what is the difference between C/N refernce and C/N margin

[FTA hunter]

i was able to get locked apstar 5 using a 3.6m mesh dish. I used a spectrum to locate the signal on the spectrum it says C/N reference 11.8 dB. But when i monitor the same feed on a tandberg ird it says C/N margin +0.9 dB. What is the difference between this i looked around website but didn't get any detail.

 

[Titanium]

So little information, but here we go...
The meter is averaging a selected bandwidth and showing and displaying 11.8dB over the noise floor within the selected range. TheTandberg is tuned to a specific frequency which only has .9dB carrier margin over the noise floor. Maybe off freqency, severe TI or other? .9dB is extremely low and likely an usable signal.

 

[FTA hunter]

so where should i look at when aligning the dish. eventually looking at the spectrum doesnt help me much i did calculated the IF frequency and my span was set to 50MHz.

 

[Titanium]

Ultimately, the receiver is the deciding factor if the signal can be processed.

A signal trace on a spectrum analyzer is one useful tool in aligning and diagnostics, but it doesn't indicate if the signal can be decoded on a receiver. The SA will identify potential signals, but the receiver must receive a usable ratio of carrier to noise in order to decode a compatible signal. If the raw signal has too many errors, the FEC will not be capable of correcting the errors and the result is a digital cliff. I have noticed that Tandbergs typically require much higher CNR and are deaf as compared to other IRDs and consumer STBs.

Maybe the SA is also displaying an interfering signal in this range?

 

[FTA hunter]

i am not sure coz on the spectrum i don't see any spikes on the transponder. today i tried a domestic coship domestic STB the Q was at 80%. but after 30 mins or so the signal breaks up. i did a loop out to tandberg IRD margin was at +3.0 dB but it dropped to 0.1db . i know that there is no interference since the dish is located at mountains miles away from TI towers even hard to get cell reception. I am trying to get asiasat 5. we are just on the edge of there footprint.
Could it be the footprint is on and off on the edge?? but if so how come i am able to lock to +3.0 db margin. I am using 4.5 meter solid dish.

 

[Titanium]

Being on the edge of a footprint, you will see every change result in the signal hitting the digital cliff. Many variables will come into play when receiving outside the link budget parameters: Atmospheric conditions (solar, particulate matter, temperature, etc.), satellite position in the box, link power management, dish surface accuracy affected by temperature and wind loads, LNB temperature and drift. I am sure others could add more variables....