# Math SAT of 800, Geometry and Distance to Clear Obstacle Question

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#### HDTVFanAtic

##### SatelliteGuys Pro
I wish I could remember who posted this website to calculate clearing obstacles, but unfortunately I cannot remember who it was:

http://www.satlex.de/en/elobst_calc-params.html?calc_by=by_e&d=4.75&h=2.0&e=21.3

I am trying to clear an obstacle and was looking at the numbers over the weekend when several questions came up.

1) Is the distance to the center of the dish or the lnb - as the diagram seems to leave it hanging at the lnb - when I would think it would be the center of the dish.

2) The diagram indicates an offset - would a prime focus be the same?

3) I am a little surprised dish size does not come into play with the calculation so the entire dish would have a view of the bird - it doesn't appear to on that page.

M

#### Mr Tony

##### SatelliteGuys Pro
Supporting Founder
2) The diagram indicates an offset - would a prime focus be the same?
Actually in some cases, a Prime Focus would need to be higher to clear the obstruction. When I was setting up my 6 foot dish, if I had it a foot off the deck, only 1/2 the signals would come in (one polarity). Converselly, a offset dish could get everything. My C-Band dish is 30" off the ground to get optimum signal on G4. My KU dish aimed at IA5 will work fine down to a foot off the deck floor.

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#### HDTVFanAtic

##### SatelliteGuys Pro
Actually in some cases, a Prime Focus would need to be higher to clear the obstruction. When I was setting up my 6 foot dish, if I had it a foot off the deck, only 1/2 the signals would come in (one polarity). Converselly, a offset dish could get everything. My C-Band dish is 30" off the ground to get optimum signal on G4. My KU dish aimed at IA5 will work fine down to a foot off the deck floor.

Well, therein lies the question I suppose....what does one use for the parameters

H

#### HDTVFanAtic

##### SatelliteGuys Pro
From the experts I deal with that do Commercial Satellite uplink and downlinks - they say in reality that polarity even comes into play:

Assuming the lower edge of the reflector is on the roof 2.5 L x 1 H= 22 degrees of elevation. If you need 6.5 feet of clearance you would need 16.25 Feet of Length.

This is know as Inogon Level / Run / Rise / Degree / Conversion

Now for reality. If this is a linear satellite and it is 18" off the base to the lower edge of the reflector you can subtract the 18" off the Height plus about 15% for polarity spread so you will only be clearing about 4.5 feet of Height obstacle or Length / Base will only need to be 11.25 Feet.

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#### HDTVFanAtic

##### SatelliteGuys Pro
I don't know how to work it, but here is another one you can try:
http://www.bjupress.com/services/bjhomesat/tech_support/obstacle.html
Maybe there's enough info on the site to teach you how it works?

Al

I seem to remember that someone posted that url as well about a year ago - and that the elevation on that page is for 97W from various cities around the USA - so you need to insert the elevation from your location of the bird you are trying to look at.

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