Cobweb Antenna Element Length Calculator
Enter one or more center frequencies in MHz (comma separated), then calculate starting cut lengths for each cobweb element.
What this cobweb calculator does
A cobweb antenna is a compact, horizontally polarized, multi-band HF antenna that uses nested wire elements arranged in a roughly square shape. This calculator gives you a practical starting point for each element by using a standard half-wave dipole length model, then applying a velocity factor and optional trim allowance.
The output includes total wire length for each band, side length for a square layout, and corner radius from center. That makes it easier to mark spreaders, cut wire, and begin tuning with an analyzer.
Core formulas used
1) Half-wave length in meters
L = 143 / f(MHz)
This is a widely used practical formula for initial dipole cuts in meters.
2) Insulation / construction correction
Lcorrected = L × Velocity Factor
Insulated wire usually needs a velocity factor less than 1. Bare wire tends to be closer to 1.00.
3) Trim allowance
Lcut = Lcorrected × (1 + trim% / 100)
Builders often start slightly long so they can trim to resonance. A common first pass is +2% to +5%.
4) Square side estimate
Side = Lcut / 4
If the element is arranged as an approximate square, each side is one quarter of total element length.
How to use the results in a real build
- Choose target center frequencies for each band you want active.
- Cut each element to the calculator’s recommended starting length.
- Assemble with your planned element spacing and feed arrangement.
- Measure SWR or impedance with an antenna analyzer at final mounting height.
- Trim in small, equal increments, starting with the lowest-frequency (outer) element.
Common target 5-band cobweb set
A very popular cobweb configuration covers 20m, 17m, 15m, 12m, and 10m. One typical frequency list: 14.2, 18.1, 21.2, 24.95, 28.4 MHz. You can replace these with your preferred SSB, CW, or digital centers.
Build quality checklist
Mechanical
- Use non-conductive spreaders with adequate stiffness in wind.
- Keep equal tension on all element sides.
- Maintain consistent spacing between adjacent elements.
Electrical
- Use a proper current choke at the feed point to reduce common-mode current.
- Protect junctions from moisture (sealant, heat-shrink, or proper weather boots).
- Confirm feedline routing does not couple strongly to elements.
Frequently asked questions
Is this calculator exact?
No calculator can be exact for every installation. Height, nearby objects, wire type, element spacing, and feed method all shift resonance. Treat output as a strong first cut, then tune in place.
What velocity factor should I use?
If you are unsure, 0.95 is a reasonable starting value for common insulated wire. For bare copper wire, values closer to 0.98–1.00 may be appropriate. Final tuning always wins.
Why add trim allowance?
Cutting slightly long gives you room to tune downward in length. If you cut too short at the start, recovering length is harder and less clean.
Final note
Good antennas are built twice: once on paper, once in the air. Use the calculator to plan carefully, then refine with measured data at operating height. That process gives the best on-air performance.