Quick Collinear Antenna Design Tool
Enter your target frequency and build assumptions to estimate section lengths, total antenna length, and a practical cut chart for a stacked half-wave collinear antenna.
What this collinear antenna calculator does
A collinear antenna stacks multiple radiating sections vertically so more energy is concentrated near the horizon. That can improve useful range for VHF, UHF, GMRS, marine, and amateur radio applications where low-angle radiation is preferred.
This calculator provides a practical first-pass design for a half-wave section collinear build. It computes free-space wavelength, corrected section lengths, optional inter-section spacing, total physical length, and a simple gain estimate. It is intentionally geared toward workshop use: measuring, cutting, assembling, and then fine-tuning with an analyzer.
Inputs explained
1) Frequency
Your design frequency drives every dimension. If you plan to cover a narrow channel set, use the center frequency. For broader use, choose the band center and expect some tuning compromise toward the edges.
2) Number of half-wave sections
More sections usually means more low-angle gain, but also more mechanical length, narrower vertical beamwidth, and tighter tuning sensitivity. Common practical builds use 2 to 8 sections.
3) Velocity factor
Velocity factor accounts for how quickly RF travels in your conductor environment compared with free space. For bare metal elements, values are often close to 0.95 to 1.00. For coaxial phasing lines, values can be much lower depending on dielectric type.
4) End-effect shortening factor
Real elements are not ideal infinite-thin conductors. End effects and diameter usually make practical lengths slightly shorter than textbook electrical lengths. A starting factor around 0.94 to 0.97 is common, then trim during tuning.
5) Gap between sections
This is the physical separation between radiating sections as a percentage of wavelength. If your topology places segments directly connected, use 0%. If your design includes small spacers or phase structures, enter the effective spacing.
Formulas used in this tool
- Wavelength: λ = 300 / fMHz (meters)
- Half-wave electrical length: λ/2
- Corrected section length: (λ/2) × velocity factor × shortening factor
- Gap length: λ × (gap% / 100)
- Total physical length: N × section length + (N - 1) × gap length
- Approximate gain trend: 2.15 + 10log10(N) - losses dBi (rule-of-thumb)
Practical build workflow
Step 1: Calculate and cut long
Use the cut chart, but leave each section slightly longer (for example 1% to 2%) to allow for final trimming. It is easier to remove material than add it back.
Step 2: Assemble straight and rigid
Mechanical alignment affects pattern and repeatability. Keep the radiator perfectly vertical, use consistent spacing, and secure all joints against movement and weather.
Step 3: Tune with instrumentation
Use a VNA or antenna analyzer to tune for your target operating window. Check SWR and impedance, then trim symmetrically where possible. Re-measure after each small adjustment.
Step 4: Validate real-world performance
After matching looks good, test on-air signal reports or calibrated field-strength methods. Height above ground and local surroundings can change apparent performance more than a small cut-length tweak.
Common mistakes to avoid
- Using theoretical lengths without velocity factor or end-effect correction.
- Ignoring connector, mounting mast, or nearby metal interactions.
- Over-trimming too quickly during tuning.
- Assuming gain increases indefinitely with more sections despite feed and loss penalties.
- Not weatherproofing joints and feed transitions for long-term stability.
Design note and disclaimer
This calculator is a design estimator for educational and project planning purposes. Final dimensions depend on conductor diameter, construction style, phasing method, mounting environment, and measurement setup. Always verify with RF test equipment before permanent installation.