SNR versus Eb/No

The formula relating S/N to Eb/No only works if the two measurements are made at the modem. Is the local spectrum analyser at the remote site ?

If the modem displayed Eb/No, compared with the Eb/No as calculated from the measured C/N, is abnormally low then some ideas..

The level into the modem is wrong, extremely high or low. E.g higher than +15dBm or lower than -50 dBm. Read modem spec for acceptable range for wanted carrier level.

There is a superimposed similar signal causing interference. E.g cross-pol interference from similar carrier on opposite polarisation or even your own signal! Turn your carrier off and check for clean noise floor. Compare co-pol and x-pol spectrums and adjacent satellites spectrums.

There is phase noise added to the signal.e.g due to ripple on LNB power supply. Use spectrum analyser to check that the modem input cable is clean between 0Hz and 200kHz.

There are other very high power carriers going into the modem. E.g You are at big teleport hub and you have weak wanted carrier and massive TV carriers on same satellite. Read modem spec about composite input power.