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Parabolic dish calculator

What actually decides things here

The gain of a dish is set almost entirely by its diameter, and that is the dullest part of the calculation. What is interesting is f/D: it decides the angle at which the feed must “see” the rim. Get that wrong and part of the power sails past the reflector, and the efficiency drops so far that a bigger dish will not save you.

feedD 1.2 mF
The feed sits at the focus; the angle at which it sees the rim of the dish sets f/D
Frequency (MHz)
Dish diameter (m)
Aperture efficiency
f/D ratio
Characteristics
Gain
−3 dB beamwidth
First pattern null
Effective area
Geometry
Focal length
Dish depth
The feed should illuminate
Surface accuracy tolerance (λ/16)

📐 The formulas​

Gain and pattern:

G[dBi] = 10 × log10( η × (π·D/λ)² )
HPBW  ≈ 70 × λ/D      [degrees]
first null ≈ 140 × λ/D
Ae = η × π·D²/4

Geometry:

F = (f/D) × D                focal length
depth  d = D² / (16·F)
angle to the rim: θ = 2 × arctan( 1 / (4 × f/D) )
surface tolerance ≈ λ/16   (costs ≈ 0.5 dB)

Typical aperture efficiencies:​

ηWhen
0.45a simple feed, noticeable spillover
0.55typical for an amateur build
0.60–0.70an engineered horn and careful geometry
Where this calculator stops:
  • The aperture formula holds for dishes of at least 5–10 λ; below that it overstates the gain
  • Aperture efficiency is entered, not calculated: it depends on the feed, not on the dish
  • Blockage by the feed and its support is not accounted for, and on small dishes that is up to 1 dB
  • Noise temperature, which matters for EME and for receiving, is not calculated at all

🎯 In practice​

  • An amateur microwave station. A 1.2 m dish at 10 GHz gives nearly 40 dBi and a 1.7° beam.
  • Receiving QO-100. A domestic 60–90 cm satellite dish, with a calculation of what it will actually do.
  • Sizing up someone else's antenna. Diameter and frequency, and you know its gain to within a couple of decibels.
Practical advice:
  1. Work out the angle to the rim first, then choose the feed — not the other way round.
  2. A 1.7° beam means aiming has to be precise: a hand-turned mount will not do.
  3. At 10 GHz a solid surface is essential; at 1.2 GHz mesh with openings up to λ/10 works just as well.
  4. A deep dish (f/D 0.3) shields the feed from the ground better; a shallow one (f/D 0.5) is easier to illuminate.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

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UR3PKI. «Parabolic dish calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/antennas/parabolic (licence CC BY-NC-SA 4.0).

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