This page provides figures that may be used in the link budget calculator
Uplink C/interference dB.
If you are using a high power spectral density uplink such as a feeder link for
broadcasting or a VSAT outlink destined for small antennas, and have the
satellite transponder set to low gain, then put a figure like 30 dB here.
If you are transmitting a low power spectral density uplink like a VSAT transmit
signal destined for a large hub antenna, and have the satellite gain set to
high, then put a figure like 20 dB. You are vulnerable to uplink
interference from other uplink antennas pointed to adjacent satellites and from
cross polar interference from co-frequency users on your satellite.
Satellite C/intermodulation dB
If the transponder is filled with 3 or more carriers, you will suffer from
transponder intermodulation noise. The level of intermodulation noise
depends on the transponder operating point. For a typical
multi-carrier transponder the input back off will be 6 dB and the output back
off will be 3.5 dB. The C/intermodulation ratio will be around 21
dB. Put 21 dB in the box.
If you are operating with 1 or 2 carriers the intermodulation falls out of band
and is negligible. Put in a number like 50 dB or higher, which is
equivalent to virtually nothing.
Downlink C/interference dB.
If you are using a large receive earth station put a figure like 30 dB here.
If you are using a medium size receive earth station put in 25 dB
If you are using a very small receive earth station put in 20 dB.
Small antennas have poor off axis discrimination and receive more interference
from adjacent satellites. Their cross polar alignment adjustment is also
often poor.
In all the above cases, if you know the proper figure, put it in of course. The figures above are intended as a good starting point.
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