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Antenna Calculators
Starting lengths for the most popular homebrew antennas. Every result is shown in metric and imperial, and the shortening factors are editable so you can match your own wire or tube.
One frequency for every calculator
Each calculator starts at a sensible default for its antenna. Apply one frequency here to compare every design on the same band, then adjust any single card as you like.
Half-wave dipole
Polarisation: horizontal as a flat-top · vertical if hung as a vertical wire
Feed-point impedance about 50–75 Ω at a decent height; a 1:1 current balun is good practice.
Inverted-V dipole
Polarisation: mostly horizontal, with some vertical from the sloping legs
An inverted-V is usually cut a few percent shorter than a flat dipole and presents close to 50 Ω. Keep the ends out of reach: they carry high RF voltage.
End-fed half-wave (EFHW)
Polarisation: follows the wire (horizontal, mixed as a sloper, or vertical)
The end of a half-wave wire sits at a few thousand ohms, so it needs a high-ratio matching transformer. Fit a common-mode choke on the coax, since the system relies on the coax shield and a short counterpoise.
Full-wave loop (square, delta, round)
Polarisation: set by the feed point (middle of a horizontal side = horizontal, middle of a vertical side = vertical)
Perimeter factor 1.0217 is the classic 1005/f (ft) rule. A resonant loop sits around 100–120 Ω; a quarter-wave section of 75 Ω coax brings that down to roughly 50 Ω.
Two-element quad
Polarisation: set by the feed point (bottom side = horizontal, side = vertical)
Classic 1005/f driven and 1030/f reflector (in feet) perimeters. A spacing of 0.15–0.25 λ is typical; feed the driven element at the bottom corner for horizontal polarisation or the side for vertical.
Ground-plane vertical
Polarisation: vertical
Four radials drooping at about 45° bring a ¼-wave vertical close to 50 Ω. A ⅝-wave radiator needs a matching coil or stub at the base, so tune that on the analyser.
J-pole
Polarisation: vertical
Use about 0.95 for bare tube or wire and roughly 0.95–0.97 for window ladder line. The tap position is a starting estimate: slide it for the lowest SWR.
Coax and velocity factor
Velocity factors are typical values; check your cable's datasheet or measure it with an analyser.
Vertical, horizontal or both?
Polarisation is the orientation of the wave's electric field, and it follows the orientation of the radiating part of the antenna: a horizontal wire gives horizontal polarisation, a vertical whip gives vertical. It matters most on line-of-sight VHF and UHF paths, where a mismatched pair of antennas is often quoted as losing up to about 20 dB in the ideal case. Reflections from buildings and terrain usually soften that in real life, but a mismatch is still worth avoiding. On HF skywave paths the ionosphere scrambles polarisation, so height above ground and take-off angle matter far more than which way the wire points.
| Antenna | Polarisation | How to change it |
|---|---|---|
| Half-wave dipole | Horizontal as a flat-top; vertical if hung as a vertical wire | Orientation of the wire |
| Inverted-V | Mostly horizontal, with a vertical component from the sloping legs | Wider apex angle gives more horizontal |
| End-fed half-wave | Follows the wire: horizontal, mixed (sloper) or vertical | Orientation of the wire |
| Full-wave loop and quad | Set by the feed point: horizontal or vertical | Move the feed to a horizontal or a vertical side |
| Ground-plane vertical | Vertical | Fixed |
| J-pole | Vertical | Fixed |
| Hentenna | Horizontal when upright; vertical when turned 90° on its side | Rotate the loop in its own plane |
Compromise installation
- Follow the mode you use most. On VHF/UHF, SSB, CW and weak-signal digital modes such as FT8 are conventionally horizontal, while FM voice, repeaters, packet and APRS are usually vertical. Check what local operators in your area use.
- Slant at 45°. Tilting a hentenna, loop or quad to 45°, or using a sloping wire, gives a mixed signal that is theoretically about 3 dB down against either polarisation. That is a modest, predictable loss compared with a full cross-polar mismatch.
- Use two antennas. A vertical ground plane or J-pole for FM and a horizontal dipole or hentenna for SSB, switched at the shack with a coax switch, is simple and cheap.
- Choose loop feed deliberately. A wire loop can be re-fed from the bottom side (horizontal) or a vertical side (vertical) later with little extra work, so you can test both.
- HF: keep it simple. A horizontal dipole or inverted-V, a quarter to half a wavelength up, is a good all-round compromise. A vertical only works well with a decent radial or ground system.
- Mind the surroundings. Keep any antenna clear of metal masts, guttering and wiring, ideally on a fibreglass or PVC mast section, because nearby metal shifts tuning and distorts the pattern. Indoors or in a loft, reflections make polarisation less predictable, so the compromise matters less.
Antenna calculator FAQ
Why is a dipole shorter than half a wavelength?
End effect and the slightly slower wave speed along the wire make a resonant dipole about 5 % shorter than half the free-space wavelength, which is where the shortening factor comes from.
What is velocity factor?
It is the fraction of free-space speed at which a signal travels in a cable. A quarter wave of solid-PE coax is therefore about 66 % of a free-space quarter wave.
Do I need to trim after cutting?
Yes. Calculators give a starting length. Cut slightly long, check resonance with an analyser, then trim a little at a time.
Also see the hentenna calculator and the homebrew antenna guide.