What is a ground plane and why is it needed?
A ground plane is the conductive surface under a quarter-wave monopole; the antenna uses that surface as a mirror and behaves electrically like a half-wave dipole. If the plane is missing or small the radiation pattern is distorted, impedance shifts away from 50 Ω and the outer braid of the coaxial cable starts to radiate. The practical target is a surface with a radius of at least λ/4: 17.3 cm at 433 MHz, 8.6 cm at 868 MHz.
A λ/4 monopole is in fact half an antenna. Its missing half is supplied by the image produced by the conductive surface beneath it. That is why a monopole's performance depends on the surface it is mounted on as much as on the antenna itself.
If the ground plane is inadequate, three symptoms appear:
- The radiation pattern lifts upwards; you lose gain in the horizontal plane, that is, in the direction you actually work.
- The feed point impedance shifts and SWR rises.
- Current jumps onto the outer braid of the coaxial cable. The cable becomes part of the antenna; range starts to change depending on how the cable is coiled and where your hand touches. In the field this is one of the classic causes of the complaint "the remote sometimes reaches, sometimes doesn't".
The practical application is simple: mount the receiver antenna on a metal enclosure, outside the cover, making good electrical contact with the body. The enclosure cover itself is often a sufficient ground plane. If you are mounting on a plastic box, add a conductive plate or radial wires at the antenna base.
When mounting on a metal body, paint and anodized layers are insulating; clean the contact surface and measure the conductivity.
Full article: 433 MHz or 2.4 GHz?