Calculator
Example Data Table
| Frequency | Sections | Coax VF | End correction | Half-wave section | Half-wave coax piece | Estimated gain |
|---|---|---|---|---|---|---|
| 1090 MHz | 4 | 0.66 | 3.0% | 133.39 mm | 90.76 mm | 6.96 dBi |
| 1090 MHz | 6 | 0.78 | 3.0% | 133.39 mm | 107.27 mm | 8.57 dBi |
| 978 MHz | 4 | 0.66 | 4.0% | 147.14 mm | 101.16 mm | 6.96 dBi |
| 1090 MHz | 8 | 0.80 | 2.0% | 134.77 mm | 110.02 mm | 9.71 dBi |
Formula Used
Wavelength: λ = c ÷ f
Quarter-wave element: Lq = λ ÷ 4 × K
Half-wave element: Lh = λ ÷ 2 × K
Coax phasing section: Lp = λ ÷ 2 × VF
Reflection coefficient: Γ = (SWR - 1) ÷ (SWR + 1)
Mismatch loss: ML = -10 × log10(1 - Γ²)
Estimated gain: G = 2.15 + 10 × log10(N × η) - ML - CL
System gain: SG = G - feedline loss + preamp gain
Here, c is light speed. f is frequency in hertz. K is the physical end correction factor. VF is coax velocity factor. N is section count. η is efficiency. CL is construction loss.
How to Use This Calculator
Enter the ADS-B frequency. Use 1090 MHz for common aircraft reception.
Add the number of radiating sections you want to stack.
Enter the coax velocity factor from the cable sheet.
Use end correction to shorten physical metal elements.
Add expected losses, SWR, and preamp gain.
Press Calculate. Review the result above the form.
Cut sections slightly long. Trim after testing the antenna.
Use CSV or PDF export for shop records.
Article
Why this calculator helps
An ADS-B collinear antenna uses repeated radiating sections. Each section works with wavelength. The target frequency is often 1090 MHz. Small length errors matter at that band. A few millimeters can shift the best match. This calculator turns frequency into practical build lengths. It also applies end correction and coax velocity factor.
Planning radiating sections
A basic section is often near one half wavelength. The physical length is shorter than the free-space value. Wire diameter, insulation, tubing, and nearby objects affect it. The end correction field lets you reduce the ideal length. Use a small reduction for bare metal. Use a larger reduction for insulated wire or thick conductors. More sections can add gain. They also make the stack taller and harder to tune.
Phasing and coax math
Collinear designs need correct phase between sections. Coax phasing pieces use the cable velocity factor. Foam coax, solid polyethylene coax, and specialty cables differ. Enter the value from the cable data sheet. A half-wave phasing piece is one half wavelength multiplied by velocity factor. Cut it slightly long at first. Then trim while checking the antenna.
ADS-B performance notes
ADS-B reception depends on line of sight. Height is often more useful than extra gain. Feedline loss can remove much of the benefit. Use short coax where possible. Use low-loss cable for long runs. A mast-mounted preamp can help. It should not overload the receiver. The system gain result includes feedline loss and preamp gain. Treat it as a planning value.
Using the result
Build from the calculated lengths, then test. Use an antenna analyzer near 1090 MHz. Keep bends clean and repeatable. Seal outdoor joints well. Keep the antenna vertical. Place it away from metal surfaces. Compare messages per second and aircraft range. Adjust one thing at a time. Record each change. Good records make tuning easier.
Common build limits
The gain estimate is not a lab result. It ignores mast coupling and imperfect solder joints. It also ignores pattern tilt from bad phasing. Use it for comparison only. Weatherproofing can change tuning after rain. Test before final sealing. Then test again outdoors. A careful build usually beats a tall but random stack and cleaner element spacing overall.
FAQs
What frequency should I use for ADS-B?
Use 1090 MHz for standard ADS-B aircraft signals. Some regions also use 978 MHz UAT. Enter the exact frequency you want to receive.
What is velocity factor?
Velocity factor shows how fast radio energy travels through coax. It changes phasing length. Always use the value listed for your cable type.
Why is end correction needed?
Real elements act slightly longer than their physical size. End correction shortens the cut length. It helps account for diameter, insulation, and construction effects.
Does more sections always mean better range?
No. More sections can add gain, but they narrow the vertical pattern. Bad phasing, feedline loss, or poor height can reduce real performance.
Should I cut elements exactly to the result?
Cut slightly long first. Then trim slowly while measuring. This is safer because metal is easier to remove than replace.
Can this calculator predict exact gain?
No. The gain value is an estimate. Real gain depends on construction accuracy, surroundings, cable loss, matching, and test setup.
Why include feedline loss?
Feedline loss reduces signal before it reaches the receiver. At ADS-B frequencies, poor coax can waste a lot of received signal.
Where should I mount the antenna?
Mount it high, vertical, and clear of metal. ADS-B is mostly line of sight. A clean view often improves range more than extra gain.