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My vertically polarized antenna’s main lobe is tilting upward. At what antenna height above ground does ground reflection transition from constructive to destructive interference, causing this pattern tilt?

I have a λ/4 monopole on a large ground plane. At low heights (e.g., λ/4 above real ground), I expect maximum radiation at the horizon. My measurement shows the peak gain at a 15° elevation. I understand this is due to the phase of the ground-reflected wave. Is there a simple formula or rule of thumb for the elevation angle of the first maximum as a function of antenna height (h) and ground constants? Does this mean for certain applications (e.g., terrestrial links), there's an "optimum" height to maximize horizontal gain?

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By Arjun Answered 1 year ago

This is classic ground reflection array theory. For a monopole of height *h* above perfect ground, the far-field is proportional to sin(kh sinθ), where θ is the elevation angle (0° at horizon). The first maximum occurs when the argument is π/2, leading to:
θ_max = arcsin( λ / (4h) ).
So, for *h = λ/4*, θ_max = arcsin(1) = 90° (straight up, which is incorrect for real ground). For real ground, the reflection coefficient has a phase shift near 180° at grazing angles, modifying the pattern. For a typical *h = 0.5λ*, the formula gives θ_max ≈ 30°; with real ground, it's closer to 15-20°. The "optimum" height for maximum horizon gain is actually very low (h

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