Side-by-side comparison
| Parameter | Dipole | Monopole Antenna |
|---|---|---|
| Physical length | λ/2 (half wavelength) | λ/4 (quarter wavelength) above ground |
| Input impedance | 73 Ω (resistive at resonance) | 36.5 Ω (half of dipole, due to image theory) |
| Gain | 2.15 dBi in free space | 5.15 dBi (3 dB higher due to ground reflection) |
| Radiation pattern | Omnidirectional in azimuth; figure-8 in elevation | Same as upper half of dipole; radiates only upward |
| Ground plane requirement | Not required; balanced structure | Required; acts as image to complete the dipole |
| Feed type | Balanced; balun needed for coax feed | Unbalanced; coax outer = ground plane, no balun |
| Typical application | FM broadcast dipoles, 868 MHz IoT modules | AM broadcast towers, rubber-duck on 2 m handheld radios |
| Bandwidth (typical) | ~10% of centre frequency | ~10% of centre frequency (similar) |
| Polarisation | Linear (along dipole axis) | Linear (vertical if monopole is vertical) |
Key differences
A half-wave dipole resonates at 73 Ω and radiates symmetrically above and below its axis. A quarter-wave monopole over a perfect ground plane resonates at 36.5 Ω — exactly half — because the ground acts as an image, effectively doubling the antenna. This halving of impedance and the confinement of radiation to one hemisphere give the monopole 3 dB more gain (5.15 dBi vs 2.15 dBi) in the upper hemisphere. Dipoles need a balun with coaxial feed; monopoles connect directly to coax with the braid grounded.
When to use Dipole
Use a half-wave dipole when no ground plane is available and a balanced, omnidirectional pattern in all directions is needed, such as in a 433 MHz ISM-band remote sensor floating in air.
When to use Monopole Antenna
Use a quarter-wave monopole when a large ground plane (PCB, vehicle roof, or earth) is available and a direct 50 Ω coax connection is preferred, as in a 2 m (144 MHz) VHF handheld radio.
Recommendation
For handheld and vehicle-mounted radios, choose the monopole — the ground plane is already there, the 36.5 Ω can be matched easily, and you save half the antenna length. Use a dipole in free-space deployments or where ground plane size is impractical.
Exam tip: University papers test image theory: be ready to show that a monopole over a perfect ground is equivalent to a full dipole in free space, giving half the impedance and twice the directivity in the upper hemisphere.
Interview tip: Interviewers will ask why a monopole has 5.15 dBi gain while a dipole has 2.15 dBi — the correct answer is that the ground plane forces all radiation into the upper hemisphere, effectively doubling the power density there.