Calculator Inputs
Formula Used
Parent paraboloid: z = (x² + y²) / 4F
Elliptical aperture area: A = πWH / 4
Rectangular aperture area: A = WH
Equivalent diameter: D = √(4A / π)
Wavelength: λ = c / frequency
Gain: G = η × (πD / λ)²
Gain in dBi: 10 × log10(G)
Beamwidth estimate: HPBW ≈ 70λ / D
Ruze factor: e^[-(4πσ / λ)²]
How to Use This Calculator
Enter the dish width and height at the aperture rim. Add the parent focal length and the offset of the aperture center from the parent axis.
Select the aperture shape. Use elliptical for most offset antennas. Use rectangular when checking a panel or cut section.
Enter frequency and efficiency for antenna estimates. Add RMS surface error when surface accuracy matters. Press calculate to show results below the header and above the form.
Use CSV for spreadsheet work. Use PDF for printing, sharing, or saving a quick design record.
Example Data Table
| Use Case | Width | Height | Focal Length | Offset | Frequency | Efficiency |
|---|---|---|---|---|---|---|
| Small satellite reflector | 0.75 m | 0.85 m | 0.45 m | 0.22 m | 12 GHz | 60% |
| Point to point radio dish | 1.20 m | 1.35 m | 0.70 m | 0.32 m | 5.8 GHz | 55% |
| Solar reflector concept | 1.50 m | 1.70 m | 0.90 m | 0.40 m | 1 GHz | 70% |
Offset Parabolic Dish Design Guide
What the Shape Means
An offset parabolic dish is a cut from a larger parent paraboloid. The feed sits away from the main opening, so it blocks less incoming energy. This layout is common in satellite antennas, radio links, and compact solar concentrators. It also helps when a clean aperture is needed.
Why Offset Geometry Matters
The calculator begins with the parent focal length. It then places the dish opening at a chosen offset from the parent axis. Each surface point follows the same parabolic rule. Because the opening is offset, the upper rim and lower rim can have different depths. That is why a normal on-axis depth value is not enough for careful layout.
Input Quality
Good inputs make better estimates. Measure the aperture width and height at the rim. Use the same unit for focal length and offset distance. Enter the frequency when antenna gain or beamwidth matters. For solar use, frequency can stay at a sample value, and the geometry outputs remain useful. Efficiency covers spillover, taper, material loss, and alignment errors.
Performance Estimates
The equivalent diameter converts an oval or rectangular opening into a round aperture with the same area. This helps estimate gain and beamwidth. A larger aperture gives more gain. A higher frequency also gives more gain, but it narrows the beam. Narrow beams require stronger pointing control.
Surface Accuracy
Surface accuracy is important at microwave frequencies. A rough reflector scatters energy. The Ruze factor estimates this loss from RMS surface error. Low frequency systems tolerate more error. High frequency links need a smoother surface.
Practical Planning
Use the offset angle and normal tilt to plan feed brackets. These values are estimates, not a replacement for full electromagnetic modeling. Still, they are useful for early design checks. Compare several rows in the example table. Small changes in focal length, offset, or efficiency can move the feed and change performance.
Final Checks
For building work, add safety margins. Real dishes bend, supports sag, and feed horns have their own patterns. Check the final design with physical measurements and manufacturer data. This calculator gives a clear starting point for practical reflector planning.
Record trials before cutting material. Saved CSV and PDF files make reviews easier. Teams can compare aperture plans and feed positions clearly. This reduces layout mistakes early.
FAQs
What is an offset parabolic dish?
It is a section cut away from the center of a larger paraboloid. The feed is placed outside the clear aperture, reducing blockage and improving usable reflector area.
Why is the offset distance important?
The offset distance changes rim depth, feed direction, and surface tilt. It helps define how far the aperture sits from the parent dish axis.
What does equivalent diameter mean?
Equivalent diameter is the round dish diameter with the same projected area. It allows gain and beamwidth estimates for oval or rectangular openings.
Can this calculator be used for solar dishes?
Yes. The geometry values are useful for solar reflectors. Antenna gain and beamwidth are mainly for radio frequency designs.
What is aperture efficiency?
Aperture efficiency estimates real reflector performance after losses. It includes feed spillover, taper, surface error, alignment, and material effects.
What is the Ruze factor?
The Ruze factor estimates loss from surface roughness. It becomes more important as frequency rises or wavelength becomes shorter.
Why are top and bottom rim depths different?
An offset aperture is not centered on the parent axis. Because of that shift, different rim points lie at different paraboloid depths.
Is this a final engineering design tool?
No. It gives strong early estimates. Final builds should include structural checks, feed pattern data, material tolerances, and field measurements.