Comprehensive Study Notes for Undergraduate Electrical Engineering
Reflector antennas are high-gain directional antennas that use a reflecting surface to concentrate electromagnetic energy into a narrow beam. They are essential components in modern communication systems, radar, radio astronomy, and satellite communications due to their ability to achieve very high gains and narrow beamwidths.
The simplest form consisting of a primary antenna and a flat reflecting surface. While useful for redirecting energy, it does not provide good collimation.
Consists of two or three mutually intersecting conducting flat surfaces (dihedral or trihedral). The feed element (typically a dipole) is placed at the apex.
Two flat plates intersecting at angle θ (typically 90°)
Three mutually perpendicular surfaces
The most common high-gain reflector antenna. The reflector surface follows a paraboloid of revolution defined by the equation:
where f is the focal length and r' is the distance from focus to reflector surface at angle θ'
| Parameter | Formula | Description |
|---|---|---|
| Focal Length | f = D² / (16 × d) | D = diameter, d = depth at center |
| f/D Ratio | f/D = 1 / (4 tan(θ₀/2)) | θ₀ = half-angle subtended by reflector |
| Half-Angle | θ₀ = 2 arctan(D / 4f) | Angle from focus to reflector edge |
| Depth | d = D² / (16f) | Depth of paraboloid at center |
Feed located at focal point, pointed back at reflector
Hyperbolic subreflector redirects energy to feed at vertex
Elliptical subreflector between focus and vertex
Feed positioned off-axis to avoid blockage
Figure: Parabolic reflector showing focal point, feed horn, and ray tracing demonstrating the collimation property
The gain of a reflector antenna is directly related to its physical aperture area and the efficiency of illumination. The fundamental relationship is derived from the effective aperture concept.
The half-power beamwidth (HPBW) is inversely proportional to the antenna diameter in wavelengths.
The exact beamwidth depends on the illumination taper across the aperture. Higher edge taper results in wider beamwidth and lower sidelobes.
The radiation pattern of the feed antenna determines the illumination across the reflector aperture. Optimum illumination requires:
where θ₀ is the half-angle subtended by the reflector
Total aperture efficiency is the product of several factors:
Typical Overall Efficiency: 50% to 70% for well-designed systems
where εᵣₘₛ is the root-mean-square surface deviation. For 1% gain loss, surface errors should be less than λ/20.
The feed antenna is critical for overall system performance. Common feed types include:
| Feed Type | Characteristics | Applications |
|---|---|---|
| Dipole with Reflector | Simple, broad beam, moderate bandwidth | Small dishes, low frequencies |
| Pyramidal Horn | Good directivity, moderate bandwidth | Standard parabolic feeds |
| Conical Horn | Symmetric pattern, good for circular polarization | Satellite communications |
| Corrugated Horn | Low sidelobes, wide bandwidth, low cross-polarization | High-performance systems |
| Choke Ring Horn | Excellent pattern control, very low sidelobes | Radio astronomy, deep space |
Uses hyperbolic subreflector
Uses elliptical subreflector
Offset reflectors eliminate aperture blockage by using a section of the paraboloid away from the axis of symmetry.
| Parameter | Small f/D (Deep Dish) | Large f/D (Shallow Dish) |
|---|---|---|
| Feed Position | Close to vertex | Far from vertex |
| Feed Beamwidth | Wide (harder to design) | Narrow (easier to design) |
| Spillover | Higher | Lower |
| Aperture Illumination | More tapered | More uniform |
| Cross-polarization | Higher | Lower |
| Typical Range | f/D = 0.25 - 0.4 | f/D = 0.4 - 0.6 |
Enter basic parameters to calculate gain, beamwidth, and other characteristics
Earth station antennas (VSAT, DBS), tracking antennas, deep space networks
Typical sizes: 0.6m to 70m
Air traffic control, weather radar, military surveillance, automotive radar
High gain for long-range detection
Single dish observations, interferometer arrays (VLA, ALMA), cosmic microwave background
Extreme surface precision required
Long-distance telephone links, backhaul for cellular networks, point-to-point data
High reliability, high data rates
Communication satellites, planetary probes, GPS satellites
Deployable and inflatable reflectors
Satellite TV dishes (DTH), 5G base stations, WiFi backhaul
Low-cost mass production
Test your understanding of reflector antenna concepts:
Select the correct answer: