Terminology: Bandwidth, LNA Noise Temperature, Saturation Flux Density and Polarization.

Bandwidth is another fundamental antenna parameter. Bandwidth describes the range of frequencies over which the antenna can properly radiate or receive energy. Often, the desired bandwidth is one of the determining parameters used to decide upon an antenna. For instance, many antenna types have very narrow bandwidths and cannot be used for wideband operation.

Bandwidth is typically quoted in terms of VSWR (Voltage Standing Wave Ratio, and sounds very complicated. But it is simply a measure of how much power is reflected from an antenna. ) . For instance, an antenna may be described as operating at 100-400 MHz with a VSWR<1.5. This statement implies that the reflection coefficient is less than 0.2 across the quoted frequency range. Hence, of the power delivered to the antenna, only 4% of the power is reflected back to the transmitter. Alternatively, the return lossS11=20*log10(0.2)=-13.98 dB.

Note that the above does not imply that 96% of the power delivered to the antenna is transmitted in the form of EM radiation; losses must still be taken into account. 

LNA Noise Temperature: It is the measure of the amount of noise generated by the LNA. It is measured in Kelvin-the lower the better. Typical performance can be achieved for 30K to 70K. LNA noise performance is sometimes specified in terms of Noise Figure which is measured in dB.

NF=10*Log (1+T/290), NF in dB, T in Kelvins. 

Saturation Flux Density: Flux density is a measure of signal strength at a point in space and is measured in Watts/meter2 or dBW/meter2. It is usually applied to signals received at a satellite.

Polarisation: The polarisation of an RF wave in space is defined by the orientation of the electric vector (E) of the wave. The polarisation of an RF wave is used in satellite systems to separate two signals at the same frequency and allows frequency reuse in satellite systems. There are circular and linear polarizations and under each there are horizontal and vertical.


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