An AM directional antenna system uses two or more towers, each fed with a precisely controlled phase and amplitude, to mold a station’s coverage into something other than a circle. The piece of equipment that does the controlling — a forest of vacuum capacitors, coils, current meters, and sampling loops, often filling its own small building at the base of the array — is called the phasor.
By adjusting how much current each tower carries, and at what phase angle each one is driven, the engineer can push power toward the listeners and away from co-channel stations a thousand miles away. Many AM stations are licensed with two completely different patterns: a daytime pattern, often broader because ground wave is the only path reaching the audience, and a nighttime pattern, usually tighter because the ionosphere starts bouncing signals around after dark and a station’s reach — and its potential to interfere with distant stations — expands dramatically.
This interactive lets you build and reshape your own array. Pick a configuration, drag the towers around, slide the per-tower phase and amplitude, and watch the radiation pattern morph in real time. The phasor strip at the bottom is the engineer’s-eye view: each tower’s drive as a vector in the complex plane — length is amplitude, angle is phase.
Start with the 2-tower config and tap Day. That’s the classic cardioid — a peak to the west and a clean null toward the east. Now grab Tower 2’s phase slider and walk it slowly from 90° to 180°. Watch the null swing south while the pattern stretches into a figure-eight. Look at the phasor strip while you do it: the two arrows are rotating relative to each other, and where they point opposite directions is where the pattern goes to zero.
Switch to the 3-tower line, Day. That’s an endfire array — the beam shoots east because each successive tower is driven at a phase that exactly cancels its eastward extra path. Now drag Tower 2 north or south and the beam tilts along with it. Geometry and phasing are two ways of saying the same thing.
Try the 4-tower square on Night. Four phasors fanned out 90° apart on the complex plane produce a satisfyingly tight, symmetric pattern with four deep nulls — exactly the kind of arrangement a station might use to protect several distant co-channels at once.
And drag a tower way out past the third ring on the layout. Spacing alone, with phases unchanged, reshapes the pattern dramatically — that’s why moving even one tower in a licensed array requires re-tuning the whole phasor.
What you’re playing with here is exactly what FCC-licensed AM directional stations do every sunset and sunrise. The phasor cabinet at a directional facility is the actual physical embodiment of every slider on this page: capacitors and inductors arranged to produce specific phase shifts, transformers to set current ratios, and meters and sampling loops so the operator can verify the pattern is what the license says it should be. Real arrays can run far beyond four towers — some legacy class-A stations operate with six, eight, or even twelve — and patterns are routinely re-verified with field-strength surveys covering hundreds of miles. The night the engineer flips the dawn-dusk switch, the phasor physically reconfigures, the towers redistribute their current, and the pattern reshapes — quietly, all at once, exactly like you reshape it here with the preset buttons.
A note on the model: this is the horizontal radiation pattern only, with the textbook assumption of identical short verticals, no mutual coupling between towers, and lossless ground. Real arrays bring complications — coupling between closely-spaced towers, common-point impedance, sampling-loop currents, ground-system irregularities — that an engineer has to chase down on the workbench. The pattern shapes here are honest about phase and amplitude relationships but ignore those second-order effects. The relative directions of peaks and nulls, and how they swing as drives change, are what to watch.