Antenna Basics
Antennas are designed and built to suit a particular frequency or frequency band. If you use an antenna designed for different frequency, then it will only radiate a small portion of the generated RF power from the transmitter, and it will only absorb a small portion of the RF signal power for the receiver. Using the correct frequency antenna is very important.
Antennas are compared to a theoretical isotropic antenna. This antenna radiates all of the power from the transmitter in a 3-dimensional spherical pattern – very much like a point source of light without any mirrored reflectors.
An isotropic antenna is theoretical only, because in the construction of antennas, the radiation pattern becomes distorted in certain directions.
A “dipole” antenna is manufactured with an active “radiator’ with a length equal to ½ the wavelength of the design frequency.
The RF power envelope radiated by a dipole is distorted by radiating more power in the horizontal plane and less in the vertical plane – that is, there is more power radiated to the sides than up and down.
The effective RF power o the sides has increased, and the effective power up/down has decreased.
The term “effective radiated power” or ERP is used to measure the power radiated in specific directions. The difference between the effective radiated power and the transmitter power is called the “antenna gain”, and is normally expressed in dB.
Antenna Gain In dB = log10 (PERP/PTX)
The gain of a dipole antenna to the sides of the antenna is + 2.14 dB. This means the effective radiated power to the sides of the antenna is 2.14dB more than the power from the transmitter. The gain in the up/down direction will be negative, meaning that the effective radiated power in these directions is less than the power from the transmitter.
A dipole mounted vertically transmits and RF wave in the vertical plane – this is called vertically polarized. If the antenna is mounted horizontally, then the RF wave will have horizontal polarity.
The radiation pattern can be distorted further by connecting multiple dipole elements together. These antennas, known as “collinears”, have a higher gain to the sides and a more negative gain up/down.
Collinear antennas are normally manufactured with gains of 5dB, 8dB or 10dB compared to the transmitter power. When antenna gains are expressed as a comparison to the transmitter power, it is called “isotropic gain”, or gain compared to an isotropic antenna. Isotropic gain are expressed as “dBi”.
Another common way to express gain is “as compared to a dipole” – these gains are expressed as “dBd”. The difference between dBd and dBi is the intrinsic gain of a dipole, 2.14dB (normally rounded to 2dB).
DBd = dBi - 2
Be careful with antenna gain specifications – manufacturers do not always specify if they are using dBi or dBd.
Dipole and collinear antennas are called omni-directional as they transmit equally is all directions in the horizontal plane. Normally omni-directional antennas are used for intra-plant applications, where the radio path lies completely within an industrial site or factory. The radio path is often made up of strong reflected signals over relatively short distances and the direction of incoming signals over relatively short distances and the direction of incoming signals is not always obvious.
Directional antennas distort the radiation patterns further and have higher gains in a “forward” direction. These antennas are often used for longer line-of-sight radio paths found in intra-plant applications.
A Yagi antenna has an active dipole element with “reflector” elements which act to focus power in a forward direction.
Yagis are normally available from 2-element up to 16-element Yagis. The more reflector elements added, the higher gain in the forward direction and the lower gain to the sides and rear. Also, as more elements are added, the directional angle becomes smaller as the gain is more tightly focused.
Yagis are mounted with the central beam horizontal and the orthogonal elements either vertical or horizontal. If the elements are vertical, then the antenna is transmitting with vertical polarity: if the elements are horizontal, the polarity is horizontal.
Antennas in the same system should have the same polarity.
For higher frequency Yagi antennas, it is physically possible to add side reflectors to increase the gain further. For 2.4GHz devices, parabolic reflectors around the dipole element yield extremely high gains and extremely narrow transmission beams.
The simples antenna commonly used is “1/4 wave whip” antenna. These antennas are simply 1/4 wavelength conductor, normally mounted directly to the wireless device. They are “ground-dependant” antennas In that they need an external reference plane to efficiently radiate power. These antennas are nominally a unity gain antenna, however gain depends on the installation and often the installed gain is approx –2dBi.