Technical data

www.westermo.com Theoretical and general applications 107
Antennas
Terminology
When discussing radio communications and antenna it is vital to understand a few
basic terms and expressions. The first basic formula to remember relates frequency (f)
to wavelength (l) by the equation: l [m] = 300 / f [MHz].
The radiation pattern is the three dimensional radiation characteristics of an anten-
na in 2 planes, the electric field (E) and magnetic field (H).The gain of the antenna is its
capability to force radiation in a specific direction in space at the expense of other
directions. Gain is expressed in dB compared to some reference: for example dBi
refers to gain compared to an isotropic antenna and dBd to a dipole antenna. The
polarization is defined as the plane of antenna’s electric field E and can be vertical, hori-
zontal, slanted or circular. Typically the antenna’s physical orientation equals the anten-
na’s polarization. Orthogonal polarization’s have a cross polarization loss of 21 dB. In
practice all the antennas in one system should use the same polarization.
The Impedance of an antenna is its AC-resistance and reactance within the operat-
ing band. Nominal impedance of 50 ohms is a standard. The bandwidth is the frequen-
cy range where the antenna’s characteristics like impedance, gain and radiation pattern
remain within the specifications. The commonly used term attenuation is mainly relat-
ed to feeders and radio propagation and is also expressed in dB.
The antenna and its components
An antenna is an electromechanical device whose purpose is to radiate as effectively as
possible the power from the feeder in a specific manner.
A power splitter matches and combines multiple loads or sources and equally splits
the power between them without disturbing the characteristic impedance of the system.
Splitters are used in antenna arrays to combine multiple antennas or in RF distribution
harnesses. A feed-line is an interconnecting cable between radio equipment and antenna.
Feeders tend to be lossy components so the type has to be carefully selected depending
on the required length and operating frequency. Lightning protectors can be inserted
between the radio equipment and feeder to help protect the radio against a lightning
strike. Typically a lightning protector is a DC short-circuited quarter wave stub. When
interconnecting antenna circuit components, impedance match has to be maintained in
order to provide ideal flow of power without additional losses due to reflections.
Impedance match is commonly measured as VSWR (Voltage Standing Wave Ratio)
where a VSWR of 1:1 is ideal and 1:1.5 is more realistic in practice.
B A C K