Re: EsHailsat QRP Operation


Ole OZ2OE
 

Andy,

Thank you for the explanations. I fully agree that with many different parameters in play - and some not really known - the analysis is not simple.

But I was only trying to comment a statement you've written in an earlier posting that: "provided any ground station has sufficient antenna gain to see the transponder noise floor, S/N will be constant".

While this is true for ground stations seeing high (ie 10 dB or more) transponder noise, it is not the case for ordinary 60 to 70 cm RX antennas, where noise level after latest transponder gain adjustments are more like 3 dB. Here there is a degradation from the ultimate S/N level otherwise possible with bigger antennas, because one noise source - from the transponder - is not dominating the other - from the LNB.

In the "simple" case of a +3 dB raise in noise level, the two contributions - within the transponder bandwith - are equal and because they are unrelated, they add up. At this level the ultimate S/N ratio is reduced by 3 dB. Improvements can be made by selecting good LNB's. Not all "0.1 dB" LNB's are equal - although all are not 0.1 dB!

Bigger RX antennas are essential for QRP operation. Those with "recommended size" antennas will have to use Goonhilly WebSDR or similar for detection of the really weak ones.

73 Ole OZ2OE




Den 17. marts 2019 kl. 13.37.37 +01.00, skrev Andy G4JNT <andy.g4jnt@...>:

Warning - this rambles on for quite a bit, and I've changed the subject line.

No, it's not as simple as you state, although LNA noise figure does matter, it's only significant when you have a small antenna.  First of all, when considering any space link  budget, we must stop talking about Noise Figure in the overall system.  When antenna temperature is anything other than a warm terrestrial 290K it becomes meaningless.   A decent antenna looking up at cold sky may have an antenna temperature of 10 - 30K,   call it 20K for now.   And now LNA noise figure becomes a lot more significant.  Eg a 1dB NF LNA, has a noise temperature of 290 * (10^(1/10) - 1) =  75K.   Assuming this dominates the complete receive chain, that means a system noise temperature of 75+20 = 95K.

Which all goes to show that LNA performance becomes a lot more critical when receiving WEAK signals from space.

But now consider a situation where a satellite transponder has enough gain that it is radiating a significant amount of it's own receiver noise but no other signals.   Lets say it's radiating sufficient noise such that our ground station is seeing that transmitted noise at a level of 5dB above the ground station's own noise level.  BUt it is a radiated signal now, and as such cannot be directly added in to the ground stations own noise in any meaningful way.   Here, for example, that 5dB increase in our 95K system defined above, 3.16 in linear terms, could be said to correspond to a new receiver noise temperature of 3.16 * 95k = 300K,  of which 95K is our own, the rest from the satellite
But if we used a bigger antenna on the ground, and saw a transponder noise level 10dB above Rx noise, that could be said to be 950K by the same argument.   Which is absurd - increasing antenna gain does NOT change system noise temperature.

So ... yes, LNA performance does contribute to overall system performance, and more than in dB for dB terms, but ONLY WHEN receiving weak signals from space.    A satellite downlink from a linear transponder IS NOT A WEAK SIGNAL if you are using a 'normal' downlink receive antenna, so LNA / LNB performance is not that critical.

I believe, although can't yet prove for myself as I have a slightly blocked view of the satellite, is that a 60cm dish and good LNA results in transponder received noise floor appearing about 5dB above receiver noise.   Goonhilly uses a larger dish

What actually IS the weak signal when operating QRP into the satellite is defined by the satellite's antenna gain and noise temperature.   Now, a satellite antenna global beam looking at the earth will be seeing warm terrestrial noise, and have a much higher noise temperature - something like 200K used to be taken in my old days working on Inmarsat as there is some attenuation of hot earth and spillover into cold sky - sort of sidelobes working in reverse!

Assume a low power uplink results in a received S/N of 10dB (using a noise bandwidth appropriate to the signal of interest).  That S/N will be radiated by the transmitter and can never be bettered, whatever you do to the ground Rx system.    Only if you have a very small antenna or poor LNA wil there be significant further degradation.  

There are so many interdependent variables in a satellite up and downlink using a bent-pipe transponder that it is only really possible to get exact contributions from any particular part by putting together a spreadsheet containing every single parameter.  And unfortunately, for this system, we don't have all the satellite parameters to hand to do this properly.  We don't know transponder gain and we can only estimate Rx noise figure / temperature and the Rx and Tx antenna gains.
 
What makes life even more complicated is when receiving LARGE signals via the satellite.  If you have a very small ground station antenna (or poor noise figure) and can't see transponder noise  you'd still get them - but now your own Rx performance really is the limiting factor - which is actually my situation at the moment.
I did say it wasn't straightforward !
 
Goonhilly, with it's 1.3m dish is seeing a transponder noise floor something like 10dB above thermal, so any QRP operation via that is entirely dependent on uplink EIRP and coding efficiency.   SO the WebSDR there is an excellent receiver goto when testing QRP uplink operations.

I've rambled on a bit here, but hope you can see that for any typical ground station, unless the operators reduce the transponder gain significantly,  anyone with a recommended system of 60cm dish or more with good LNA should be seeing transponder noise.   Ground station LNA noise figure does matter, but certainly not in a one-for-one dB related way. 



On Sun, 17 Mar 2019 at 10:19, Ole OZ2OE <ole@...> wrote:
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




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