How Antennas Work: Electromagnetic Wave Propagation Explained

5G antennas use phased array technology, consisting of 128 individual emitters within a square housing. These emitters work cooperatively to form a directional beam of electromagnetic radiation. This system, called beamforming, allows the antenna to direct signals toward specific users. However, this technology creates measurement challenges because the beam direction changes randomly as users move, making it difficult to accurately measure radiation exposure levels.

Beamforming, developed by Cabron in 1905, uses multiple antennas separated by specific distances with controlled signal phases to create focused energy beams through constructive and destructive interference. Modern 5G phased arrays contain hundreds of active elements (e.g., Bell Labs' 384-element array), with millimeter wave frequencies enabling compact antenna designs. The system rapidly steers beams between users without physically moving antennas, creating continuous coverage illusions. At 28 GHz over 2 meters, free space path loss reaches 67 dB (signal weakens 5 million times). This extreme attenuation means successful millimeter wave links represent extraordinary engineering achievements rather than excessive power outputs.

The cavity magnetron gave radar its power, but not the ability to steer its beam without physically moving any parts. Phased array beamforming works by lining up many small antennas and feeding each one the signal at a slightly different moment, allowing the beam to be swung across the sky by changing timing alone. When the Soviets launched Sputnik in 1957, the United States needed radar that could watch the entire sky at once. In 1958, Huggins demonstrated beam steering using ferrite phase shifters instead of motors. The AN/FPS-85 in Florida (1969) was the world's first large phased array with 5,000 transmitters. PAVE PAWS (1970s) was the first solid-state array scanning for submarine-launched missiles. In 1983, the Navy sent the SPY-1 radar to sea aboard the USS Ticonderoga, with four fixed antenna faces each holding 4,500 radiating elements, giving a warship a full 360° stare. The mathematics had actually been ready for decades; in January 1943, Bell Labs engineer Sergei Schelkunoff published A Mathematical Theory of Arrays. A modern 5G base station uses massive MIMO with dozens to over a hundred small antenna elements, using digital beamforming to aim narrow beams at phones. The flat white Starlink dish has roughly 1,300 tiny antenna elements that steer the beam electronically to lock onto satellites at 27,000 km/h. As of early 2026, 5G had passed 3.1 billion subscriptions worldwide and Starlink crossed 10 million active users. Today, fighter jets carry AESA radar with every individual element being its own tiny transmitter and receiver. The latest wave swaps old semiconductor material for gallium nitride, which handles far more power in a smaller space and can roughly double how far a fighter's radar can see.

Phased array antennas electronically steer beams by applying controlled phase shifts to individual antenna elements, enabling constructive interference for desired directions and destructive interference for unwanted angles; the beam steering angle is determined by the equation φ = (2πd/λ)sin(θ), where d is element spacing, λ is wavelength, and θ is the steering angle, allowing applications in 5G communications, radar, and satellite systems without mechanical movement.

This section covers phased array antenna systems and beamforming. Phased arrays consist of many small antenna elements (hundreds to thousands) that can be individually controlled. By adjusting the phase of each element, the beam can be electronically steered without mechanical movement. This enables rapid beamforming and tracking, used in radar, satellite communications, and 5G systems. The section explains that beamforming can also be achieved using multiple antennas with phase control, where signals from different antennas are combined with specific phase relationships to create constructive interference in desired directions.

The RF front end system in a radio receiver performs four essential functions: signal amplification, noise suppression, frequency selection, and impedance matching. The system processes incoming RF signals through sequential stages: sampling via antenna induction, capacitive coupling with 0.1μF capacitors, signal regulation using potentiometers or attenuators, noise filtering through reversely connected diodes, bandpass filtering using tuned transformers with variable capacitors, and final amplification using low-noise NPN transistors in common emitter configuration. Each stage transforms the signal progressively toward the mixer stage while maintaining signal integrity and rejecting unwanted interference.

Impedance matching networks transform impedances between stages to achieve maximum power transfer. When viewed from the load side, a series LC network appears as a series resonant circuit with Q = X/R. When viewed from the source side, the same network appears as a parallel resonant circuit with Q = R_p/X. The relationship between these perspectives allows matching networks to transform impedances. For maximum power transfer, the input impedance seen by the source should match the source's output impedance, which is achieved through proper selection of L and C values in the matching network.

RF systems share common architectural components including mixers, local oscillators, bandpass filters, power amplifiers, and low noise amplifiers. The RF front end represents the most critical and challenging portion of receiver design, encompassing all circuitry from the antenna through the mixer stage. This section processes signals at original high radio frequencies before down-conversion to intermediate frequencies. Key components include the antenna, RF filter, and low noise amplifier. The front end must handle weak incoming signals while rejecting interference, making it essential for receiver sensitivity and selectivity. Understanding front end design is fundamental to creating effective RF communication systems.

RF amplifiers consist of three core components: the central transistor, input impedance matching network, and output impedance matching network. The standard impedance value for RF systems is 50 ohms, enabling maximum power transfer between interconnected modules. For optimal performance, source and load impedances must be complex conjugates—equal real parts with opposite imaginary signs. S-parameters replace traditional parameters for RF characterization, describing wave relationships at each port. Reflection coefficients (gamma) quantify impedance mismatches, and for maximum power transfer, input and output gammas must be complex conjugates. Single-stub matching networks employ combinations of series and shunt elements (inductors and capacitors) to transform impedances to the desired value. The design process involves plotting initial impedance on the Smith chart and moving toward the center by adding reactive components—series elements change impedance along constant resistance circles, while shunt elements move along constant conductance circles. The complete workflow includes selecting an appropriate RF transistor, obtaining S-parameters from manufacturer documentation, calculating reflection coefficients, determining their complex conjugates, and designing matching networks using Smith chart techniques.

The RF front-end is the portion of the radio circuit between the antenna and baseband processor, handling all signal conditioning for transmission and reception. For reception, signals from the antenna pass through filters and amplifiers before reaching the transceiver. For transmission, signals from the transceiver pass through power amplifiers (PA) and filters before being sent to the antenna. The transceiver decodes incoming signals and encodes outgoing signals, converting between digital baseband and analog RF domains. The Power Amplifier requires proper voltage supply (typically 1.8V), clock signals, and data communication with the CPU. The PA output is monitored by sensors that can trigger device shutdown if excessive power is detected, protecting the system from damage.

Waveguides are metallic tubes that confine and transmit electromagnetic energy in the microwave frequency range (gigahertz), offering low signal loss and high power handling capacity for applications like antenna systems and telecommunications; they come in elliptical, rectangular, and circular shapes, with elliptical waveguides being most common for frequencies between 3.4-23.6 GHz, and operate using Transverse Electric (TE) and Transverse Magnetic (TM) modes. Satellite microwave communication systems function as relays that receive, amplify, correct, and retransmit signals between terrestrial stations, enabling high-speed data transfer across long distances and difficult-to-reach geographic areas, though they face challenges including propagation delays, atmospheric effects, and potential signal interruptions.

Transmission lines (two parallel wire, coaxial cable, microstrip) and waveguides (rectangular/circular hollow metallic tubes) differ fundamentally in structure, propagation mechanism, and performance: transmission lines support TEM mode with unlimited frequency range and use circuit theory analysis, while waveguides support only TE and TM modes above a cutoff frequency and require field theory analysis; waveguides offer lower attenuation and higher power handling capability compared to transmission lines, making them preferable for microwave and satellite communications applications.

Microwave waveguides are hollow metallic pipes used to transmit high-frequency electromagnetic waves (microwaves) instead of coaxial cables because at microwave frequencies (around 1 GHz and above), the skin effect causes current to concentrate only in a very thin surface layer (about 2 micrometers), making traditional cables highly lossy; waveguides work by confining electromagnetic waves between their conducting walls, allowing them to travel along the pipe's length through transverse electric and magnetic fields that oscillate perpendicular to each other at approximately the speed of light, with the waveguide having a minimum cutoff frequency determined by its physical dimensions.

Waveguides are essential components in microwave systems including microwave ovens, radar systems, TV transmitters and receivers, and satellite communication equipment. They connect the magnetron to the food chamber in microwave ovens, transmit signals between antennas and equipment in radar systems, and carry signals from satellites to ground stations. Waveguides offer significant advantages over transmission lines: they can handle power levels 100 times greater, have lower losses due to confined electromagnetic fields, and provide guided transmission with minimal radiation loss. Multiplexing allows multiple signals to be transmitted over a single waveguide through frequency multiplexing (different frequencies) or mode multiplexing (different propagation modes). Waveguides support TE (Transverse Electric) and TM (Transverse Magnetic) modes. In closed waveguides with conducting walls, TE modes are possible but TM modes are not, due to boundary condition requirements at conducting walls.

This lecture introduces the fundamental concepts of microwave communication, covering the microwave frequency spectrum divided into designated bands (L-band, S-band, C-band, X-band, Ku-band, K-band, Ka-band) with specific frequency ranges and applications in satellite communication, radar, mobile phones, navigation, medical treatment, and household appliances. It also explains waveguide fundamentals as metallic conduits designed to carry high-frequency radio waves and microwaves, comparing them with transmission lines in terms of power handling, signal attenuation, and frequency characteristics.