View Full Version : Obstacle Height
09-25-2010, 01:12 PM
According to Dishpointer, I have an obstacle that's the correct distance away (a tree) but it's nowhere near accurate on the height. It says the height is 38.1 metres or 125 feet. The tree is question is only about 50 feet in height. Could an error be occurring when the height is converted to metres. I'm also not sure why it would give details in metric although maybe it defaults to metric because I'm in Canada?
Your proceedure for adding the obstacle height & the antenna height doesn't seem to make sense. Would you not subtract the antenna height?
Is there a general rule of thumb to use for LOS? Like a satellite with an elevation of a certain number of degrees or above will most likely be received relevant to your position.
09-25-2010, 01:37 PM
It is not measuring the height of trees, buildings, etc... cannot do that from a 2D satellite image.
What it tells you though is that IF there is an obstacle at the distance d, then the obstacle must not be higher than h, measured from the base of the dish, otherwise it will interfere with the signal.
You will need to determine how hight the obstacle really is - which you did. You say the tree is 50ft in height. The website told you that the tree would be interfering with the signal if it is higher than 125 feet (if the dish is at ground level, higher if on roof level). So, there is no problem in your case.
And yes, the measurements are displayed in metric units except in the U.S.
A rule of thumb would always involve d and h, like 1 in 2, for a specific satellite and location. You can get this rule from the obstacle marker if you place it at say 1m away.
09-25-2010, 02:17 PM
Ah, I see. Thanks for that.
As for the general rule, what I meant was that I know we get signal for 83W & it's just above 30 degrees elevation (relative to my position). I just wanted to know if I would have a better probability of getting other sats to the west if they are also at a 30 degree elevation or above (not accounting for obstacles). I have noticed that the elevation seems to go down for the sats I want the further west I go.
09-27-2010, 07:17 AM
I changed the position of the dish to the backyard, selected obstacle, and moved the red balloon to just in front of the dish. It indicates an obstacle at: h=4.7m (15.5') h=3.6m (12'). If I understand you correctly, if my antenna is at ground level, this obstacle (another tree) would block the signal if it is 12 feet or higher and I would have to erect a mast of at least 12' (or what ever its' actual height is) for reception.
09-27-2010, 07:22 AM
Kind of correct. Yes, the tree would block the signal if its higher than 12 feet. The mast should be at least height of the tree minus 12 feet high. So, if the tree is 15 feet, the mast should be 3 feet high. If the tree is only 9 feet high, the equation would give a negative number which indicates that a mast is not required. Add to the mast height any possible growth of the tree.
09-27-2010, 08:42 AM
Thanks Alan. I think I finally got it. But....
I wonder why (at least in my location), Dishpointer doesn't recognize these as obstacles per say. If I select 83W, the first obstacle indicated is that tree across the street (shown in Post #1). It's only when I move the red balloon that the question of these other obstacles arise. I would have thought that Dishpointer would start at a distance of say 3' (however close you can get the mouse to move it in this 2D) .... Is it more of what's the closest object it can find in a 2D environment and this is the maximum height you have to consider?? ... if that's true then I guess Dishpointer isn't actually saying that these are obstacles ....
If I put the dish a North/East part of my back yard approx. 50' away from the house. The height of my roof is approx. 25' (described as a 1 & 1/2 story - a bungalow with an extra large roof - the upstairs occupying most of the roof) and attached garage of approx. 21' in height. All I can see above the point of the roof (garage) is about 4' of the top of a tree. This may or may not be in the way of any of these satellites. It is the rough LOS view from my back yard from the 83W to 103W sats.
I can't see any other obstacle from this point of view (perspective) not even the tree across the street. I'm trying to get this into my head with some sort of visual representation...
I wonder if I made a T (about 6' tall) with a pivoting cross piece. If I sighted down the pivoting arm, and measured the distance with a protractor, would that give me a relative LOS elevation? I could note if there are any obstacles seen and at what position they are magentic. I think 83W was something like 270W (magnetic) relative to me. And then compare these to what the relative position is to the other sats?
Am I making this too complicated??? Something tells me that Dishpointer is doing all this for me - but only to a certain point???
09-27-2010, 09:10 AM
Have you seen the DishPointer apps video to the right of the map? If you have an iPhone or an Android based phone, you need to get that app - everything will be obvious then.
As for the DishPointer website: It does not process the images. It does not know that it's house, a tree or how high these items on the image are. It will never be able to do that.
The obstacle marker is just placed 50m from the main dish icon. You need to move it around. The obstacle marker would also be placed on open see if you put the dish icon there.
That the first obstacle governs the height is not true. An obstacle behind it might be a lot higher and that one would block the signal. Though, the further the obstacles are, the higher they have to be to block the signal.
You can create all sort of tools to check for LoS issues, you just need to transfer the elevation angle on your tool.
The magnetic azimuth values and the elevation are given below the map. But you can use alignment line on the dishpointer map too.
Hope this helps.
09-27-2010, 09:24 AM
Doh! Thanks for that. It is Monday morning after all.....
P.S. I don't have any of those fancy IPhone or Android thingies .....