Satellite Communications/uplink power losses
QUESTION: Dear Sir,
can you please mention the main points cause losses to satellite uplink signal?
I already made a comparisons between my sng system and a similar systems and found that my uplink power is too much higher than the others, i.e.:
for 12 MHz bandwidth dvb-s2 8psk mpeg-2 parameters uplinked to Eutelsat-3B my system requires 55Dbm while the another sng requires 48Dbm for the same received signal margin.
I have directly connected the hpa to the feedhorn with a single waveguide (without waveguide switch ) to find that only 0.8db is the difference in losses.
both hpas are working properly and look ok.
note: the hpa controller reads 32Db as a reflected/reverse power, its around 1.6 watt which I think its negligible, can you help me plz?
ANSWER: Hi Mr Bassam,
I am not sure what you meant by Margin you have quoted for Two SNGs. Because Margin is NOT measurable but the provision of margin is given when Link Budgeting is calculated. Also the link equations have several parameters that need be considered for deciding safe operating Margin in Rain attenuation and also are based on Satellite PFD and EIRP. Also the IBO and OBO need to be strictly adhered. So based on the quality of service offered by Satellite User, they will uplink the power.
Also please note that Link Margin is in terms of dBs and NOT in dBms.
And if you meant that the Power Output is the difference quoted, the figure of 7 dB is very high for the same Margin. May be the coupler specification used in HPA can be checked and calibrated.
You didn't mention whether the same satellite is used by both SNGs?
In Ku band operation sometimes because of rain attenuation, Automatic Power Control will uplink much higher output than in free space to take care of Uplink Rain Margin.
Consider the above and you may get practical values.
---------- FOLLOW-UP ----------
QUESTION: dear Mr. TIRUMALA,
thanks a lot for replying and my question was about the losses points that caused by my system not the environment losses. here are the information required:
1.(I am not sure what you meant by Margin you have quoted for Two SNGs.)
I meant the downlink c/n margin as a reference to compare between the two SNGs where both are using the same dish size (1.5m).
2. (May be the coupler specification used in HPA can be checked and calibrated)
do you mean by COUPLER the waveguide switch?
if so, I already bypassed it by connecting the waveguide directly from the hpa to the feedhorn as I explained later.
3. (You didn't mention whether the same satellite is used by both SNGs?)
yes, both SNGs were linked to the same satellite and same channel used sequentially and in a clear sky.
hope if you can help me solving this issue.
Hi Mr Bassam,
If all technical parameters are the same then the only difference bwtween power levels to be transmitted (after due Link Budget calculations considering satellite parameters, Earth Station and Modulation) the only parameter remaining is the CONTOUR OF SATELLITE FOOT PRINT. HERE the ES POWER IS DECIDED BASED ON the location of the SNG which indicates what foot print contour it is trying to access and this is purely based on the location and the foot print. This can be as high as what you mentioned in the power level.
Except this there is NO other technical explanation but for any hardware ambiguity.
For example if one SNG is in zero dB contour and the other is at minus 3 dB then the resultant transmitted power surely will be higher for the later to meet the same margin figures.