Electronics Guide

Antenna Elements

The radiating elements form the core of any antenna system. These may be simple dipoles, complex arrays, reflector antennas, or specialized structures depending on application requirements. Element selection depends on frequency, gain requirements, beamwidth, polarization, and physical constraints.

Feed Networks

Feed networks distribute RF energy between the transmitter/receiver and antenna elements. For single-element antennas, this may be a simple coaxial cable. For arrays, complex corporate feeds, series feeds, or hybrid networks distribute signals with proper amplitude and phase relationships to achieve desired radiation patterns.

Matching Networks

Impedance matching ensures efficient power transfer between the transmission line and antenna. Matching networks may use lumped elements, transmission line stubs, transformers, or combinations to transform impedances and minimize reflections across the operating bandwidth.

Support Structures

Towers, masts, poles, and mounting brackets provide physical support for antennas. These structures must withstand environmental loads (wind, ice, seismic) while minimizing interference with antenna patterns. Height, stability, and access for maintenance are key design factors.

Transmission Lines

Coaxial cables, waveguides, or other transmission lines connect antennas to transmitters and receivers. Line selection involves tradeoffs between loss, power handling, flexibility, cost, and environmental durability. Long runs or high frequencies may require waveguide or low-loss cable to maintain efficiency.

Linear Arrays

Linear arrays arrange elements along a line, typically with uniform spacing. The resulting pattern has high directivity in the plane containing the array axis. Linear arrays are common in broadcast antennas, base station antennas, and radar systems. Key parameters include element spacing, number of elements, and excitation taper.

Planar Arrays

Planar arrays arrange elements in a two-dimensional grid, enabling beam control in both azimuth and elevation. These arrays are fundamental to modern radar, satellite communications, and 5G millimeter-wave systems. The array factor is the product of the individual row and column array factors.

Circular and Conformal Arrays

Circular arrays provide 360-degree azimuthal coverage with direction-finding capability. Conformal arrays mount elements on curved surfaces (aircraft fuselages, ship hulls) to maintain aerodynamic profiles while providing antenna functionality.

Array Feed Techniques

Feeding array elements with proper amplitude and phase requires careful network design:

• Corporate feed: Binary power division tree providing equal path lengths to all elements. Offers bandwidth but requires complex layouts.
• Series feed: Elements fed sequentially along a transmission line. Simpler but introduces progressive phase shift with frequency (beam squint).
• Space feed: A feed antenna illuminates the array through free space, similar to reflector antenna feeds. Used in large arrays where corporate feeds become impractical.
• Active arrays: Each element has its own amplifier and phase shifter, enabling precise control and graceful degradation with element failures.

Phase Shifter Technologies

Phase shifters are critical components in phased arrays:

• Switched-line phase shifters: Select among transmission lines of different lengths. Simple and predictable but limited to discrete phase states.
• Loaded-line phase shifters: Vary effective transmission line length using switchable reactive elements.
• Reflection-type phase shifters: Use variable-impedance terminations to create phase-shifted reflections.
• Ferrite phase shifters: Magnetic materials provide continuous phase control at high power levels.
• Digital phase shifters: Provide discrete phase states typically in 3-6 bit resolution.
• Analog/vector phase shifters: Enable continuous phase variation for precise beam positioning.

Beam Steering Principles

To steer the main beam to angle theta from broadside, elements must be excited with a progressive phase shift:

delta phi = (2 pi d / lambda) sin(theta)

Where d is element spacing and lambda is wavelength. This creates a plane wave front directed at the desired angle. Practical systems must consider grating lobes (which appear when element spacing exceeds half wavelength), reduced effective aperture at scan angles, and element pattern effects.

Active Electronically Scanned Arrays (AESA)

AESA systems integrate transmit/receive modules at each element position. Each module contains power amplifier, low-noise amplifier, phase shifter, and control electronics. AESA benefits include:

• Graceful degradation with element failures
• Multiple simultaneous beams
• Rapid beam agility
• Adaptive nulling for interference rejection
• High reliability through redundancy

Digital Beamforming

Modern systems increasingly digitize signals at each element, performing beamforming in software. Digital beamforming enables unlimited simultaneous beams, optimal combining algorithms, and adaptive processing that can continuously optimize performance based on the signal environment.

Parabolic Reflectors

Parabolic reflectors focus incoming parallel rays to a single point, or convert a point source into a collimated beam. Key configurations include:

• Prime focus: Feed at the geometric focal point. Simple but the feed blocks some aperture.
• Cassegrain: A convex hyperbolic subreflector redirects energy to a feed behind the main reflector, eliminating feed blockage and simplifying receiver placement.
• Gregorian: Similar to Cassegrain but uses a concave elliptical subreflector, placing the virtual focus behind the main reflector.
• Offset feed: The feed is positioned outside the aperture, eliminating blockage entirely at the cost of some asymmetry.

Feed Systems for Reflectors

The feed illuminates the reflector with appropriate amplitude and phase distribution. Common feeds include:

• Horn antennas: Provide controlled beamwidth and low sidelobes. Corrugated horns offer excellent cross-polarization performance.
• Dipole feeds: Simple and compact, often with reflectors or directors to shape the pattern.
• Splash plate feeds: Direct feeds that reflect energy off a small plate to illuminate the main reflector.
• Array feeds: Multiple feed elements enable beam shaping, multiple beams, or adaptive pattern control.

Feed design involves tradeoffs between illumination taper (affecting aperture efficiency and sidelobe levels), spillover (energy missing the reflector), and cross-polarization.

Shaped Reflectors

Departing from pure parabolic geometry enables customized beam patterns. Shaped reflectors in satellite systems create contoured coverage areas matching geographic regions. Single or dual reflector shaping can optimize gain, minimize spillover, or shape the beam footprint.

Fixed Mount Systems

Fixed mounts position antennas permanently toward specific targets. Common in point-to-point links and broadcast applications, these systems require accurate initial alignment but no ongoing positioning control. Mounting hardware must maintain alignment despite environmental loads and thermal expansion.

Azimuth-Elevation Mounts

Az-El mounts provide rotation about vertical (azimuth) and horizontal (elevation) axes. This intuitive system is common in satellite earth stations, radar, and radio astronomy. Near zenith, rapid azimuth rotation may be needed to track objects passing overhead (the keyhole problem).

Polar (Equatorial) Mounts

Polar mounts align one axis parallel to Earth's rotation axis, enabling satellites in geostationary arc to be tracked with single-axis rotation. Common in amateur and TVRO satellite systems, polar mounts simplify tracking but require accurate polar alignment.

Tracking Systems

Tracking systems keep antennas pointed at moving targets:

• Program tracking: Follows pre-computed trajectories based on orbital elements or flight plans.
• Monopulse tracking: Uses comparison of signals from multiple feed elements to derive pointing errors.
• Conical scan: A nutating feed creates amplitude modulation indicating pointing error.
• Step tracking: Periodically adjusts pointing to maximize received signal.
• GNSS-aided: Uses GPS/GNSS for approximate pointing, refined by signal tracking.

Tower Types

Antenna support structures must provide height, stability, and access:

• Self-supporting towers: Free-standing lattice structures requiring no guy wires. Higher cost and larger footprint but avoid land requirements for guy anchors.
• Guyed towers: Lighter lattice or tubular structures supported by guy wires. Cost-effective for tall installations but require significant land for guy anchors.
• Monopoles: Single tubular structures with clean appearance. Common in urban areas where aesthetics matter.
• Stealth structures: Antennas concealed in artificial trees, flagpoles, or architectural elements to minimize visual impact.

Structural Loading

Tower design must account for multiple load types:

• Dead loads: Weight of tower, antennas, cables, and equipment.
• Wind loads: Often the dominant design factor, particularly for large antennas.
• Ice loads: Ice accumulation increases weight and wind loading area.
• Seismic loads: Ground motion effects, particularly important in earthquake-prone regions.
• Thermal loads: Expansion and contraction with temperature changes.

Safety and Access

Tower installations require safe climbing systems, work platforms, fall protection, and lighting for aircraft warning. Regulatory requirements (such as FAA in the United States) may mandate lighting patterns and tower marking. Ground systems provide lightning protection and safe grounding.

Coaxial Cable Systems

Coaxial cables are the most common antenna feed lines. Key considerations include:

• Attenuation: Increases with frequency and cable length. Low-loss cables use larger diameters and foam dielectrics.
• Power handling: Limited by heating and voltage breakdown. Increases with cable size and decreases with frequency.
• Connectors: Must maintain impedance match and weatherproofing. Common types include N, 7/16 DIN, and various proprietary designs.
• Weatherproofing: Outdoor installations require UV-resistant jackets and sealed connections.

Waveguide Systems

Waveguides offer lower loss than coaxial cable at microwave frequencies. Rigid rectangular or circular waveguide is common in high-power and low-loss applications. Flexible waveguide sections accommodate movement. Pressurization prevents moisture ingress and arcing.

System Accessories

Complete transmission line systems include:

• Surge protectors: Protect equipment from lightning-induced surges.
• Power dividers/combiners: Split or combine signals for multiple antennas or redundant equipment.
• Circulators and isolators: Direct signal flow and protect transmitters from reflected power.
• Filters: Suppress interference, harmonics, and out-of-band signals.
• Line stretchers: Provide adjustable electrical length for phase matching.

Effective Isotropic Radiated Power (EIRP)

EIRP characterizes transmit system performance as the equivalent power from an isotropic antenna:

EIRP = Pt - Lf + Ga

Where Pt is transmitter power (dBW), Lf is feed line loss (dB), and Ga is antenna gain (dBi). EIRP determines the signal strength available at distant receivers.

System Noise Temperature

For receive systems, the system noise temperature determines sensitivity:

Tsys = Ta + Tr + Tline

Where Ta is antenna noise temperature (from sky, ground, and interference), Tr is receiver noise temperature, and Tline accounts for losses in the feed system. Low-noise amplifiers mounted at the antenna minimize the contribution of feed line losses.

G/T Figure of Merit

The ratio of antenna gain to system noise temperature (G/T) characterizes receive system performance. Expressed in dB/K, G/T enables comparison of different antenna systems independent of specific signal parameters. Satellite earth station specifications commonly use G/T requirements.

Broadcast Systems

Broadcast antenna systems provide wide-area coverage for radio and television. FM and TV antennas typically use vertical arrays of radiating elements on tall towers to achieve coverage requirements. Circular polarization reduces multipath effects in mobile reception.

Cellular and Wireless Networks

Base station antenna systems use sector antennas with carefully controlled patterns to provide coverage while minimizing interference. Modern systems include remote electrical tilt (RET) for pattern optimization and massive MIMO arrays for capacity enhancement.

Satellite Communications

Earth station antenna systems range from small consumer dishes to large tracking antennas for deep space communication. Key requirements include precise pointing, low noise temperature, and appropriate G/T for the link budget.

Radar Systems

Radar antenna systems must handle high peak powers while maintaining pattern quality. Phased arrays enable rapid beam steering for target tracking and search functions. Specialized requirements may include monopulse feeds for precision tracking.