4. THE ANTENNA FEED

Figure 12: Example of an antenna feed and LNA assembly [Ref 3].

The purpose of an antenna feed is to couple the incoming RF energy reflected from the dish into the LNC. It affects the performance of the overall antenna system in several ways.

• Thermal noise temperature. The feed should desirably "see" as much of the dish surface as possible. For prime and offset focus dishes there should desirably be no overspill of the feeds reception pattern otherwise unwanted thermal noise from the ground behind the dish will increase the antenna equivalent thermal noise temperature.

• Efficiency. For maximum antenna efficiency an optimum feed design would have zero attenuation for the illuminated part of the dish but infinite attenuation for angles beyond the dish. This is known as a uniform illumination law. Practical feeds can only approximate this ideal. However if the design goal is to minimise sidelobe levels an illumination law with "aperture edge taper" may be used. This reduces sidelobe levels but also reduces on axis gain.

• Beamwidth. The "inverse taper illumination" laws may be used to obtain a main lobe that has a narrower beamwidth. But this is achieved at the expense of main lobe gain and sidelobe rejection performance.

Figure 13: Diagram showing feed overspill and inadequate feed illumination [Ref 3].

Figure 14: Diagram showing "uniform illumination" and "aperture edge taper" feed illumination laws [Ref 7].

Satellite television signals like all radio waves are polarised. This property of satellite signals is often used to improve spectrum efficiency in satellite bands. Almost all satellite operators make use of oppositely polarised signals to provide a greater number of transponders within a given amount of available bandwidth.

A limitation on the use of oppositely polarised transponders is the cross polarisation discrimination (XPD) of the receiving system. With suitable reception equipment and the absence of adverse weather conditions XPD ratios in excess of 30dB can be achieved.

Figure 15: Transponder frequency plan of Optus B series satellites. Note use of vertical and horizontal polarisations to achieve frequency re-use.

Two different types of polarisation have been employed in satellite television applications these are: circular (right hand or left hand circular); or linear (horizontal or vertical) polarisations. Most experience in Australia is with linear polarised signals.

Note that a linear polarised feedhorn can recover circularly polarised signals but will suffer a 3 dB signal loss.

Generally a feed will be optimised for reception of one polarisation of satellite signal and will offer high attenuation to the opposite polarisation. Where satellites carry signals of interest on both vertical and horizontal polarisations the following strategies may be adopted to recover signals on both polarisations.

Manual rotation of the feed and LNC assembly through 90 degrees. (In practical situations it is easier to find the exact angle by nulling the unwanted polarisation signal rather than by peaking the wanted polarisation signal)

this process can be mechanised by adding a motor to the polarisation rotation hardware

a similar but more subtle approach is to rotate the antenna probe inside the feed rather than the whole assembly

another approach is a dual polarisation antenna feed. This uses a coil of wire around a circular waveguide containing ferrite material. When current is passed through the coil the Faraday effect resulting from the induced magnetic field changes the polarisation of the received signal. This method introduces a small insertion loss, perhaps 0.1 to 0.3 dB. But because this precedes the LNC it could have a bearing on very low noise temperature systems.

another approach is to have two probes mounted at right angles to each other and select the appropriate probe by diode switches (or to feed both signals on separate cables back to the satellite receiver(s). Where diode switching has been used cross polarisation discrimination levels have been degraded (approx. 15 dB).

A device called orthomode transducers (OMTs) can be used in antenna feed. OMTs allow two LNCs to be connected to a single waveguide feed so that both vertically and horizontally polarised signals can be received simultaneously.

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