Principles and functions
The concept of printed antennas or microstrip appeared in the 1950s, but it was not until twenty years later that the first achievements appeared. At first mainly reserved for military applications and then for industry from the 1990s. They are now installed in many electronic devices and are the preferred type of antenna for VHF (Very High Frequency) applications and in particular for Wi-Fi and amateur radio.
A printed antenna consists of a ground plane and a dielectric substrate, the surface of which bears one or more metallized elements, called a “patch”. The patch come in different shapes which will influence the nature of the radiation from the antenna. In practice, radiating elements are found in the form of a rectangle or a square, a circular disc or a ring, a triangle or a dipole. The most used element is the rectangular one.
The dimensions of the patch are usually in the order of λ / 2 to λ, defined by the resonant frequency to be given to the antenna. The characteristics of the substrate influence those of the antenna, it must be of low permittivity ( εr<3 ) to allow radiation. In addition, its thickness must be negligible compared to the wavelength.
The excitation is an important point for the patch antenna, in fact, depending on the method, it is possible to directly influence its radiation and modify its performance. The antenna power depends on how the antenna is going to be integrated into a device.
Feeding methods can be classified into two categories:
- Contact power supplies (coaxial probe or microstrip line).
- Proximity power supplies (electromagnetic coupling by line or slot).
Patch antenna (FR4 substrate) powered by microstrip line
The patch antennas were developed to meet constraints such as cost and size. Their applications cover a wide frequency band, from 100 MHz to 100 GHz.
Among the advantages of these antennas, we can mention:
- Low weight, small footprint, possibility of being placed as close as possible to the systems.
- Planar structure that can be shaped.
- Low cost, easy to perform, suitable for mass production.
- Multi-band antenna, multi-polarization possible (linear and circular)
- Possibility of putting several antennas in arrays to improve the overall radiation characteristics (several tens or hundreds of elements)
However, they also have their limitations:
- Narrow bandwidth (1 to 5%).
- Average gain
- Low power (a few tens of watts)
- sensitive to technological dispersions
Characterization of an antenna
- The radiation pattern of an antenna makes it possible to visualize these lobes in three dimensions, in the horizontal plane or in the vertical plane including the most important lobe. The antenna radiation pattern therefore makes it possible to assess the directivity of the antenna. Figure n ° 3 shows a radiation pattern of a patch antenna in horizontal and vertical section planes and in 3 dimensions.
E-plane and H-plane radiation diagram (b) 3D radiation pattern
- The aperture angle of an antenna characterizes the portion of space (the direction angle) in which the radiated power is half (-3dB) the radiated power in the direction of maximum radiation. It is therefore representative ofthe directivity of the antenna. if this angle is narrow, the antenna is directional.
- The gain of an antenna is the gain in the direction of maximum radiation. This property characterizes the ability of an antenna to focus radiated power in one direction. This property characterizes the ability of an antenna to focus radiated power in one direction.