MSG-1 Dish alignment and wind (Was solar outage)


a_van_belle
 

David wrote:
Interestingly, during wind and rain, when I have missed segments,
the indicators have not dropped anything like that low, suggesting
perhaps that the indicators average over too long a period to show
problems, and that momentary drops due to wind (moving the dish) may
be quite important.

My findings:
The Signal Quality readout is a 1-second average value that does not detect
short signal drops.
As the name implies it does not show signal level but the quality of the
received signal !
When I put an additional line-amplifier in, the readout remains the same
and during a solar outage the readout dropped where as my Satellite signal
meter showed and increase.

I did some calculations based on receiving Hotbird with an 88 cm dish.
This dish has a "half power" beam-width of only 2 degrees at 11096 MHz. So
if the dish moves in windy conditions, a deflection of 1 degree left and 1
degree right (same applies for up and down movement) could already cause a
loss of half of the signal (-3dB)

When the left rim is pushed back 7mm (and the right rim comes forward 7mm)
you have a 1degree misalignment!
If your mounting brackets are not that stiff (like the one from Robert
Moore) they could bend too much in the wind.
As Robert discovered the dish will start to oscillate and the signal drops
and rises too fast to be detected on the Signal Quality readout.
I have not tried this but I guess you can detect the signal drops using a
Satellite signal meter because this does not average.

But now an interesting case: Suppose the dish is already 0.5 degree
misaligned, normally this will only give a small signal reduction but now
you only have an allowable deflection of 3.5 mm in one direction ! That
really requires a very stiff dish mounting !

I welded a wall mounting myself because I was not satisfied with the
commercially available mounts, they are too long and the bolts fixing the
mounting to the wall are placed very close to each other.
I placed the dish as close to the wall as possible to keep the mount as
short as possible.
Take a look at the file I uploaded to the Files area.
80cmMSGdish.jpg (288 kb)

So if you suffer from missing segments at high winds, watch for movement of
the dish..
Don't mount the dish on a high or unprotected spot if there is no need.
Have the dish re-aligned with a Satellite signal meter because
wind-movement will aggravate any error.
If necessary add additional bracing(s) to a fixed point to prevent the dish
from oscillating in high winds.

Compared to receiving Meteosat 7 on 1691 MHz we need a more accurate
alignment and a stiffer mount.
There is not much difference between receiving digital TV satellites and
MSG over DVB, but every short "freeze" on a TV satellite set will result in
a lost segment on MSG !

Greetings,
Arne van Belle