Ground Planes for Monopole Antennas

A monopole antenna (vertical rod) radiates efficiently only when paired with a ground plane - a conducting surface that acts as the electrical other half of the antenna.

What counts as a ground plane

• Vehicle roof: Excellent. A metal roof is an ideal ground plane for NMO-mount antennas.
• Metal enclosure: A metal equipment housing near the feedpoint serves as a ground plane.
• PCB ground plane: The copper ground layers on a dev board act as the ground plane for the rubber-duck SMA antennas supplied with dev boards.
• Radials: For antennas on non-conductive masts, attach 3-4 quarter-wave radials (8.2 cm at 915 MHz) at the base, angled 45 degrees downward.

Do commercial LoRa antennas need a ground plane?

Most commercial 915 MHz verticals designed for LoRa use a balanced dipole structure or have built-in radials. These do not require an external ground plane. Check the manufacturer mounting instructions.

A poorly grounded monopole can have its radiation pattern tilted upward rather than horizontal, reducing effective range. This matters mainly for DIY wire antennas, not commercial products.