Calculator Input
Example Data Table
| Dish Width | Dish Height | Depth | Unit | Frequency GHz | Efficiency % | Use |
|---|---|---|---|---|---|---|
| 60 | 66 | 6.5 | cm | 11.7 | 60 | |
| 75 | 85 | 8.5 | cm | 11.7 | 62 | |
| 90 | 100 | 10.8 | cm | 12.2 | 65 |
Formula Used
Equivalent diameter: D = √(width × height)
Focal length: F = D² ÷ (16 × depth)
Focal ratio: F/D = focal length ÷ equivalent diameter
Offset angle: θ = atan(height ÷ (4 × focal length))
Dish face reading: face angle = satellite elevation − offset angle + mount correction
Wavelength: λ = 0.299792458 ÷ frequency in GHz
Gain: G = 10 log10(efficiency × (πD ÷ λ)²)
Beamwidth: HPBW ≈ 70λ ÷ D
How To Use This Calculator
Measure dish width across the widest rim points.
Measure dish height from top rim to bottom rim.
Place a straight edge across the rim.
Measure depth from the straight edge to the deepest reflector point.
Enter the frequency used by the satellite band.
Enter realistic efficiency. Many small dishes use 55% to 70%.
Add satellite elevation for a mount reading estimate.
Use mount correction when your bracket scale has a known error.
Press calculate. Review the result above the form.
Download CSV or PDF for installation records.
Offset Dish Planning
An offset dish looks simple. Its geometry is not always simple. The reflector is a section of a larger parabola. The feed sits below the visible center. This design keeps the feed arm away from the signal path. It also changes how the dish face is aimed.
Why The Numbers Matter
Small measurement errors can move the feed away from the focal point. That can reduce signal strength. It can also widen the search area during alignment. Width, height, and dish depth help estimate the effective diameter. They also help estimate focal length. A good focal ratio makes feed placement easier.
Depth And Focal Length
Dish depth is the sag from the rim plane to the deepest point. Measure it with a straight edge across the rim. Then measure down to the reflector surface. The calculator uses an equivalent diameter for oval dishes. It then applies a parabolic focal formula. This gives a practical focal distance for setup work.
Offset Angle And Mount Reading
The offset angle explains why the dish face often points lower than the satellite. A dish with a 25 degree offset may receive a signal that is 25 degrees above its face direction. The mount reading estimate subtracts the offset angle from satellite elevation. A correction field helps match local brackets or inclinometer habits.
Signal Gain And Beamwidth
Gain depends on aperture size, wavelength, and efficiency. Higher frequency means shorter wavelength. A larger dish gives more gain. Efficiency accounts for surface shape, feed match, and blockage. Beamwidth gives an aiming tolerance estimate. A narrow beam needs slower movement and better tools.
Best Use
Use this tool before field alignment. Measure carefully. Enter the same unit for width, height, and depth. Add frequency and efficiency for gain estimates. Compare results with the example table. Save the output as CSV or PDF for your installation notes. Recheck all dimensions if the focal ratio looks unusual. Many home offset dishes fall near moderate focal ratios, but models vary. This calculator gives planning values. Final peaking should still use a signal meter and receiver quality reading.
Safety Checks
Check pole plumb before adjustment. Tighten bolts gradually. Record final readings after wind so future service becomes faster later.
FAQs
What is an offset dish?
An offset dish is a parabolic reflector section. Its feed sits below the signal path. This reduces blockage and helps improve practical reception.
Why does the dish face point low?
The reflector has an offset angle. The signal can arrive higher than the visible face direction. This is normal for many satellite dishes.
How do I measure dish depth?
Place a straight edge across the dish rim. Measure from that edge to the deepest point of the reflector surface.
What is focal length?
Focal length is the distance from the reflector geometry to the focus. The feed should be near this point for better signal collection.
What efficiency should I enter?
Use 55% to 70% for many small home dishes. Use a higher value only when the reflector and feed are known to be efficient.
What does beamwidth mean?
Beamwidth estimates how narrow the aiming window is. Smaller beamwidth needs slower dish movement and more careful final peaking.
Can this replace a signal meter?
No. It gives planning and setup values. Final alignment should use receiver quality readings or a satellite signal meter.
Why use a custom focal length?
Use it when the manufacturer provides exact focal data. It can improve results when depth measurements are hard to take accurately.