Offset Fed Dish Input Form
Example Data Table
| Width |
Height |
Depth |
Frequency |
Efficiency |
Focal Length |
Net Gain |
Beamwidth |
| 90 cm |
100 cm |
8.5 cm |
11.7 GHz |
65% |
53.6 cm |
38.8 dBi |
1.89° |
| 75 cm |
84 cm |
7.4 cm |
12.5 GHz |
60% |
42.8 cm |
36.8 dBi |
2.11° |
| 120 cm |
132 cm |
11.0 cm |
10.7 GHz |
68% |
89.3 cm |
41.0 dBi |
1.55° |
Formula Used
Offset focal length: F = W³ / (16 × d × H). W is dish width, H is dish height, and d is depth.
Equivalent aperture diameter: De = √(W × H).
Wavelength: λ = c / f. The calculator uses c = 299,792,458 m/s.
Aperture gain: Gain dBi = 10 × log10(η × (π × De / λ)²) − total losses.
Beamwidth: HPBW ≈ 70 × λ / De degrees.
Near field: R ≈ 2 × De² / λ.
Auto offset angle: angle ≈ cos⁻¹(min(W,H) / max(W,H)). A manual value overrides this estimate.
How to Use This Calculator
- Measure the clear dish width, height, and center depth.
- Select the same unit used for all physical dimensions.
- Enter operating frequency and aperture efficiency.
- Add realistic loss values for cables, radomes, pointing, and polarization.
- Enter a manual offset angle if the dish data plate provides one.
- Press Calculate to view the result above the form.
- Use the CSV or PDF button to save the current calculation.
Offset Fed Dish Planning
An offset fed dish is a shaped part of a larger paraboloid. Its feed sits below the main beam path. This layout reduces blockage from the LNB and support arm. It also makes the face look lower than the true pointing angle. Good setup needs geometry, frequency, and efficiency together.
Key Geometry Ideas
Width, height, and depth describe the reflector face. The calculator uses those values to estimate focal length. For common offset dishes, the apparent height is often larger than width. That shape gives a practical estimate of offset angle. The focal ratio then shows how deep or shallow the parent parabola is. A low ratio needs a wider feed pattern. A high ratio needs a narrower feed pattern.
Signal Estimates
Gain depends on aperture size, frequency, and efficiency. Larger dishes collect more energy. Higher frequency also raises theoretical gain for the same diameter. Real systems lose signal through surface errors, feed mismatch, radomes, pointing error, and cable paths. The loss fields help make the result more realistic. Beamwidth gives the rough angular width of the main lobe. Narrow beams give more gain, but alignment becomes less forgiving.
Feed and Mount Use
The feed target distance is an initial mechanical guide. Real brackets and LNB throats can shift the best point. After setting the distance, peak the signal slowly. Move azimuth first, then elevation, and then skew. Offset dishes do not point where the face appears to point. The mount elevation is usually the true elevation minus the offset angle. This helps installers avoid aiming too high.
Practical Limits
Use these results as planning values. Small measuring errors can change focal length. A bent reflector can reduce gain. Wet radomes and poor connectors can add loss. At microwave frequencies surface accuracy matters. A common target is a surface error below one sixteenth of a wavelength. Always confirm final alignment with a meter, receiver, or spectrum display.
Data Quality
Measure the dish with a straight edge across the rim. Record depth at the deepest point. Use the same unit for every dimension. Enter losses as positive values. Save the exported files with site notes. Later comparisons are easier when each survey uses the same method carefully.
FAQs
What is an offset fed dish?
It is a reflector cut from a larger paraboloid. The feed sits away from the main beam path. This reduces feed blockage and helps improve useful aperture performance.
Why is dish height different from width?
Many offset dishes appear taller because the reflector face is tilted from the parent paraboloid. That visible shape helps estimate the offset angle when no data plate exists.
What does F/D ratio mean?
F/D compares focal length with equivalent aperture diameter. It helps select feed pattern width. Lower values need wider illumination, while higher values need narrower illumination.
Is the focal length exact?
It is an estimate based on measured width, height, and depth. Manufacturing shape, measurement error, and feed throat position can shift the best real focus slightly.
Why enter aperture efficiency?
Efficiency adjusts ideal gain for real reflector behavior. Feed taper, spillover, surface error, blockage, and mismatch reduce usable gain below the perfect theoretical value.
What is the mount elevation result?
It estimates the physical dish mount angle. For many offset dishes, mount elevation equals true satellite elevation minus the offset angle.
Can I use inches?
Yes. Choose inches as the dimension unit. Keep width, height, depth, and feed correction in the same unit for consistent geometry results.
Why are CSV and PDF downloads useful?
They save the input and result set. This helps compare installations, keep site notes, and repeat the same method during later antenna adjustments.