E9B: Antenna Patterns and Designs

An antenna's radiation pattern is a map of how it distributes energy in different directions. Reading and interpreting these patterns — and understanding how they are modeled mathematically — is essential for choosing and comparing antennas.

This lesson covers azimuth and elevation radiation patterns, 3 dB beamwidth, front-to-back and front-to-side ratios from Figures E9-1 and E9-2, total radiated power versus isotropic, the far field, and antenna modeling using the Method of Moments.

Azimuth and Elevation Patterns

Antenna radiation patterns are presented in two complementary cross-sections:

• Azimuth pattern: A horizontal slice through the radiation pattern viewed from above, showing how the antenna radiates in different compass directions. Used to assess directionality in the horizontal plane.
• Elevation pattern: A vertical slice showing how the antenna radiates at different angles above the horizon. The angle of peak response is called the elevation angle or takeoff angle, and it strongly affects which propagation paths the antenna favors.

Beamwidth, Front-to-Back, Front-to-Side

Three key numbers describe a directional antenna's pattern performance:

• 3 dB beamwidth: The angular width of the main lobe measured between the two points where power drops to half (3 dB below) the peak. A narrower beamwidth means higher directivity.
• Front-to-back ratio: The difference in dB between the signal level in the direction of maximum radiation (front) and directly behind the antenna (back). A higher ratio means less interference from signals coming from the rear.
• Front-to-side ratio: The difference in dB between the main lobe and the strongest signal 90 degrees to the side of the main beam.

Figure E9-1: Azimuth Pattern Readings

Figure E9-1 shows an azimuth radiation pattern. Three exam questions ask you to read specific values from this pattern.

Figure E9-2: Elevation Pattern Readings

Figure E9-2 shows an elevation radiation pattern. Three exam questions ask about the pattern type and its measured values.

Total Radiated Power: Directional vs Isotropic

A common misconception is that a directional antenna amplifies power. It does not. A lossless directional antenna and an isotropic radiator driven by the same transmitter power radiate the same total amount of power — the directional antenna simply redistributes that power, concentrating more of it in the desired direction at the expense of other directions. Total radiated power is determined by transmitter output and antenna losses, not by pattern shape.

The Far Field

The far field of an antenna is the region where the radiation pattern's shape no longer varies with distance from the antenna. In the far field, the electric and magnetic fields are perpendicular to each other and to the direction of propagation, and the ratio of E to H equals the impedance of free space (377 Ω). All practical antenna measurements and pattern specifications are made in the far field.

Antenna Modeling: Method of Moments

The most common mathematical technique for computer antenna modeling is the Method of Moments (MoM). In this approach, each wire in the antenna structure is divided into short segments. Each segment is assigned a uniform current value, and the interactions between all segments are solved simultaneously to find the current distribution across the entire antenna.

The accuracy of a Method of Moments model depends on using enough segments. If the number of wire segments drops below 10 segments per half-wavelength, the computed feed point impedance may become incorrect — one of the key disadvantages of using too few segments. More segments produce more accurate results but require more computation.

E9B Practice Questions