Free Coaxial Collinear (CoCo) Antenna Calculator
Build high-gain omnidirectional vertical antennas for ADS-B 1090 MHz, LoRa 868/915 MHz, and amateur radio. Calculate λ/2 segment cut lengths and choke sleeves.
📡 Frequency & Coaxial Cable Specifications
Consecutive half-wave coaxial cables are connected by transposing center conductor to outer shield. This 180° phase reversal forces RF currents on all outer shields to flow in phase, synthesizing an array with high broadside omni gain.
Total Active Array Length: 971 mm (38.2")
• Choke Sleeve Length: 65.3 mm (Copper pipe over base feedline).
• Enclosure: Mount inside 3/4" Schedule 40 PVC pipe for waterproof UV protection.
The Velocity Factor Mandate: The internal wave speed inside the dielectric determines the physical length: $L_{seg} = rac{c cdot VF}{2 f}$. Cutting coax to the free-space wavelength makes the antenna completely deaf!
Recommended Tools & Equipment
Tested hardware and components for high reliability
How a Coaxial Collinear (CoCo) Works
A standard half-wave dipole has a toroidal radiation pattern with 2.15 dBi gain. By stacking half-wave dipoles vertically in phase, the elevation radiation pattern squashes toward the horizon like a flat pancake, projecting maximum signal power toward distant aircraft or ground stations.
Construction Calculations
Coax Half-Wave Segment: L_seg = (λ_0 / 2) × VF
Top Quarter-Wave Whip: L_whip = λ_0 / 4 (VF = 1.0 in air)
Base Decoupling Sleeve: L_sleeve = (λ_0 / 4) × 0.95
Array Gain Approximation:Gain ≈ 2.15 + 10 × log10(N × 0.70) dBi
Assembly Instructions
- Cut $N$ identical segments to the exact millimeter length $L_{seg}$.
- Strip 5mm of outer jacket and braid from both ends of each segment, exposing the center dielectric and conductor.
- Solder center conductor of Segment 1 to outer shield of Segment 2, and shield of Segment 1 to center conductor of Segment 2. Insulate with heat-shrink tubing.
- Slide a copper pipe sleeve over the bottom feedline connected to ground at the top to choke common-mode RF currents off the feeder.
Frequently Asked Questions
Why is RG-6 75 Ohm coax preferred over 50 Ohm coax for ADS-B 1090 MHz?
RG-6 uses foamed polyethylene dielectric with an ultra-low loss velocity factor of ~0.82 to 0.85 and has low attenuation at 1 GHz. Despite the slight nominal 75 Ohm to 50 Ohm SWR mismatch (1.5:1), the lower dielectric loss and stiff aluminum/copper shielding provide superior ADS-B range over RG-58.
Why should I not build a 20 or 24-element CoCo antenna?
As the number of elements increases, internal dielectric attenuation dissipates power before it reaches the top elements. Furthermore, the vertical elevation beamwidth becomes razor-thin (< 5°). If the antenna sways in the wind, the signal beam tilts into the clouds or ground, causing erratic reception.
What is the function of the bottom decoupling sleeve?
Without a choke or quarter-wave sleeve balun, RF current flows down the outer shield of your feedline cable into your SDR receiver, distorting the omnidirectional pattern and introducing indoor electrical noise into the receiver.