Fundamentals of ISM-Band and short range device antennas, Part 1

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Part 3 covers RF Propagation issues.
Click here for Part 4: Examples

Antenna Basics
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 ε_r, 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 ε_r. We can distinguish three field regions where the electromagnetic wave develops: reactive near field, radiating near field, and far field:

• In the reactive near field, reactive field components predominate over the radiated field. This means that any variations in the electrical properties (for electrical antennas) or magnetic properties (for magnetic antennas) have a strong influence on the antenna's impedance at the antenna feed point. The distance from the antenna to the boundary of the reactive near field region is commonly assumed as:
  
  
• In the radiating near field, the radiated field predominates, and the antenna impedance is only slightly influenced by the surrounding media in this region. But the dimensions of the antenna cannot be neglected with respect to the distance from the antenna. This means that the angular distribution of the radiation pattern is dependent on the distance. For measurements of the radiation pattern, the distance from the antenna should be larger than the radiating near field boundary, otherwise the measured pattern will be different from that under real life conditions. The diameter of the radiating near field is
  
  

  with D as the largest dimension of the antenna.

• For distances larger than R₂, the radiation pattern is independent of the distance, meaning we are in the far field region. In a practical application, the distance between transmitter and receiver antennas is usually in this region.

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.

Antenna Characteristics
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.

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