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Low Eb/No

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Ex Member

Sep 1st, 2012 at 8:52am  
Good dear house.

I did a VSAT installation some months back and i discovered that i cannot get beyond 6.0 db as my Eb/no. I check every thing that needed to be check including the dish mount and etc but seems not to get beyond this.

What else can i do to get a higher Eb/No.


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Eric Johnston
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Reply #1 - Sep 1st, 2012 at 2:38pm  
Check the polarisation:

If your downlink carrier is linear polarised, what is the name of the polarisation: horizontal or vertical?. Do you have a linear polarisation feed system ? Have you set the nominal polarisation (horizontal or vertical) and then applied the calculated adjustment amount of degrees (clockwise or anticlockwise)?

If your downlink carrier is circular polarised, do you have a circular polarisation feed (i.e. with a polariser section). If this is missing you will have at least 3 dB degradation.  The pins in the polariser section must be at 45 deg to the LNB polarisation plane.   If the pins are in line with the LNB polarisation you will have linear polarisation and at least a 3 dB reduction.  If 45 deg the wrong way, you will get no wanted signal at all.

In all cases of wrong or misadjusted polarisation you will suffer interference from, and cause interference to, carriers on the opposite polariation (cross-pol) transponder.

Check the feed for water, insects, nuts, washers, bolts etc inside.

Check for clear view of the satellite.
Get just under the lower edge of the dish and look towards the satellite at the beam elevation angle. Is there any obstruction: trees, walls etc?.

Low power from the satellite and a too small dish.
Ask your service provider about the beam contour over your site and your dish size.  Should the Eb/No be higher ?
Are other similar sites better ?  Maybe you need a larger dish.

Dish distortion.
Does the weight of the feed/LNB/BUC cause the sides of the dish to be pulled forwards ? Put two tight threads up-down and across the front of the dish.  They should just touch where they cross.   Aim for 1mm gap at 14 GHz.  

It would be helpful to say what satellite and show us a picture of your antenna and feed system.

If you have a dedicated carrier, just for you, and none of the above helps then reduce the information rate and adjust the FEC coding rate till you get a satisfactory service.   In general, a larger dish will allow an increase in the information rate capacity for the same satellite power and bandwidth used, and thus lower cost per bit.  

Best regards, Eric.
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