A vertical AM tower isn’t just a mast that holds an antenna up — the tower is the antenna, and how tall it stands is one of the most consequential choices a station makes. But the best height is not simply “as tall as you can afford.” What actually matters is the tower’s height measured in wavelengths — its electrical height — and there is a sweet spot beyond which a taller tower puts less signal along the ground, not more.
Because a wavelength depends on frequency, the very same physical tower behaves completely differently across the band. A quarter of a wavelength (90 electrical degrees) is the classic, dependable height. Keep going and the ground-wave signal actually strengthens, reaching a maximum a little past half a wavelength. Push beyond that and the signal along the ground falls away, because the energy starts going up at high angles instead of out toward listeners.
Set a frequency, then slide the tower up and down and watch where it lands on the curve.
Ground-wave field
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Electrical height
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vs quarter-wave
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Wavelength
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Leave it on 740 kHz and slide the tower up from short. The field climbs through the quarter-wave point near 90° — the standard AM height — and keeps rising. Around 190° you reach the anti-fade region, and near the labeled peak you are getting just about the most ground-wave signal a single tower can produce. Keep going and the marker crests and starts back down: the tower is taller, yet the signal along the ground is weaker, because power is now leaking into a high-angle lobe that does nothing for ground coverage.
Now change the frequency without touching the height. The marker jumps anyway — because what counts is electrical height, the tower measured in wavelengths. A 350-foot stick is a modest quarter-wave at 740 kHz but an electrically tall mast up near 1700 kHz. That is the whole lesson of the curve: the bottom axis is degrees, not feet.
This is why a station near the bottom of the dial needs an enormous tower to reach even a quarter-wave, while a station near the top can do the same job with a stub a few hundred feet tall. It is also why some stations deliberately build to around 190° — tall enough for a strong ground wave, but shaped to throw little signal skyward, which reduces the nighttime fading that happens when a station’s own sky wave interferes with its ground wave.
A note on the model: these figures are idealized — a single base-fed tower with the textbook sinusoidal current distribution, over perfect ground, with no losses. The quarter-wave value is anchored to the standard reference of about 195 mV/m at 1 km for 1 kW, and the rest of the curve follows from the tower’s gain toward the horizon. Real towers over real ground come in lower, and the exact height of the peak shifts a little with ground conductivity — but the shape, and the lesson, hold.