RF ENGINEERING // ANTENNA DESIGN // OFFSET PARABOLIC

Offset Dish Calculator

Complete offset parabolic dish antenna calculator with corrected geometry. Computes focal length, F/D ratio, true offset angle, subtended half-angle, feed distances, tilt mount angle, gain, HPBW, sidelobe estimate, and illumination cone. Includes interactive cross-section visualization and gain vs frequency sweep.

Parent Parabola Geometry True Offset Angle F/D Ratio Feed Rim Distances Mount Tilt Angle Gain / HPBW Subtended Half-Angle Sidelobe Level 5 Amateur Bands mm / inches

Antenna Parameters

Input Units

Width W (horizontal aperture) Horizontal dimension of the dish Height H (vertical aperture) Vertical dimension of the dish Max Depth d (sag across width) Measured from aperture plane to deepest point

Frequency Aperture Efficiency η (%) Typical offset dish: 60–75%

Desired Elevation Angle (°) Used to compute mount tilt angle

Focal Length f = W² / (16·d)	—
F/D Ratio (f / W)	—
Parent parabola diameter equivalent	—

True offset angle θ_off (bottom-rim polar angle)	—
Subtended half-angle θs (feed cone half-width)	—
Full illumination cone angle 2·θs	—
Center of dish polar angle from focus	—
Top rim polar angle from focus	—

Distance from focus to bottom rim	—
Distance from focus to top rim	—
Distance from focus to center of dish	—
Feed path difference top→bottom	—

Desired elevation angle	—
Dish back tilt from vertical (mount angle)	—
Dish axis elevation angle (= desired elev.)	—
Feed pointing tilt from dish axis	—

Effective aperture diameter (W)	—
Wavelength λ	—
Aperture efficiency η	—
Gain G = η·(πD/λ)²	—
HPBW ≈ 70·λ/D	—
First sidelobe level (Airy-pattern approx.)	—17.6 dB
Noise temperature contribution (sky + spillover est.)	—