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Eb/No seasonal variations?

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Ex Member

Apr 20th, 2013 at 8:09pm  

Anybody who has been around this kind of stuff for a while could enlighten me: my signal this am shows an increase of well over a half dB overnight!!!!  This is averaged over a couple hours so far, as it is very jumpy.

My Hawaii Teleport is getting into spring: have seasons anything to do with it????

What else?

Thank you

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Oasis Networks
Ex Member

Reply #1 - Apr 21st, 2013 at 9:52am  
Hello Chris,

This can happen because of a few reasons, some reasons comes into my mind:
1. The dish is not pointed exactly to the center of box of the satellite, so your EbNo varies.

2. At night your LNB performs better because of lower temperature.

3. You have some humidity in your feed system which drains into water at night, which improve your performance.

4. Less likely, but especially at lower elevation and at special places around the globe, at certain seasons, you might be affected from scintillation. Try to take a look at this link for example.

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Ex Member

Reply #2 - Apr 22nd, 2013 at 8:34am  

Thanks Oasis, interesting thoughts.

1=My Ku EbNo went up abt 3/4 of a dB overnite, higher than ever! so I'll assume that part is man-made, my provider messing around with my brains, behind my back... Or could be a "box" or attitude coarse adjustment at the satellite? I'd hope they do those often enough they couldn't be noticed at the customer end?

2=We seem to loose perhaps 1/2 of a dB during the day, and couldn't that be caused by sun noise, just un-focused or whatever the right term is?

3=Now for the constant jumpiness of the Ku signal (perhaps less so at nite?) what could cause this? Usually + n - 0.3dB or so, but also with some sort of steps....  Like hanging higher for a few min, still moving, then stepping a bit down and hovering around lower. I usually watch and write down numbers for at least 10 min, and then do an eyeball average.

How do the scintillation scales translate into Eb dBs? I'm 2 North and 157WEST, on a daily basis? Anything to do with #3 above?

Thanks for the comments.

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Oasis Networks
Ex Member

Reply #3 - Apr 22nd, 2013 at 9:19am  
HI Chris,

As far as I know, scintillation will not affect Ku band, but maybe Im wrong.

What is your dish size and what satellite are you pointed at?

I would recommend you to check for condensation in the feedhorn window during the day as well.

Kind regards,
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USN - Retired
Ex Member

Reply #4 - Apr 22nd, 2013 at 1:12pm  
Rain causes a scintillation effect on Ku-band, the effect increasing with latitude. The real scintillation occurs at lower frequencies, and is caused by electrical activity in the ionosphere. So if it's not raining.......

You seem to be describing a cyclic phenomenon. If it's a consistent  12/24 hour cycle, it's likely a COB issue. It's not unusual when Az or El gets altered slightly when the dish is accidentally nudged too hard - or gets moved in a strong wind. Determine which satellite you're on, and consult the manufacturer website. Most have COB charts or graphs to tell you what time the satellite will cross the center of the box. That's the most opportune time to optimize your pointing angles.

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« Last Edit: Apr 23rd, 2013 at 10:40am by N/A »  
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Eric Johnston
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Posts: 2107
Reply #5 - Apr 22nd, 2013 at 4:46pm  
Pointing errors, with fixed dish, allow variations of signal level over the day to occur due to satellite movement.
Ideally you should peak up the dish pointing when the satellite is at the centre of the stationkeeping box.  This is  often not reliably practical as it may occur only once every few weeks or daily, depending on the east-west position which drifts slowly over many days. Contact the satellite operations center for advice.

The satellite movement in this case is an ellipse, gradually changing size and drifting sideways. If north-south station-keeping is ceased, the daily movements gradually become an elongated north-south pattern, as the inclination increases at the rate of 0.8 deg per annum, to form a classic figure of eight pattern.  Little fuel is needed to maintain east-west stationkeeping, but it is quite normal to allow satellites to use a full daily range of +/- 0.1 or +/-0.15 deg in all directions.

Typically a satellite might be launched with 'negative' 0.5 deg inclination, this being allowed to decrease to zero during the first few months, and, towards end of life, after say 15 years, the inclination will be allowed to increase above 0.15 deg, to say 3.2 deg over a further 4 years of 'inclined orbit' operation.  

Tropospheric scintillation occurs particularly at low elevation angles.
This example is Ku band tropospheric scintillation at 5.4 deg elevation, due to boundries between layers of moist and dry air in the lower atmosphere.  To minimise rain attenuation, rain noise and tropospheric scintillation effects, avoid C band operation below 5 deg, and Ku band operation below 10 deg elevation.

Ionospheric scintillation is noticed in the tropics and at low frequencies like L band and C band.  It is associated with sun-spot activity and peaks every 11 years.

Best regards, Eric.
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« Last Edit: Apr 22nd, 2013 at 10:26pm by Admin1 »  
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