Calculator Inputs
Formula Used
The parabolic dish focal length is calculated from dish diameter and dish depth.
f = D² / 16d
Here, f is focal length, D is dish diameter, and d is dish depth. Radius is D / 2. The focal ratio is f / D. Aperture area is πr². Estimated gain uses dish diameter, wavelength, and efficiency.
Gain dBi = 10 log10(η × (πD / λ)²) - losses
How to Use This Calculator
- Measure the full open diameter of the parabolic dish.
- Measure the center depth from rim plane to vertex.
- Select the correct measurement unit.
- Add feed offset only when your feed holder needs adjustment.
- Enter frequency and efficiency for estimated signal gain.
- Press the calculate button.
- Place the feed near the calculated focal length.
- Download CSV or PDF for records.
Example Data Table
| Diameter | Depth | Unit | Focal Length | F/D Ratio | Use Case |
|---|---|---|---|---|---|
| 120 | 18 | cm | 50.00 cm | 0.4167 | Satellite reflector |
| 90 | 14 | cm | 36.16 cm | 0.4018 | WiFi dish |
| 48 | 7 | in | 20.57 in | 0.4286 | Solar concentrator |
| 2.4 | 0.35 | m | 1.03 m | 0.4286 | Large antenna |
Parabolic Dish Focal Point Guide
Why the Focal Point Matters
A parabolic dish reflects incoming parallel waves toward one focus. That focus is the best place for a feed horn, receiver, sensor, or absorber. A small placement error can reduce signal strength. It can also create uneven heating in solar projects. Correct focal placement improves performance and saves adjustment time.
Key Geometry
The calculator uses diameter and depth. These two values describe the dish curve. The diameter is measured across the rim. The depth is measured from the rim plane to the deepest center point. The focal length is then found with a standard parabola relation. A deeper dish usually has a shorter focal length. A shallow dish usually has a longer focal length.
Feed Position Planning
The feed should sit near the calculated focal point. The value is measured from the dish vertex along the center axis. Some projects need a small offset. This calculator includes that adjustment. Use it for bracket thickness, feed housing length, or measured mounting differences. Always check the physical structure before final fitting.
Signal and Gain Estimate
The tool also estimates gain. It uses dish diameter, frequency, efficiency, and loss allowance. This is useful for rough antenna planning. Real gain can change because of surface accuracy, feed design, blockage, cable loss, and alignment. Use the estimate as a planning value, not as a certified lab result.
Practical Measuring Tips
Use a straight bar across the rim. Measure the depth from that bar to the center vertex. Keep the tape vertical. Measure diameter through the true center. Repeat measurements from different directions. Average the values when the dish is not perfectly round. Good measurements create better focal point results.
Design Use
This calculator helps with satellite dishes, radio antennas, acoustic reflectors, solar cookers, and light concentrators. It also helps compare dish shapes. The F/D ratio is useful for judging dish type. A balanced ratio can make feed mounting easier. Review the rim angle before choosing brackets or supports.
FAQs
1. What is a parabolic dish focal point?
It is the point where reflected parallel rays meet. For antennas, this is usually where the feed horn or receiver should be placed.
2. What measurements do I need?
You need the dish diameter and dish depth. Measure diameter across the rim and depth from rim plane to center vertex.
3. What formula does this calculator use?
It uses f = D² / 16d. D is dish diameter, d is dish depth, and f is the focal length.
4. Where should I place the feed?
Place the feed near the calculated focal length from the dish vertex. Then fine tune using signal strength or heat concentration.
5. What is the F/D ratio?
It is focal length divided by dish diameter. It helps describe whether the dish is deep, moderate, or shallow.
6. Does frequency affect focal length?
No. Focal length comes from dish geometry. Frequency affects wavelength and estimated antenna gain, not the physical focus.
7. Why add feed offset?
Feed offset adjusts for brackets, housings, or measured mounting differences. Use zero when the feed sits exactly at the focus.
8. Is the gain estimate exact?
No. It is a planning estimate. Real gain depends on surface quality, feed match, blockage, losses, and final alignment.