This 4-part report discusses antenna fundamentals and the various types of antennas used for short range devices. Fundamentals are presented along with practical design principles. It is excerpted from the report: ISM-Band and Short Range Device Antennas.
Click here for Part 2.
Part 3 covers RF Propagation issues.
Click here for Part 4: Examples
Antennas are the connecting link between RF signals in an electrical circuit, such as between a PCB and an electromagnetic wave propagating in the transmission media between the transmitter and the receiver of a wireless link.
In the transmitter, the antenna transforms the electrical signal into an electromagnetic wave by exciting either an electrical or a magnetic field in its immediate surroundings, the near field. Antennas that excite an electrical field are referred to as electrical antennas; antennas exciting a magnetic field are called magnetic antennas. The oscillating electrical or magnetic field generates an electromagnetic wave that propagates with the velocity of light c. The speed of light in free space c0 is 300000 km/s. If the wave travels in a dielectric medium with the relative dielectric constant εr, the speed of light is reduced to:
We can calculate the wavelength from the frequency f of the signal and the speed of light c using the formula:
Using common units, the equation:
is often used for the wavelength in free space. If the wave travels in a dielectric medium, for instance in the PCB material, the wavelength has to be divided by the square root of εr. We can distinguish three field regions where the electromagnetic wave develops: reactive near field, radiating near field, and far field:
In the receiver, the antenna gathers energy from the electromagnetic wave and transforms it into an electrical voltage and current in the electrical circuit. For better comprehension, the antenna parameters are often explained on a transmit antenna, but in most cases, if no nonlinear ferrites are involved, the characteristics of an antenna are identical in receive and transmit modes.
Polarization describes the trace that the tip of the electrical field vector builds during the propagation of the wave. In the far field, we can consider the electromagnetic wave as a plane wave. In a plane electromagnetic wave, the electrical and the magnetic field vectors are orthogonal to the direction of propagation and also orthogonal to each other. In the general case, the tip of the electrical field vector moves along an elliptical helix, giving an elliptical polarization. The wave is called right-hand polarized if the tip of the electrical field vector turns clockwise while propagating; otherwise it is left-hand polarized.
If the two axis of the ellipse have the same magnitude, the polarization is called circular. If one of the two axis of the ellipse becomes zero, we have linear polarization. Similarly, polarization is vertical if the electrical field vector oscillates perpendicularly to ground, and it is horizontal if its direction of oscillation is parallel to the ground plane.
A transmission system has the best performance (ideal case) when the polarization of the transmitter and the receiver antenna are identical to each other. Circular polarization on one end and linear polarization on the other gives 3-dB loss compared to the ideal case. If both antennas are linearly polarized but 90° turned to each other, theoretically no power is received. The same phenomenon happens if one antenna is right-hand circularly polarized and the other one is left-hand circularly polarized.