Link Power Budgetting
3.Calculation of Transmit EIRP (in dB W)& operating FD
1. Transmit power - per carrier It is the power given to the antenna at the Earth station to radiate power. It's range lice in the watts.
2. Transmitter feeder loss - Losses will occur in the connection between the transmitting antenna and the transmitter proper. Such losses will occur in the connecting wave-guides, filters and couplers.
3. Antenna pointing loss - When a satellite link is established, the ideal situation is to have the earth station and satellite antennas aligned for maximum gain. There are two possible sources of off axis loss , one at the satellite and one at the earth station . The off axis loss at the satellite is taken in to account by designing the link for operation on actual satellite antenna contour. The off axis losses at the earth station is referred to as the antenna pointing loss. Antenna pointing losses are usually only a few tenths of a decibel.
4. No of carriers - By frequency multiplexing scheme we can transmit any number of carriers through an antenna.
5. Transmit EIRP - This is the product of actual power given to transmitting antenna and antenna power gain of transmitting antenna.
That the equivalent isotropic power (EIRP) may be defined as
EIRP is often expressed in decibels relative to 1 watt, or dBW. Let Pt be in Watts then; EIRP = [PT]+[GT] dBW
If we consider transmitter feeder loss & antenna pointing loss in to account
Transmitter EIRP= 10 log (Transmitter power per carrier )-Transmitter Feeder loss + Transmitter antenna gain - antenna pointing loss (db W) ......
Example:Calculation of transmit EIRP for an antenna having the following data. Transmit power per carrier- 0.28 W, antenna feeder loss-1.00 dB, antenna pointing loss-0.70 dB And transmitter antenna gain 45.21dB.
Solution : Transmit EIRP = 10 log (0.28)-1 + 45.21- 0.70
= 37.98 dB
6. Satellite operating flux density (dB /Sq m) -The traveling tube amplifier in a satellite transponder exhibits power output saturation. The flux density required in the receiving antenna to produce saturation of the TWTA is termed the saturation flux density.
Satellite operating flux density = Transmitter EIRP -Up link Rain attenuation -162.2 (dB /Sq m)......
Example: calculate satellite operating flux density (dB /Sq m)for the satellite having EIRP 37.98 dB and no rain attenuation .
Solution : Satellite operating flux density dB/Sq m = 37.98- 162.2
= -124.21 dB/Sq m
7. Amplifier back off -To reduce inter-modulation distortion, the operating point of the TWT must be shifted to the linear portion of the curve, the reduction in input power being referred as input back off. When multiple carriers present, the power around saturation, for any carrier, is somewhat less than the achieved with single carrier operation.
The input back-off is the difference in decibels between the carrier input at the operating point, and saturation input that would be required for single carrier operation.
8. Power rating of ground transmitter -It the power delivered by the ground transmitter. Power rating of ground transmitter can be expressed as
Power rating of ground transmitter =10((10 log (Number of carriers * Power per carrier)+amplifier backoff)/10) ........
Example: calculate power rating of ground transmitter operating at following conditions. Number of carriers- 1, Power per carrier- 0.28 W, Amplifier back off-0 dB.
Solution : Power rating of ground transmitter
= 10((10 log (Number of carriers * Power per carrier)+amplifier backoff)/10)
= 0.28 W