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Normal Topic Ku at 6.7 deg elev, AND from an EQUATORIAL location.... (Read 856 times)
chriswlan
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Ku at 6.7 deg elev, AND from an EQUATORIAL location....
7th Aug, 2017 at 7:47pm
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Hello all: looking for a scntillation guru!

I understand that very low elev is a trouble to begin with, but I can't find any research paper about having this compounded with a VSAT located at a 2deg Latitude? How much of a worse case scenario is this: it maximizes the length of "boiling tropical atmosphere" that Ku got to go through on its way to a small island! Is scintillation worse with antenna overlooking hot land, or when shooting from the beach over the ocean?!?!?

I've been watching this for a while now, and there is deep fades of course and they seem to have components from seconds to minutes to daily.... I'm guessing at least one dB/sec for 2-6 dBs! Might be hard for ModCod to follow? And a few fades are just too deep.

First question would be:
Assuming the incoming wave diffraction constantly varies also, how much does that contribute to the scintillation fades?

Stated another way: going to bigger and bigger reflectors is there a point of decreasing return, where the varying APPARENT arrival elevation gets too far outside the fixed antenna very narrow main lobe center.

Perhaps kinda a "Center of Box" issue, compounded by a time scale in seconds. Anybody ever tried multiple antennas for "elevation diversity"???

Chris
  
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Nimrod Kapon - Oasis Networks
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Re: Ku at 6.7 deg elev, AND from an EQUATORIAL location....
Reply #1 - 8th Aug, 2017 at 12:03pm
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Hi,

elevation of ~7 degrees on Ku, in a tropic region doesnt sounds like a joy.

About scintillation, I remember I saw once a nice diagram of the scintillation belt around the globe, some areas around the equator are affected more than others. Try to google for scintillation belt maybe you can find it.

Good luck!
Nimrod
  
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Re: Ku at 6.7 deg elev, AND from an EQUATORIAL location....
Reply #2 - 9th Aug, 2017 at 2:48pm
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Based on experience in the UK, I don't recommend Ku band operation at below 10 deg elevation. You will get problems both due to scintillation and rain. In tropical areas, I would expect scintillation to be worse as the warmer air can hold more water and thus higher refractive index differences between dry and moist layers in the atmosphere.

If you do want to try at between 5 and 10 deg elevation, then extra large link budget margins are my first suggestion.  Use an excessively large receive dish and pay for more satellite power towards the hub.

Fast open-loop Up Path Power Control (UPPC), based on downlink measurements, can be added to your return link to the hub so as to avoid extra satellite power cost. Ideally you need a beacon receiver and UPPC controller. Be aware of the risk of sending excessive signals to the satellite if your LNB or LNB cable develops a fault. Specify for up to 6 dB/sec and use a smoothly varying transmit gain control. To successfully receive a beacon you will need a PLL type LNB.

Scintillation variations on the 11 GHz downlink will not be exactly the same as on the 14 GHz uplink so applying fast UPPC will not be 100% cure.  Try getting the hub end to make measurements with and without the UPPC applied. I would be interested in the results.

If you are sharing a giant DVB-S2 download carrier, the adaptive closed-loop variable Modulation and Coding (modcod) feature of DVB-S2 RCS is appropriate for slow rain fades, but is not quick enough to deal with scintillation. Conceivably you could ask the hub to put you permanently in their most robust modcod frame option. 

Larger dishes, say 1.8m dia or greater, are beneficial as they provide a degree of 'aperture averaging'. Very large dishes, greater then 10m dia, have problems with reductions due to angle of arrival changes.

Height diversity with two antennas has been used on terrestrial point to point radio relay links.  I don't know how they combine or choose the best signal. Greater than 25m vertical spacing is suggested. 15m vertical spacing is not enough. 
« Last Edit: 10th Aug, 2017 at 7:45am by Admin1 »  

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