Antenna modeling

What it is

Antenna modeling is predicting how an antenna will behave by calculation, before you cut any wire. You describe the wires, their height, the ground and the frequency; the program returns the pattern, gain, feed-point impedance and SWR. It lets you try a dozen variations in an evening for no cost, and see effects such as height that are hard to measure.

How it works

The program divides every wire into short segments and assumes the current is roughly uniform within each one. Each segment's current produces a field, which acts on every other segment, so the solver sets up and solves a large set of simultaneous equations for all the currents at once. From those currents it computes the far-field pattern, and from the current at the source segment, the feed-point impedance. That is the Method of Moments.

The wire has to be cut finely enough to follow the true current. Using fewer than about 10 segments per half wavelength is a classic mistake: the computed feed-point impedance can come out wrong. Segments that are extremely short compared with the wire diameter, or sudden size changes between neighbours, cause errors too.

What it can and cannot tell you

Reliable for	Be careful with
Trends: what happens when you raise, lengthen or add an element	Absolute gain and impedance, which depend on the inputs
Pattern shape, lobes and nulls, front-to-back	Real soil: it is usually treated as one uniform layer, and your yard is not
Where current flows, including on the feed line or tower	Buildings, trees and wiring unless you model them
Comparing candidate designs on equal terms	Loss in coils, traps and connectors unless you enter it

The model contains exactly what you typed. A good model of the wrong antenna is still wrong, and the results come out with many decimal places whether or not they deserve them.

In practice

• Validate first. Model a plain half-wave dipole and check it against known results (roughly 70 to 73 Ω in free space, 2.15 dBi) before trusting the program on a design you care about. Compare relative changes more than absolute figures.
• Change one thing at a time, so you know what caused each difference.
• Use a sensible ground such as poor, average or good soil, and see how much the answer moves. If it moves a lot, treat the result as a range.
• Then build and measure with an analyzer, and feed what you find back into the model. Modeling reduces surprises; it does not replace measuring.
• Match what you want to know. Modeling can predict a pattern, not whether the band will be open; see Skywave, hops and the skip zone.

Related

• Radiation patternsReading azimuth and elevation plots, beamwidth and front-to-back.
• Ground effects and heightHow earth and height shape the pattern.
• Antenna impedance and radiation resistanceWhat the feed line sees at the feed point.
• Gain and directivitydBi, dBd and where the energy goes.
• Antenna analyzers and VNAsMeasuring impedance, SWR and resonance across a band.
• The half-wave dipoleThe reference antenna: pattern, impedance and length.