Ground Plane
The ground plane is the conductive surface beneath a monopole antenna that produces the mirror image of the missing second arm. The practical target is an uninterrupted conductive area with a radius of at least λ/4 from the antenna base; in free-space terms that is roughly 17 cm for 433 MHz and 8.6 cm for 868 MHz. An inadequate plane distorts both the radiation pattern and the input impedance.
When the plane shrinks, two things happen: the antenna resonance shifts and the radiation pattern tilts upward. In addition, a system that cannot find enough surface for the return current pushes that current onto the outer shield of the coaxial cable; the cable then starts radiating as if it were part of the antenna.
- Symptom: the range changes when you touch the cable or when its routing is altered.
- A painted surface is not conductive: for mounts on an enclosure, the paint must be scraped away at the contact point or a serrated washer must be used.
- Alternative: if an adequate plane cannot be provided, a dipole-type antenna that needs no ground plane gives more stable results.
In AXI kits, when the receiver antenna is brought outside the enclosure, the mounting surface directly forms this plane. Placing the antenna in the middle of a wide, flat metal surface, away from the edge, gives markedly better results than hanging it off the end of a small bracket.
Context where this term is used: Wireless remote control installation: pairing, relay, safety