Monopole Antenna
A monopole is a single-armed antenna erected on a conductive ground plane; the plane creates the image of the missing second arm. It is the most common solution in receiver enclosures and panel mountings. Its performance depends largely on the size of the conductive surface beneath it, and if the plane is inadequate both the pattern and the impedance are degraded.
The working principle of a monopole is the mirror image: the arm of length lambda/4, together with the virtual arm reflected by the conductive plane beneath it, behaves electrically like a dipole. This is why the mounting surface is part of the antenna, not an accessory.
- Advantage: being single-armed it is short and mechanically robust, and it is mounted directly on the enclosure.
- Weakness: without a sufficient ground plane the outer jacket of the coaxial cable starts to radiate; range can change even by touching the cable.
- Practical target: a conductive surface of at least lambda/4 radius. For 433 MHz that means roughly 17 cm in free space terms (the physical cut length of the wire is 16.4 cm because of the velocity factor).
In AXI's ST and SF sets the receiver antenna is supplied externally and can be taken outside the enclosure with a cable gland. The critical point in mounting is to seat the antenna on the outer surface of the metal enclosure, on a wide and clean conductive area; if contact cannot be made on a painted surface, it does not act as a ground plane.
Context in which this term is used: 433 MHz or 2.4 GHz?