenna that provides an image of the antenna mirroring the source currents, effectively doubling the antenna length in the direction perpendicular to the ground and redirecting radiation that would go downward into the upper hemisphere. A quarter-wave monopole antenna (λ/4 = 75 mm at 1 GHz) mounted on a large ground plane behaves identically to a half-wave dipole above the plane, with radiation resistance of 36.5 Ω (half of the dipole's 73 Ω) and gain of 5.15 dBi. The size of the ground plane significantly affects performance — a ground plane smaller than λ/4 degrades both the radiation pattern and impedance.
Follow-up: How does a finite-size ground plane affect the radiation pattern of a monopole antenna?
Q8. What is a parasitic element in an antenna and how does it work in a Yagi antenna?
A parasitic element is a conductor placed near a driven antenna element that is not directly connected to the feed but is excited by inductive/capacitive coupling, reradiating power to shape the pattern. In a Yagi-Uda TV antenna, the reflector element (slightly longer than λ/2) is placed behind the driven element to reinforce forward radiation, while director elements (slightly shorter than λ/2) are placed in front to progressively focus the beam forward, achieving 7–15 dBd gain with 4–10 elements. The Yagi's gain increases with the number of director elements, but decreasing element spacing beyond a point causes mutual coupling that reduces efficiency.
Follow-up: What is the difference between a reflector and a director parasitic element in a Yagi antenna?
Q9. What is the radiation pattern of a half-wave dipole?
A half-wave dipole radiates in a donut-shaped pattern symmetric around its axis, with maximum radiation in the broadside plane (perpendicular to the dipole) and zero radiation off the ends, described mathematically as the pattern function F(θ) = [cos(π/2 · cosθ)/sinθ]². The -3 dB beamwidth in the elevation plane is about 78°, and the gain is 2.15 dBi relative to an isotropic radiator, making the dipole the reference antenna for dBd gain specifications. This pattern is why horizontal TV dipole antennas radiate equally in all horizontal directions but have nulls directly above and below — perpendicular to the antenna axis.
Follow-up: What is the radiation pattern of a short dipole compared to a half-wave dipole?
Q10. What is antenna bandwidth and what limits it?
Antenna bandwidth is the frequency range over which the antenna maintains acceptable performance, typically defined as the range where VSWR ≤ 2:1 (|Γ| ≤ 1/3) or gain drops by no more than 3 dB from the peak value. A printed F-antenna (PIFA) in a smartphone may have a 3G/4G bandwidth of 700–960 MHz (roughly 30% fractional bandwidth) despite being only a few centimeters in size, achieved through careful slot tuning and substrate selection. The fundamental limit on small antenna bandwidth is set by the Chu-Wheeler limit: Q ≥ 1/(ka)³ for an antenna fitting in a sphere of radius a, meaning physically smaller antennas are inherently narrowband.
Follow-up: What is the Chu-Wheeler limit and what does it say about small antenna design?
Q11. What is circular polarization in an antenna and when is it used?
An antenna radiates circular polarization when the electric field vector rotates at the wave frequency (right-hand or left-hand) rather than staying in a fixed plane, achieved by feeding two orthogonal elements 90° apart in phase. GPS patch antennas are designed for right-hand circular polarization (RHCP) because the GPS signal is RHCP — this also means a linearly polarized receive antenna loses 3 dB compared to an RHCP antenna regardless of orientation. Circular polarization eliminates the polarization mismatch problem that affects linear antennas as their orientation changes relative to the transmitter, which is critical for mobile satellite communication.
Follow-up: What is axial ratio and how is it used to characterize a circularly polarized antenna?
Q12. What is antenna array and how does it improve performance?
An antenna array combines multiple antenna elements with controlled amplitude and phase excitation to achieve higher gain, shaped beams, or spatial multiplexing than any single element could provide. A 4-element uniform linear array of half-wave dipoles spaced λ/2 apart with equal-phase excitation achieves about 6 dBd gain over a single dipole and produces a main lobe perpendicular to the array axis with 2 grating lobes. In 5G massive MIMO base stations, 64 or 128 antenna elements are combined with digital beamforming to simultaneously serve multiple users with independent beams, which requires precise phase and amplitude control across all elements.
Follow-up: What is the array factor and how does it combine with the element pattern to give the overall array pattern?
Q13. What is the near field and far field of an antenna?
The near field (also called reactive near field) is the region within about λ/(2π) of the antenna where reactive (stored) energy dominates and the E/H ratio departs from 377 Ω; the far field starts at distance R > 2D²/λ (Fraunhofer condition) where the radiation pattern becomes fixed and follows the inverse-square law. For a 30 cm parabolic dish at 10 GHz (λ = 30 mm), the far-field boundary is at 2×(0.3)²/0.03 = 6 m — measurements taken closer than 6 m give an incorrect radiation pattern. Antenna range measurements (antenna ranges, compact ranges, near-field scanners) must ensure the far-field condition is met to get accurate pattern data.
Follow-up: What is the Rayleigh distance and how does it relate to the far-field boundary?
Q14. What is mutual coupling in an antenna array and how does it affect performance?
Mutual coupling is the electromagnetic interaction between adjacent antenna elements in an array, where current induced on one element by another's radiation changes the element's impedance and pattern, affecting both matching and array factor. In a 4G LTE smartphone antenna with two MIMO antennas separated by 20 mm at 1.8 GHz (λ ≈ 167 mm), the coupling between antennas is typically -10 to -15 dB, degrading envelope correlation coefficient and MIMO throughput. Decoupling techniques include neutralization lines, defected ground structures (DGS), and carefully designed feeding networks that force currents to cancel between elements.
Follow-up: What is the envelope correlation coefficient (ECC) and why must it be low in MIMO antennas?
Q15. What is a microstrip patch antenna and what are its advantages and limitations?
A microstrip patch antenna is a resonant metallic patch on a grounded dielectric substrate, fed by a microstrip line or coaxial probe, resonating when its length is approximately λ/(2√εᵣ_eff) in the dominant mode. A rectangular patch on Rogers RO4003C (εᵣ = 3.55) at 2.45 GHz is about 29 mm long and 37 mm wide, providing 6–8 dBi gain with a broad beamwidth of roughly ±40°. The main limitations are narrow bandwidth (typically 2–5% for a single layer) and low power handling compared to waveguide antennas, both of which can be improved using stacked patches, air substrates, or aperture coupling at the cost of additional height and manufacturing complexity.
Follow-up: How does increasing the substrate thickness affect the bandwidth and efficiency of a patch antenna?
Common misconceptions
Misconception: Antenna gain refers to amplification of the transmitted power.
Correct: Antenna gain describes the concentration of radiated power in a preferred direction relative to an isotropic radiator — a passive antenna cannot amplify total power, only redistribute it spatially.
Misconception: A larger antenna always has higher gain.
Correct: Gain depends on aperture efficiency and directivity; a large antenna with poor aperture illumination can have lower gain than a smaller, well-designed antenna with the same physical aperture.
Misconception: The radiation pattern of an antenna is the same in transmit and receive modes.
Correct: By the reciprocity theorem, the radiation pattern — directivity, beamwidth, and gain — is identical in transmit and receive modes for any passive antenna; the effective aperture and gain are related through Ae = Gλ²/(4π) in both directions.
Misconception: Polarization mismatch between transmit and receive antennas only causes 3 dB loss.
Correct: A 3 dB polarization mismatch loss occurs only for cross-linear polarization; for orthogonal linear polarizations (horizontal vs. vertical) the theoretical mismatch loss is infinite (zero power transfer), and real-world effects like multipath reduce this to a finite but large value.