How does a monopole antenna work?
A quarter-wave monopole over a perfectly conducting infinite ground plane forms an image that completes a half-wave dipole. Its input impedance is half the dipole’s, about 36.5 Ω, and its directivity is doubled (5.15 dBi) because it radiates only into the upper half-space.[1]
What happens with a finite ground plane?
Real ground planes of a fraction of a wavelength tilt the pattern upward, change impedance and reduce gain; radials (ground-plane antennas) and drooping radials raise impedance toward 50 Ω. On small devices the PCB ground itself is part of the antenna, so ground size strongly affects efficiency.
What are inverted-F and PIFA antennas?
Bending a monopole parallel to ground (inverted-L) lowers height; adding a shorting stub near the feed (inverted-F) provides an impedance match. Printed IFAs are common for 2.4 GHz devices; PIFAs (planar inverted-F) are used in phones. Chip antennas are dielectric-loaded variants that depend heavily on board layout and ground clearance.[1]
What design guidance applies to PCB antennas?
- Follow the reference layout and keep-out area exactly.
- Keep metal (batteries, displays, enclosures) away from the radiating element.
- Provide a pi-matching network footprint for tuning in the final enclosure.
- Measure efficiency in an antenna chamber, not just return loss.
Frequently asked questions
Why 36.5 Ω?
Half of the half-wave dipole's 73 Ω, because the monopole sees only half the radiating structure.
Can a monopole work without ground?
It needs some counterpoise; otherwise the feed cable becomes part of the antenna.
What is an IFA?
An inverted-F antenna, a bent monopole with a shorting stub for matching.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.