Re: EsHailsat Location Beacon running


Ole OZ2OE
 

Hi Andy

I enjoy reading technical comments on this group. But I think you have missed the effect of background LNB noise?

Just "seeing" the transponder noise floor is not enough to ensure S/N will be constant. In (the simple) case of a 3 dB transponder noise floor, what you see is actually the LNB noise floor plus an identical amount of transponder noise - adding up to a 3 dB increase. Increasing transponder noise floor from here will reduce influence of LNB noise - to a a point where LNB noise is really in the "background".

But at a low values of transponder noise floor there is some degadation in S/N. And LNB noise figure becomes important.

- or is it me missing something - hi.

73 Ole OZ2OE

Den 16. marts 2019 kl. 19.29.20 +01.00, skrev Andy G4JNT <andy.g4jnt@...>:

The critical factor for QRP operation is S/N into the satellite receiver.  Then, provided any ground station has sufficient antenna gain to see the transponder noise floor, S/N will be constant, and a function of uplink  EIRP only.    Changing transponder gain will move the downlink received signal up or down, and the noise floor will go with it, maintaining downlink S/N.

Things only hurt when you have a low gain downlink antenna or poor noise figure (*).  If transponder gain is dropped too much, the received signal could fall below the ground station's own noise floor and that would degrade S/N

(*) There is a figure of merit used in spacecraft link terminology for the receiver performance, called G/T; gain of the antenna divided by system noise temperature in Kelvin.  It is usually expressed in dB, a fact that confuses many who meet the term for the first time (like me when I first met it !).  G/T expressed in dB is either 10.LOG(linear antenna gain / temperature)    or    Antenna gain in dB - 10.LOG(temp) 

Andy


On Sat, 16 Mar 2019 at 17:39, Martin - G8JNJ via Groups.Io <martin_ehrenfried=yahoo.com@groups.io> wrote:
On Sat, Mar 16, 2019 at 05:35 PM, Martin - G8JNJ wrote:

Yes that's another problem, as the Goonhilly dish may also have moved with the wind (as mine did too) so it's difficult to find a common reference point.
I think Phil may also have adjusted the Goonhilly SDR gain too, so that's another factor.

Regards,

Martin - G8JNJ




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