Yagi, from your own design tables.

A Yagi trades width for gain — a driven element plus a reflector and directors that focus the signal one way.

How to use it

1. Set the band / frequency.
2. Choose the number of elements (3–15) — the diagram and the feedpoint detail redraw to match.
3. Set the boom and mounting. If the boom is metal, give its diameter — this changes every element length and is not a rounding detail.
4. Set your transmit power, which sets the voltage rating the gamma capacitor needs.

What you get

• Tapered element lengths (each director a little shorter than the last) and progressive boom spacings, from a proven long-Yagi design.
• Overall boom length and an approximate gain, which follows the boom length rather than the element count.
• The boom correction added to every element, with the factor it came from.
• The full gamma match: rod length and spacing, rod diameter, the series capacitor value and the voltage it has to stand.

Tips

• More elements means more gain and a narrower beam — you have to aim it.
• A metal boom makes every element longer, not shorter. It is counter-intuitive, but that is what the measurements say — mount elements through a metal boom without correcting and the antenna comes out high in frequency.
• Bonding the elements to the boom needs twice the correction that insulating them through it does. A non-conductive boom needs none, which is why wood and fiberglass booms are the easy build.

The gamma match

Starting-point proportions, all published rules of thumb: rod 0.05 λ long, spaced 0.007 λ off the element center to center, rod diameter half the driven element, and a series capacitor of about 7 pF per meter of wavelength.

Because that capacitance is proportional to wavelength, its reactance works out the same on every band — near 76 Ω — so the voltage across it depends only on your power, not your frequency. The tool computes the peak from the line current and recommends three times that, which covers the mismatch you will see while you are still tuning. Both the rod length and the capacitor are adjusted for lowest SWR on the actual antenna; these get you into range.

Good to know

This is a design table, not a tuned build — expect to fine-tune the driven element for your feed. Gain figures are free space; over real ground and with real construction, expect a little less.

The boom correction is the G3SEK curve fit of Günter Hoch’s (DL6WU) measurements. It only covers booms thinner than 0.055 wavelengths; past that the tool says so rather than quietly extrapolating. Square booms are normally taken at their across-flats dimension.