Fresnel Zone Calculator

The cool Fresnel zone calculator helps you determine the first Fresnel zone so you can keep your signal transmission as strong as possible. It also determines the other Fresnel zones, if you need them.

First Fresnel zone radius at midpoint11.371 m17.32√(d ÷ 4f) with d in km and f in GHz

VSWR1.5:1a good match

Reflection coefficient |Γ|0.2(75 − 50) ÷ (75 + 50)

Return loss13.979 dBhigher is better, which is why it is quoted as a positive number despite being a loss

Power reflected4%|Γ|² — note this is the SQUARE, so a VSWR of 2:1 reflects only 11% of the power, not a third

Power delivered to the load96%

Mismatch loss0.1773 dBusually far smaller than people expect from the VSWR figure

Reflected power at 100 W4 W

Power reaching the load96 W

Skin depth in copper856.5685 nm√(ρ ÷ πfµ) — the depth at which the current density has fallen to 1/e

At 100× the frequency85.6568 nma tenth — skin depth goes as the inverse square root of frequency, which is why RF conductors are plated rather than solid

Wavelength51.6884 mma quarter wave is 12.9221 mm

Clearance usually required6.823 m60% of the first zone is the customary minimum — obstructing more than that costs signal even with clear line of sight

The formula

Γ = (Z_L−Z₀)/(Z_L+Z₀); VSWR = (1+|Γ|)/(1−|Γ|); δ = √(ρ ÷ πfµ)

VSWR sounds worse than it is

A standing wave ratio of 2:1 reflects only about 11% of the power, not half. The confusion arises because VSWR is a voltage ratio while power goes as voltage squared — so the reflected power is |Γ|², and the mismatch loss for a 2:1 VSWR is only about 0.5 dB. Perfectly usable links run at 2:1 routinely.

What a high VSWR does threaten is the transmitter, not the link budget: reflected power returns to the output stage, and standing waves produce voltage peaks along the line that can exceed its rating. That, rather than lost signal, is why VSWR is worth minimising.

Skin depth shrinks as the inverse square root of frequency. At 100 MHz current in copper flows in roughly the outer 6 micrometres, so the interior of a conductor is simply dead weight — hence silver plating, hollow conductors and litz wire.

The Fresnel zone matters even when line of sight is clear. Radio energy travels in a spreading ellipsoid, not a ray, and obstructing more than about 40% of the first zone costs signal through diffraction. A link that looks clear on a map can still fail on terrain clearance.