Antenna Location & Height Predictions

Every JK model starts from its free-space pattern, which is how the antenna radiates with no ground at all. For a Yagi, that’s the main forward lobe in elevation plus the smaller lobes off the back and rear (front-to-back and front-to-rear).

Real ground changes that pattern. Signal reflected off the ground in front of the tower combines with the direct signal. At some take-off angles the two add up and form lobes, and at others they cancel and form nulls. ALHP reads the real USGS ground heights along your beam heading, out to the distance you pick, and works out those reflections for your soil type. It includes slopes, hills, and the signal that bends over a hilltop. For a stack of two or three antennas, it combines each antenna at its own height.

The solid navy line is the pattern at your site. The dashed amber line is the same antenna over flat ground at the same height. The Difference column shows your site’s gain or loss compared with flat ground. Ground that falls away in front of the tower lowers the take-off angle and adds low-angle gain, while rising ground or a nearby hill does the opposite.

How stacking and gain are calculated

ALHP works out the gain at every take-off angle from 0.25° to 90°, in quarter-degree steps. At each angle it adds the direct signal to every copy reflected off your terrain, taking into account how much each one weakens and how far out of step it arrives. The total is the gain in dBi at that angle. Over flat ground the strongest low-angle lobe can reach about 6 dB above the free-space figure.

In a stack of two or three antennas, each one is the same JK model at its own height, fed in phase with the power split equally. ALHP traces the direct and reflected signals from every antenna and adds them all together. Because the antennas sit at different heights, their signals line up at some angles and not others. That sharpens the main lobe and usually adds about 2 to 3 dB for two antennas, and a bit more for three.

Why half a wavelength apart: antennas closer than half a wavelength on their lowest band interact with each other. That pulls their tuning and pattern away from the published figures, and the stack gains little. So ALHP only offers stacking at half-wave spacing or more. The prediction doesn’t include that interaction or phasing-line losses, which is why it’s a close estimate rather than an exact figure.

The peak is the highest gain and the angle where it occurs. The lobe spans the angles within 3 dB of the peak.