MISL Aerospace

Primary Frequency

Most antennas are designed to operate efficiently within a 10% bandwidth.

2.5GHz ±5% means a frequency of 2.375 to 2.625GHz, which is a 250MHz bandwidth.
At 10GHz, an antenna will operate from 9.5 to 10.5GHz.

Primary - Gain

Antennas passively increase the radiated power by concentrating it into certain directions. The gain of an antenna is a measure of its directionality.  Gain is measured in dB (isotropic) or dBi and is usually expressed in dB or dBi relative to an isotropic source (equal in all directions).  Antennas are not 100% efficient and have internal losses.

The gain of an antenna includes these losses:

Gain = Directivity - Internal losses.  Gain is the additional signal strength that the antenna provides in one direction at the expense of a signal strength in other directions.  An antenna is normally a passive device providing gain by directing the energy to a required pattern.  The result is a higher signal strength in one direction and lower in other directions.

Primary - Bandwidth

The bandwidth of an antenna is the range of frequency where the antenna is operating effectively.  Typically, our antennas have a nominal 10% bandwidth.
This may increase or decrease depending on the antenna and the performance required.  When considering an antenna specification it is important to specify the exact bandwidth required.
Specifying L-band, for example, is not sufficient as the L-band covers 1 to 2GHz which is much larger than a 10% bandwidth.

Primary - HPBW (Half Power Beam Width)

HPBW for Omni AntennasA dipole is the simplest type of omni-directional antenna.  It has 360° azimuth coverage;  the energy is squeezed from top to bottom to provide gain on the horizon.  The elevation pattern is measured by taking a vertical cut through the beam.  The antenna's beamwidth is defined by the angle over which the radiated energy falls to half its peak level.  This is known as the Half Power Beam Width (HPBW), or the -3dB point.  This will apply to the elevation pattern whilst the azimuth beamwidth will be 360°.
HPBW for Directional and Sector AntennasEnergy is directed from vertical and horizontal sides to produce a directional lobe or sector beam. For directional and sector antennas, azimuth and elevation HPBW are specified separately.

Primary - Sidelobes

In antenna engineering, sidelobes are the lobes of the radiation pattern that are not the main beam.  An antenna radiation pattern is more commonly called a beam pattern.
The power density in the sidelobes is generally much less than that in the main beam.  Sidelobe levels are measured in dBs relative to the peak of the main beam.  It is possible to control sidelobe levels, depending on the application.

Primary - Polarisation

All electromagnetic radiation is polarised.  The figures below show the electric (E) vector in a propagating wave for various types of polarisation.  The polarisation of an antenna describes the orientation of its electrical field or E-Plane.
The polarisation can be linear or circular.
Linear polarisation is usually vertical or horizontal.  Dual polar antennas can produce vertical and horizontal polarisation via separate ports.   A further extension of this are dual slant antennas;  these are essentially the same as dual vertical and horizontal antennas but with the polarisation rotated by 45°.  
Circular polarisation is produced when the E-plane of the antenna spins.   Depending on the direction of spin, the polarisation is right or left.

Primary - Cross Polar

Antennas are never perfectly polarised.  Cross polar is a measure of how much energy is in the plane perpendicular to the E-plane or opposite hand of circular polarisation.    Cross Polar is typically quoted relative to the peak gain of the antenna.

Primary - Return Loss

This is the amount of signal that is reflected by the antenna at the connector.  This is expressed either as the relative level of the return signal in dB, or in terms of The Voltage Standing Wave Ratio (VSWR) present on the input to the antenna as a result of the reflection.  Return Loss and VSWR are related.