Emerging Antenna Technologies

Introduction

Antenna engineering is moving from passive, fixed-function radiators toward electronically controlled, highly integrated and context-aware electromagnetic systems. This transition is driven by massive multiple-input multiple-output (MIMO), millimetre-wave and terahertz communications, satellite networks, biomedical telemetry, wearable electronics, the Internet of Things (IoT), and joint communication–sensing systems.

An emerging antenna can no longer be evaluated only through impedance bandwidth, gain and radiation pattern. The designer must also consider electronic control, calibration, packaging, thermal behaviour, propagation, user safety, signal processing, energy consumption, cybersecurity, manufacturability and regulation. This chapter critically examines phased arrays, active integrated antennas, reconfigurable and cognitive antennas, metasurfaces, intelligent reflecting surfaces, wearable and implantable antennas, millimetre-wave and terahertz systems, IoT antennas, and antenna-in-package technologies.

Learning Objectives

After studying this chapter, the reader should be able to:

• derive and interpret the array factor of a phased array;
• assess beam scanning, beam squint, grating lobes, coupling and calibration;
• distinguish active integrated, reconfigurable and cognitive antennas;
• explain metasurface and intelligent reflecting surface operation without violating energy conservation;
• evaluate wearable and implantable antennas using detuning, efficiency and specific absorption rate;
• construct link budgets for millimetre-wave and terahertz systems;
• select appropriate antenna architectures for IoT and antenna-in-package applications; and
• identify regulatory, ethical, reliability and cybersecurity implications.

Electronically Scanned Phased Arrays

Array factor

Consider N radiating elements at position vectors rₙ, excited by complex weights aₙeʲφⁿ. In the far field, the array factor is

AF(r̂) = Σₙ₌₀ᴺ⁻¹ aₙ exp[j(k r̂·rₙ + φₙ)],

where k = 2π/λ is the free-space wavenumber, λ is wavelength and r̂ is the observation direction. Pattern multiplication—total pattern approximated by the product of the isolated-element pattern and array factor—is accurate only when element patterns are sufficiently similar and mutual coupling is either weak or incorporated through embedded element patterns.

For a uniform linear array along the x-axis, with spacing d and observation angle θ measured from broadside,

AF(θ) = Σₙ₌₀ᴺ⁻¹ aₙ exp[jn(kd sin θ + β)],

where β is the progressive phase shift. A principal beam at θ₀ requires

β = −kd sin θ₀.

Uniform excitation gives high directivity but a first sidelobe near −13.3 dB for a large broadside array. Amplitude tapers such as Dolph–Chebyshev or Taylor distributions reduce sidelobes at the expense of beamwidth and directivity.

Grating lobes and scan range

Grating lobes are additional visible-region maxima caused by spatial sampling. For symmetric scanning to |θ| ≤ θmax, a sufficient spacing condition is

d ≤ λ/(1 + |sin θmax|).

The familiar d ≤ λ/2 condition permits scanning over the complete visible region in the ideal one-dimensional model. Practical arrays may require smaller spacing because element patterns, coupling and fabrication constraints alter scan performance.

Beam squint

An analogue phase shifter imposes approximately constant phase rather than constant time delay. If it is set at centre frequency f₀ to steer toward θ₀, the beam at frequency f approximately satisfies

sin θ(f) = (f₀/f) sin θ₀,

provided the right-hand side remains within [−1,1]. Wide fractional bandwidth therefore produces beam squint. True-time-delay networks, subband beamforming and frequency-dependent digital precoding reduce this effect, although with additional cost, loss or power consumption.

Implementation impairments

Real arrays are affected by:

• mutual coupling and scan impedance variation;
• phase and amplitude quantisation;
• oscillator phase noise and timing skew;
• gain and phase drift with temperature;
• element failures and manufacturing tolerances;
• feed-network loss and thermal gradients; and
• calibration errors between radio-frequency chains.

An important coupling metric is the active reflection coefficient at port m:

Γactive,m = Σₙ Sₘₙ(aₙ/aₘ),

where Sₘₙ are array scattering parameters and aₙ are incident excitation waves. Unlike an isolated-port S₁₁ measurement, this quantity depends on the simultaneous excitation state and scan direction.

Hybrid beamforming uses fewer radio-frequency chains than antenna elements. It reduces hardware cost and power consumption compared with fully digital beamforming, but limits the number of independent beams and constrains precoding flexibility. Fully digital arrays provide superior multiuser and interference-control capability where converter cost and power are acceptable.

Active Integrated Antennas

An active integrated antenna (AIA) combines a radiator with active devices such as power amplifiers, low-noise amplifiers, mixers, oscillators or frequency multipliers. Eliminating a lossy interconnection between device and antenna is especially advantageous at millimetre-wave and terahertz frequencies.

For a transmitter,

EIRP = PtGt,

in linear units, or

EIRP(dBm) = Pt(dBm) + Gt(dBi) − Lfeed(dB),

where Pt is power delivered before feed loss, Gt is antenna gain and Lfeed is loss between the stated power reference plane and antenna. Regulatory limits may constrain EIRP, power spectral density or both.

AIAs offer compactness and potentially improved noise or power-transfer performance, but create difficult co-design problems. Antenna impedance affects amplifier stability and efficiency; active-device harmonics can radiate; and heat changes substrate properties and phase alignment. Nonlinear power amplifiers also produce spectral regrowth and beam-dependent distortion. Electromagnetic, circuit and thermal simulations should therefore share consistent reference planes and material models.

Distributed active arrays can be designed for graceful degradation, but failures may distort beams rather than merely reduce gain. Built-in self-test, temperature sensing and over-the-air calibration are consequently important.

Reconfigurable and Cognitive Antennas

A reconfigurable antenna changes one or more characteristics—frequency, polarization, pattern or effective aperture—through controllable components or materials. Common tuning mechanisms include PIN diodes, varactors, RF MEMS switches, liquid crystals, ferroelectrics, phase-change materials and mechanically altered structures.

Frequency reconfiguration supports multiband operation; polarization reconfiguration mitigates orientation mismatch; and pattern reconfiguration improves coverage or interference rejection. Practical evaluation must include switch loss, bias-line radiation, tuning speed, linearity, reliability, power consumption and the number of genuinely distinct operating states.

A cognitive antenna system goes beyond commanded switching. It senses or receives contextual information, estimates channel and interference conditions, selects an antenna state, observes performance and updates its policy. The control loop may use Bayesian inference, reinforcement learning or conventional optimization. The physical antenna remains reconfigurable, while cognition resides in the sensing and decision architecture.

Machine learning is useful where a full electromagnetic model is unavailable or real-time optimization is difficult. However, training data may not represent hand effects, ageing, weather or adversarial interference. Safe deployment requires constrained action spaces, fallback states, uncertainty monitoring and traceable performance criteria. A cognitive controller must never infer that an unoccupied measured channel is legally available without applying spectrum regulations and database requirements.

Metasurfaces and Metantennas

A metasurface is an electrically thin arrangement of subwavelength scattering elements engineered to control reflection, transmission, phase, amplitude or polarization. Implementations include passive patterned surfaces, transmitarrays, reflectarrays, Huygens surfaces, leaky-wave structures and programmable surfaces.

For a local phase-gradient reflector in a homogeneous medium, a commonly used generalized reflection relation is

dΦ/dx = k₀(sin θr − sin θi),

where Φ is the imposed surface phase, θi and θr are incidence and reflection angles measured from the surface normal, and k₀ is the free-space wavenumber. The sign changes if a different phasor or coordinate convention is adopted.

This local formula is a design approximation, not a universal efficiency theorem. Strong anomalous reflection may require nonlocal power redistribution, bianisotropy, carefully engineered surface impedance or auxiliary evanescent fields. Discretization, conductor loss, dielectric loss and limited phase range reduce efficiency. In addition, a passive time-invariant metasurface cannot provide net power gain.

Metasurface antennas can offer low profile, conformability and inexpensive aperture fabrication. Their limitations include narrow resonances, angular dispersion, fabrication sensitivity and complex full-wave modelling. Tunable versions add bias networks, control latency and nonlinear effects.

Intelligent Reflecting Surfaces

An intelligent reflecting surface (IRS), also called a reconfigurable intelligent surface, is a controllable electromagnetic surface intended to reshape a radio channel. A simplified narrowband baseband model is

heff = hd + Σₙ₌₁ᴺ hr,n Γₙ ht,n,

where hd is the direct-channel coefficient, ht,n and hr,n are the transmitter-to-element and element-to-receiver coefficients, and Γₙ = ρₙeʲφⁿ is the controllable reflection coefficient of element n.

If the cascaded paths have comparable amplitudes and phases are perfectly aligned, the reflected field can grow approximately in proportion to N, implying an N² received-power scaling relative to one element. This is a conditional coherent-combining result—not a general law that allows unlimited power. Fixed physical aperture, increasing element density, element coupling, angular response, finite efficiency, transmitter–surface–receiver distances and near-field behaviour constrain scaling. Aperture conservation and energy conservation must be maintained. Rigorous treatments distinguish increasing physical aperture from merely subdividing a fixed aperture (Björ­nson and Sanguinetti, 2020).

IRS operation also requires channel estimation and control signalling. Estimating every cascaded element channel may create excessive overhead, particularly for fast-moving users. Grouped elements, compressed sensing, location-assisted control and codebook methods reduce overhead. Near-field surfaces require spherical-wave focusing rather than plane-wave steering.

An IRS is not automatically preferable to an active relay. A relay can amplify and regenerate signals but consumes more power and introduces receiver noise. An IRS is potentially low power and low noise, yet suffers a multiplicative two-hop path loss and normally cannot compensate for severe geometric attenuation through active gain.

Wearable and Implantable Antennas

Wearable antennas

Wearable antennas operate close to a lossy, irregular and moving human body. Textile permittivity changes with moisture, bending modifies current paths, and the body detunes resonances while absorbing power. Design objectives may include conformability, low mass, washability, mechanical durability and radiation directed away from the body.

Common structures include planar inverted-F antennas, patches, monopoles, slot antennas and substrate-integrated waveguide cavities. Artificial magnetic conductors or electromagnetic band-gap structures can improve isolation from the body, but add thickness and may remain narrowband.

Evaluation should use representative phantoms and multiple body postures. Free-space matching alone is not sufficient. Total efficiency, on-body path loss, polarization stability, bending radius and performance under sweat or laundering are relevant metrics.

Implantable antennas

Implantable antennas operate inside dispersive, high-permittivity and conductive tissue. Their electromagnetic wavelength is shorter than in free space, enabling physical miniaturization, but conduction loss severely reduces efficiency. Biocompatible encapsulation also changes impedance and must be included in simulation.

Implants may use the Medical Device Radiocommunications Service, Bluetooth-related links, inductive coupling or ultrasonic alternatives, depending on depth, data rate and jurisdiction. Reliability, battery life, hermetic sealing, fail-safe operation and patient privacy are as important as gain.

Specific absorption rate

Specific absorption rate (SAR) quantifies the rate at which RF energy is absorbed per unit tissue mass. The local sinusoidal steady-state expression is

SAR = σ|Erms|²/ρ,

where σ is tissue conductivity in S/m, Erms is RMS electric-field magnitude in V/m and ρ is mass density in kg/m³. SAR is measured in W/kg and is spatially averaged over a specified tissue mass for compliance.

For σ = 1 S/m, Erms = 10 V/m and ρ = 1000 kg/m³,

SAR = 1(10)²/1000 = 0.1 W/kg.

This pointwise calculation is not by itself a compliance assessment. Under FCC rules commonly applied to portable devices in the United States, the localized public limit is 1.6 W/kg averaged over 1 g of tissue. ICNIRP (2020) specifies, for the general public in its applicable 100 kHz–6 GHz range, localized limits of 2 W/kg averaged over 10 g for head and torso and 4 W/kg for limbs. Above 6 GHz, absorbed or incident power-density metrics become important. The applicable jurisdiction, frequency, averaging procedure and exposure scenario must always be identified.

Millimetre-Wave Antennas

Millimetre-wave systems commonly occupy approximately 30–300 GHz, although communications practice often includes bands such as 24–30 GHz under the broader “mmWave” label. Short wavelengths enable many array elements in a small aperture and support wide bandwidths. They also increase sensitivity to conductor roughness, dielectric tolerances, connector transitions and package discontinuities.

For polarization-matched antennas in the far field with an unobstructed free-space path, the Friis equation is

Pr = PtGtGr(λ/4πR)²,

where Pt is transmitted power, Gt and Gr are directional gains toward each other, R is separation and λ is wavelength. Additional atmospheric, rain, blockage, polarization and implementation losses must be introduced separately.

Worked example: verified 60 GHz link

A 60 GHz link has Pt = 20 dBm = 0.1 W, Gt = Gr = 20 dBi = 100, and R = 50 m. Taking c = 3 × 10⁸ m/s gives λ = c/f = 0.005 m.

Pr = 0.1(100)(100)[0.005/(4π × 50)]²

= 6.33 × 10⁻⁸ W.

Converting to dBm,

Pr(dBm) = 10 log₁₀(6.33 × 10⁻⁸/10⁻³) ≈ −42.0 dBm.

Equivalently, free-space path loss is approximately 101.98 dB, so Pr = 20 + 20 + 20 − 101.98 = −41.98 dBm. This value excludes atmospheric absorption, rain, radome loss, pointing error and receiver implementation loss.

Millimetre-wave links are susceptible to blockage by people, vehicles and building materials. Beam training, multi-connectivity, diversity paths and blockage prediction are therefore system-level requirements rather than optional enhancements.

Terahertz Antennas and Propagation

The terahertz region is often taken as approximately 0.1–10 THz, although definitions vary. Candidate technologies include photoconductive antennas, plasmonic radiators, dielectric resonators, silicon lens-integrated antennas, on-chip arrays and frequency-multiplied electronic sources.

Terahertz links promise very large bandwidth, high-resolution spectroscopy, imaging and precise localization. Challenges include low source power, receiver noise, packaging loss, phase noise, beam alignment and limited efficient modulation.

Propagation cannot be represented by free-space loss alone. Molecular absorption—especially from water vapour and oxygen—creates frequency-selective attenuation and transmission windows. ITU-R P.676 provides authoritative methods for gaseous attenuation, while spectroscopic databases such as HITRAN support line-by-line analysis. Rain attenuation may be estimated using ITU-R P.838 within its stated frequency range. At THz frequencies, surface roughness also changes reflection and scattering, making material characterization essential.

Absorption values depend on frequency, pressure, temperature, humidity and path length. Consequently, statements such as “THz loss equals a fixed number of decibels per kilometre” are academically inadequate unless environmental conditions and frequency are specified. IEEE 802.15.3d demonstrates standardization of very high data-rate wireless links in the 252–325 GHz range.

IoT Antennas

IoT antenna design is dominated by constraints rather than peak gain. Devices may have tiny ground planes, plastic or metal enclosures, batteries, sensors, cables and unpredictable installation orientations. Relevant criteria include:

• reliable coverage rather than maximum free-space gain;
• multiband or regional frequency support;
• low bill-of-material cost;
• tolerance to nearby objects and manufacturing variation;
• low-power wake-up and localization support;
• coexistence among cellular, Wi-Fi, Bluetooth, GNSS, UWB and low-power wide-area radios; and
• certified performance in the final product.

Electrically small antennas face fundamental bandwidth–efficiency trade-offs associated with stored reactive energy. Matching networks can improve port matching over a limited band but cannot recover power dissipated in lossy conductors or absorbed by nearby materials. Total radiated power, total isotropic sensitivity and over-the-air testing are therefore more meaningful than S₁₁ alone.

For long-lived sensor nodes, antenna inefficiency increases transmit time or required power and thus reduces battery life. Antenna placement is consequently an energy-management decision. Diversity or polarization-insensitive designs may provide greater network reliability than a narrow high-gain beam.

Antenna-in-Package Technology

Antenna-in-package (AiP) integrates one or more antennas into the same package as radio-frequency integrated circuits. It is particularly attractive at millimetre-wave and sub-terahertz frequencies because conventional board traces and connectors become highly lossy.

AiP differs from antenna-on-chip. Antenna-on-chip places the radiator on the semiconductor die, where lossy silicon and limited aperture may reduce efficiency. AiP uses package substrates, redistribution layers, embedded waveguides, mould compounds or low-loss laminates to obtain greater aperture and better isolation.

Advantages include short interconnects, repeatable assembly, compact arrays and wafer- or panel-scale testing. Challenges include package warpage, thermal expansion, dielectric uncertainty, coupling to heat spreaders, limited repairability and the need for co-design across chip, package, antenna, radome and product enclosure.

At these frequencies, reference-plane errors can invalidate measurements. Best practice combines full-wave package modelling, calibrated probe or connector transitions, over-the-air verification, thermal cycling and statistical tolerance analysis.

Comparative Technology Assessment

No emerging antenna architecture is universally superior. Phased arrays provide rapid beam control but require calibration and substantial power. AIAs minimize interconnect loss but intensify thermal and nonlinear coupling. Reconfigurable antennas conserve space but may exhibit state-dependent loss and reliability problems. Cognitive control can optimize operation but introduces data and validation risks. Metasurfaces offer thin apertures, while IRS deployments require favourable geometry and control overhead. Wearable and implantable antennas must prioritize safety and robustness over free-space efficiency. AiP is attractive at high frequencies but transfers risk to packaging and manufacturing.

A defensible selection process should compare:

1. electromagnetic performance across realistic channels;

2. energy consumption, thermal loading and duty cycle;

3. control, calibration and channel-estimation overhead;

4. manufacturing yield and lifecycle reliability;

5. exposure, spectrum and product-certification requirements;

6. cybersecurity and software-update mechanisms; and

7. total system cost rather than antenna cost alone.

Illustrative Case Study: 60 GHz Warehouse Link with Blockage Resilience

Background

This is an illustrative engineering case. A warehouse requires a 2 Gbit/s wireless link over 50 m between a fixed access point and a robotic inspection station. Cable installation is impractical. A 60 GHz phased-array solution is considered, with a programmable reflecting panel at an aisle intersection as a backup path.

Problem

The direct line of sight is intermittently blocked by loaded vehicles. The design must provide adequate received power, rapid beam recovery and controlled installation cost while avoiding unjustified assumptions about IRS gain.

Analysis

The transmitter supplies 20 dBm to a 20 dBi array, and the robot uses a 20 dBi receive array. The verified free-space result is

Pr = 6.33 × 10⁻⁸ W ≈ −42.0 dBm.

Assume 3 dB combined implementation loss and an environment-specific 0.8 dB allowance for gaseous attenuation over 50 m. The predicted unobstructed power becomes approximately −45.8 dBm. For a 2 GHz receiver bandwidth, thermal noise at 290 K is

N = −174 + 10 log₁₀(2 × 10⁹) ≈ −81.0 dBm.

With a 7 dB noise figure, the receiver noise level is about −74.0 dBm, giving a nominal pre-fade SNR near 28.2 dB. Coding and modulation selection must nevertheless use measured error-vector magnitude and implementation margins, not SNR alone.

A vehicle may impose more than 20 dB of blockage loss, so link margin based only on the unobstructed path is insufficient. The proposed IRS cannot be credited with an arbitrary N² gain. Its physical aperture, incidence angle, reflection efficiency, two-hop distances, quantized phase states and warehouse scattering must be included in a ray-based or full-wave-supported channel model.

Approach

The adopted architecture uses:

• electronically scanned arrays with hierarchical beam training;
• temperature-aware calibration and stored fallback codebooks;
• a geometrically positioned reflecting panel with line of sight to both endpoints;
• channel-quality monitoring that switches to the reflected route before complete blockage;
• robust modulation during route transitions; and
• site measurements under moving-vehicle and loaded-shelf conditions.

Outcome

The direct path supports the target rate under clear conditions. The reflected route is retained as a diversity path rather than represented as a passive amplifier. Installation acceptance is based on measured outage probability and packet latency across representative traffic patterns.

Learning Points

A correct free-space calculation is necessary but not sufficient. High-frequency antenna systems must include atmospheric and implementation loss, blockage dynamics, beam-training delay and realistic surface aperture. IRS performance must be evaluated through conservation-consistent electromagnetic and channel models.

Governance, Safety and Professional Practice

Emerging antennas raise issues extending beyond electromagnetic design. Beamforming systems collect channel and location information that can reveal user movement. Cognitive radios and programmable surfaces create software-controlled attack surfaces. Unauthorized control could redirect coverage, degrade service or increase unintended exposure.

Professional practice should include secure boot, authenticated control messages, signed firmware, access logging and safe default states. Biomedical devices additionally require privacy protection, risk management and clinically appropriate failure modes. Environmental considerations include rare materials, battery waste and the energy cost of dense infrastructure.

Exposure compliance must be demonstrated for the actual operating modes, duty cycles and simultaneous transmitters. Compliance with an EIRP limit does not automatically establish human-exposure compliance. Conversely, high instantaneous array gain does not imply excessive exposure without considering distance, averaging, power control and applicable standards.

Contemporary Research Directions

Current research includes holographic MIMO surfaces, extremely large aperture arrays, near-field beam focusing, integrated sensing and communication, fluid antennas, movable antennas, non-terrestrial arrays, sub-THz AiP, digital metasurfaces and AI-assisted inverse design. Major unresolved questions concern scalable channel estimation, physically consistent surface models, wideband control, thermal efficiency, reproducible over-the-air measurements and trustworthy autonomous adaptation.

Digital twins can combine electromagnetic, circuit, thermal and channel models for lifecycle calibration. Their value depends on validated material parameters and continuous comparison with measurements. AI should complement rather than replace Maxwell-consistent modelling and regulatory testing.

Selected Authoritative Sources

• Balanis, C. A. Antenna Theory: Analysis and Design, 4th ed., Wiley, 2016.
• Björnson, E. and Sanguinetti, L. “Power Scaling Laws and Near-Field Behaviors of Massive MIMO and Intelligent Reflecting Surfaces,” IEEE Open Journal of the Communications Society, 2020.
• FCC, 47 CFR §§1.1310 and 2.1093, radiofrequency exposure requirements.
• ICNIRP, “Guidelines for Limiting Exposure to Electromagnetic Fields (100 kHz to 300 GHz),” Health Physics, 2020.
• IEEE Std C95.1-2019, Safety Levels with Respect to Human Exposure to Electric, Magnetic, and Electromagnetic Fields, 0 Hz to 300 GHz.
• IEEE Std 802.15.3d-2017, amendments for 100 Gbit/s switched point-to-point wireless networks.
• ITU-R P.676-13, Attenuation by Atmospheric Gases and Related Effects, 2022.
• ITU-R P.838-3, Specific Attenuation Model for Rain for Use in Prediction Methods, 2005.
• Yu, N. et al., “Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction,” Science, 2011.

Chapter Summary

Emerging antennas are integrated electromagnetic systems whose performance depends on hardware, control, propagation, packaging and regulation. Phased arrays achieve electronic scanning through controlled element excitation but encounter coupling, grating lobes, beam squint and calibration error. Active integrated antennas reduce high-frequency interconnect loss while creating nonlinear and thermal co-design problems. Reconfigurable antennas alter physical operating states, whereas cognitive systems select states using sensed context and decision logic.

Metasurfaces manipulate wavefronts through engineered surface responses. IRS power scaling is conditional on coherent combining and must respect finite aperture, efficiency and path loss. Wearable and implantable antennas require body-aware design and verified SAR compliance. Millimetre-wave and terahertz links offer large bandwidth and compact apertures but are vulnerable to blockage, molecular absorption and manufacturing tolerances. IoT and AiP designs emphasize total system efficiency, robustness, manufacturability and secure control. Reliable engineering therefore requires multiphysics simulation, realistic measurement, standards compliance and critical evaluation of idealized performance claims.

Key Terms

Active integrated antenna: A radiator directly integrated with active RF or microwave circuitry.

Active reflection coefficient: Port reflection under simultaneous excitation of an array’s elements.

Antenna-in-package: Integration of antennas and radio-frequency circuitry within a common package.

Beam squint: Frequency-dependent change in beam direction caused by phase-shifter-based steering.

Cognitive antenna: A context-aware system that senses conditions and autonomously selects antenna states.

Embedded element pattern: Radiation pattern of one array element with the remaining ports terminated under specified conditions.

Grating lobe: An undesired array maximum produced by excessive spatial sampling interval.

Hybrid beamforming: Beamforming divided between analogue and digital processing networks.

Intelligent reflecting surface: A controllable surface that modifies radio propagation through adjustable scattering states.

Metasurface: An electrically thin engineered surface composed of subwavelength scattering elements.

Millimetre wave: Electromagnetic frequencies approximately between 30 and 300 GHz.

Near-field focusing: Formation of a spatial focus using spherical-wave phase control within an aperture’s radiative near field.

Reconfigurable antenna: An antenna whose frequency, pattern, polarization or related characteristic can be controlled.

Specific absorption rate: RF power absorbed per unit mass of tissue, expressed in W/kg.

Terahertz radiation: Electromagnetic radiation commonly identified with approximately 0.1–10 THz.

True-time delay: Beamforming delay that produces an approximately frequency-independent steering direction.

Total radiated power: Surface-integrated power radiated by a transmitting device.

Wearable antenna: An antenna designed for operation on or near clothing or the human body.

Review Questions

Short Answer Questions

1. What distinguishes a reconfigurable antenna from a cognitive antenna system?

2. Why can an isolated-element S₁₁ measurement be inadequate for a scanned array?

3. State two causes of beam degradation in millimetre-wave phased arrays.

4. Why must implant encapsulation be included in electromagnetic simulation?

5. What is the principal difference between antenna-in-package and antenna-on-chip?

Long Answer Questions

1. Derive the array factor of a uniformly spaced linear array and explain scanning, sidelobes and grating lobes.

2. Critically compare fully digital, hybrid and analogue beamforming architectures.

3. Explain metasurface and IRS operation, including the physical limitations of ideal N² power scaling.

4. Discuss the electromagnetic, biomedical and regulatory requirements for wearable and implantable antennas.

5. Evaluate the opportunities and limitations of terahertz antennas for communication, sensing and imaging.

Analytical / Application Questions

1. Design an eight-element linear array scanning to 45° without visible grating lobes over a specified frequency band. Determine a suitable maximum spacing and discuss beam squint.

2. Construct a 60 GHz link budget including free-space loss, atmospheric attenuation, implementation loss, receiver noise figure and blockage margin.

3. Develop a technology-selection framework comparing an active relay, IRS and additional access point for a blocked indoor millimetre-wave link.

Multiple Choice Questions with Answers

Question 1

For a uniform linear array along the x-axis, with angle measured from broadside, what progressive phase steers the main beam to θ₀?

A. β = kd cos θ₀

B. β = −kd sin θ₀

C. β = kd/sin θ₀

D. β = −k/d

Correct Answer: B. β = −kd sin θ₀

Question 2

Which technology most directly mitigates wideband beam squint in a phased array?

A. A true-time-delay network

B. A narrower element spacing alone

C. A resistive matching pad

D. A larger radome

Correct Answer: A. A true-time-delay network

Question 3

Why does an array element's active reflection coefficient depend on scan direction?

A. Wavelength becomes independent of frequency

B. The substrate loses all dielectric dispersion

C. Relative element excitations interact through mutual coupling

D. Antenna gain is identical in every direction

Correct Answer: C. Relative element excitations interact through mutual coupling

Question 4

Under which condition can an idealized IRS reflected power exhibit approximately N² scaling relative to one element?

A. When element fields combine coherently with comparable amplitudes

B. Whenever N elements occupy a fixed aperture, regardless of coupling

C. Only when every element contains an active power amplifier

D. When the surface phases are independently randomized

Correct Answer: A. When element fields combine coherently with comparable amplitudes

Question 5

At 60 GHz, a free-space link uses 20 dBm transmit power, 20 dBi gain at each end and a 50 m range. Excluding other losses, what is the received power?

A. Approximately −22.0 dBm

B. Approximately −32.0 dBm

C. Approximately −42.0 dBm

D. Approximately −62.0 dBm

Correct Answer: C. Approximately −42.0 dBm

Question 6

For normal incidence, λ = 10 mm and desired reflection at 30° in free space, what phase gradient magnitude follows from dΦ/dx = k₀ sin 30°?

A. Approximately 31.4 rad/m

B. Approximately 157 rad/m

C. Approximately 314 rad/m

D. Approximately 628 rad/m

Correct Answer: C. Approximately 314 rad/m

Question 7

Which factor is especially important when modelling an implantable antenna?

A. Tissue dispersion, conductivity and encapsulation

B. Only the antenna's free-space directivity

C. Only the metallic connector loss

D. Ionospheric fading

Correct Answer: A. Tissue dispersion, conductivity and encapsulation

Question 8

For tissue conductivity 1 S/m, RMS electric field 10 V/m and density 1000 kg/m³, what is the local SAR?

A. 0.01 W/kg

B. 0.05 W/kg

C. 0.1 W/kg

D. 10 W/kg

Correct Answer: C. 0.1 W/kg

Question 9

What is a major advantage of antenna-in-package technology at millimetre-wave frequencies?

A. It eliminates atmospheric absorption

B. It shortens otherwise lossy RF interconnections

C. It guarantees unlimited antenna bandwidth

D. It removes the need for thermal analysis

Correct Answer: B. It shortens otherwise lossy RF interconnections

Question 10

Which authoritative recommendation provides methods for estimating attenuation by atmospheric gases?

A. ITU-R P.676

B. ITU-R P.838 only

C. IEEE 802.3 Ethernet

D. IEC 60529

Correct Answer: A. ITU-R P.676