Axial Ratio

This parameter is used to describe the performance of circularly polarized antennas. The axial ratio of an antenna characterises the quality of circular polarization an antenna has at a specified angle. For GPS antennas this is normally given as the zenith angle.

The closer the axial ratio is to unity, the better the circular polarization. As the value of the axial ratio increases, the antenna changes from being circular, to elliptical, and finally to linearly polarised as the ratio heads towards infinity.

Example: Dielectric-loaded quadrifilar helix antennas have been reported to have an axial ratio of unity, which make them ideal for reception of circularly polarized signals emanating from satellites.

Bandwidth

Each communications band in the electromagnetic spectrum has a specified frequency range. This frequency range is normally specified for the communications band uplink, downlink or for a combination of the two. The frequency range given in MHz is known as the bandwidth.

Example: UMTS-FDD down link is allocated as 2110 to 2170 MHz, and therefore has an allocated bandwidth of 60 MHz.

Balanced Feed

When someone is using a handheld device, the user can significantly perturb the antenna performance. To avoid this if the antenna system/topology can stop currents flowing down the outer conductor of its feed then the antenna will be isolated from the noisy handset environment and significantly reduce the hand and head impairments.

Example: All Sarantel antennas possess an integral balun as part of their topology which means that they all have balanced feeds and have extremely low susceptibility to proximity effects of human tissue.

Beamwidth

The beamwidth of an antenna is usually measured from the antennas far field pattern. The -3dB beamwidth or half power beamwidth is the characteristic that is usually measured, as this is where the field intensity drops by -3dB from the far field patterns maximum value. This gives an angular measurement in degrees and demonstrates how broad the far field pattern is.

Example: The Sarantel antenna is has a minimum half power beamwidth of 120 degree which makes it ideal for low elevation satellite reception.

Directivity

International Standards define the directivity of an antenna as "the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions. The average radiation intensity is equal to the power radiated by the antenna divided by 4p." However when antennas for mobile telecommunications are considered, Gain and not directivity is normally specified.

Efficiency

In very basic terms an antennas job is to take in electrical power and transmit electromagnetic energy. If all the energy supplied to the antenna is radiated then the antenna is said to be 100% efficient. Therefore it can be considered as a comparison between the Power in to Power out, or alternatively Gain to Directivity.

Example: Embedded antennas used within a mobile handset are typically 5%-8% efficient when the handset is in use; however the Sarantel antenna can give 20% efficiency due to its unique properties.

Gain

International Standards define absolute gain as "the ratio of the intensity, in a given direction, to the radiation intensity that would be obtained if the power was accepted by the antenna isotropically.

The radiation intensity corresponding to the isotropically radiated power is equal to the power accepted (input) by the antenna divided by 4p". The gain is a measure of both the directive property of the antenna, as well as how efficiently it transforms the input power into radiated power. It is measured in units of dBi (decibels as referenced to an isotropic antenna element).

Isolation

When an antenna has a balanced feed it is said to be isolated from the device to which it is attached. Isolation of an antenna significantly reduces the impairments that are suffered in antenna performance when held by human tissue. A gain loss of 15dB has been reported in past investigations, though as devices become smaller more loss could be incurred if the antenna is not isolated.

Far Field Radiation Pattern

These are 2D or 3D patterns of the fields radiated from the antenna. These are measured in the far field region of the antenna; this is the field region where the angular field distribution is independent of distance.

Example: the far field radiation patterns for the GPS antenna are normally 2D elevation plots, for the quadrifilar helix antenna a robust and predictable cardioid far field radiation pattern is observed in free space and 'in-use' environments.

Polarization

By definition polarization of an antenna is "the polarization of the wave transmitted (radiated) by the antenna". Linearly polarized signals oscillate in a plane (often vertically or horizontally) as the wave propagates.

The polarization of the radiated wave is "that property of an electromagnetic wave describing the time varying direction and relative magnitude of the electric-field vector; specifically, the figure traced as a function of time by the extremity of the vector at a fixed location in space, and the sense in which it is traced, as observed along the direction of propagation".

Examples: GPS Satellite signals are right hand circularly polarized, therefore the vertical and horizontal components are equal in magnitude but their phases are such that the field is seen to rotate clockwise from the antenna as the wave propagates.

Return Loss

The Return Loss looks at the power inputted into the antenna and compares it to the amount reflected back. The reflected power is normalised by the incident power and is known as the return loss. This value is measured in decibels.

The lower the negative value for the Return Loss, the better the antenna radiates. I.e. an antenna with -20dB radiates better than an antenna with only -10dB Return loss, however as the scale is logarithmic the radiation efficiency of the antenna can be misleading. The Return loss at the frequency for which the antenna was designed is also known as the Match of the antenna.

Specific Absorption Rate (SAR)

Specific Absorption Rate measures the amount of power radiated and absorbed into a unit mass of body tissue. This issue has become of increasing interest in the last 10 years as people have claimed that mobile phones can cause damage to the user. Another consideration is that if the hand or head is absorbing energy then this is energy that is lost.

Example: Sarantel have shown that the use of bifilar and quadrifilar structures realised around a high dielectric ceramic help control the near fields of the antenna and less exposed to human tissue, making the antennas power efficient. Also the bifilar structure has an electromagnetic null directed towards the users head therefore creating a low SAR topology.

Voltage Standing Wave Ratio (VSWR)

The Voltage Standing Wave Ratio or VSWR is a measure of the impedance mismatch between the transmission line and the antenna. The greater the VSWR the greater the mismatch. When the VSWR is unity this signifies perfect match. This is often used instead of the return loss to show how well an antenna is matched.