E9B

Antenna patterns and modeling

Read azimuth and elevation plots for beamwidth, front-to-back and peak angle, and explain far field and Method of Moments modeling.

11 pool questions1 on the exam

Syllabus: Antenna patterns and designs: azimuth and elevation patterns; gain as a function of pattern; antenna modeling

Read an azimuth plot: rings are dB below the peak

The outer ring is 0 dB, the main lobe's peak. Each inner ring is that many dB weaker. In E9-1 the main lobe points to 0°.

3 dB beamwidth: find where the main lobe crosses the −3 ring on each side, then count the degrees between them.
Front-to-back: peak minus the level at 180°.
Front-to-side: peak minus the level at 90°.

Rings are dB below the peak. Read the ring the line crosses, then subtract.

RememberSubtract the ring level from 0. E9-1: beamwidth 50°, front-to-back 18 dB, front-to-side 14 dB.

An elevation plot shows lobes above the horizon

Azimuth plots go all the way round (0 to ±180°) and show compass direction. Elevation plots show only the upper half, from the horizon (0°) to straight up (90°), because ground sits below. In E9-2 the lowest lobe is the strongest, peaking at 7.5°. Front-to-back works the same way: 0 dB ahead versus the level at 180°, here 28 dB.

Elevation plot: 0° is the horizon, 90° is straight up. The ground blocks the lower half.

RememberAngles 0° to 180° with ground below = elevation. Peak angle = tip of the longest lobe.

Gain moves power; it does not add power

A lossless antenna and an isotropic radiator driven by the same power radiate the same total power. The directional antenna just squeezes it into some directions, so gain in one direction means less elsewhere.

Dashed circle = isotropic reference. Same power in each case: gain is signal moved into one direction, not power added.

RememberSame total radiated power. Gain is where it goes.

The far field has a fixed pattern shape

Close to an antenna the pattern changes with distance. Beyond some distance the shape no longer varies; signal only gets weaker. Patterns, gain and beamwidth are specified in the far field.

Far field = the region where the shape of the radiation pattern no longer varies with distance.

RememberFar field = pattern shape stops changing with distance.

Antenna models chop wires into segments

Antenna modeling software uses the Method of Moments. It models each wire as a series of segments, each with a uniform current, then solves how they all interact. Too few segments follow the real current poorly. Use at least 10 per half wavelength, or the computed feed point impedance may be incorrect.

Method of Moments: wire = a chain of short segments, each with one uniform current.

Segments per half wavelength7

RememberMethod of Moments = many short segments of uniform current. Fewer than 10 per half wave risks a wrong feed point impedance.

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Practise the 11 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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