Dual-Band 2m/70cm Yagi — Design Notes

Overview

This dual-band Yagi provides high gain on both the 2-meter (144 MHz) and 70-centimeter (430 MHz) bands. It is ideal for satellite tracking, repeater access from a distance, and weak-signal VHF/UHF work.

Design Principles

A Yagi-Uda antenna uses a driven element, a reflector behind it, and one or more directors in front. The 70cm elements are interlaced on the same boom as the 2m elements, taking advantage of the harmonic relationship between the bands.

The element lengths and spacings depend on the element diameter and mounting method. Use the Configuration selector above the specs table to switch between variants and see the exact dimensions for each build option.

Element Diameter and Mounting

Element diameter affects resonant length — thinner elements are longer, thicker elements are shorter. This antenna is available in several configurations:

• 10mm / insulated (base) — standard build, elements mounted on insulated clamps above the boom. No boom correction needed.
• 6mm / insulated — lighter build with longer elements. Good for portable or temporary setups.
• 10mm / through-boom — elements pass through drilled holes in the boom. Mechanically strongest, but requires a boom correction factor (elements shortened by approximately the boom diameter).
• 5mm / portable — ultra-light field version with split boom for transport.

Feed System

The driven element uses a gamma match for impedance matching to 50 Ω. Adjust the gamma rod length and capacitor for minimum SWR at 145 MHz. The gamma match dimensions are the same across all element diameter variants.

Boom Correction

When elements pass through a conductive boom, the boom acts as a short between the two halves of each element. This effectively lengthens the element electrically, so the physical length must be reduced. The correction is approximately equal to the boom diameter for round booms, or the boom width for square booms.

For insulated mounting (elements isolated from the boom), no correction is needed.

Mounting

Mount the Yagi on a rotatable mast for directional operation. Use a compass and signal reports to aim the antenna toward your target station or satellite pass.

Tips

• Seal all connections with self-amalgamating tape to prevent water ingress.
• Use stainless steel hardware to prevent corrosion.
• A preamp at the antenna can significantly improve receive performance on 70cm.
• For satellite work, consider mounting at 45° for circular polarization.