Dishes and obstructions in their field of capture

Digital ATV - The latest generation, cutting edge ATV - Please discuss it all here.
Forum rules
This forum is run by the BATC (British Amateur Television Club), it is service made freely available to all interested parties, please do not abuse this privilege.

Thank you
Post Reply
Basil
Posts: 71
Joined: Tue Sep 10, 2013 7:28 pm

Dishes and obstructions in their field of capture

Post by Basil » Sun Sep 12, 2021 8:43 pm

Oscar 100 dish. X meters diameter, looking at the satellite. How do I calculate if a structure such as a bungalow or workshop roof of height Y will degrade the signal? I am thinking in terms of how far away from the structure must a given diameter dish be for the structure to have negligible effect on the received or transmitted signal? I Googled but haven't found how to calculate this. Thanks. (I picked up a big Andrew dish from Northamptonshire today and want to see where it might go).

g0mjw
Posts: 1684
Joined: Sat Sep 20, 2014 9:15 am

Re: Dishes and obstructions in their field of capture

Post by g0mjw » Sun Sep 12, 2021 9:23 pm

Google should have yielded an answer.

You need Fresnel clearance but to a first approximation just make sure there is clearance from the base of the dish to the obstacle taking account of the elevation angle. Just do some simple trigonometry, h = d x tan(elevation). Typically in Southern UK it's going to mean the height of the building has to be quite a bit less than half the distance assuming the dish is on the ground.

You can also use the sun transit which happens twice a year if the sky is clear. The transit happens when the sun appears to pass directly behind the satellite - i.e. sun, satellite and dish are all aligned.


Mike

Edit - size of dish doesn't really matter, you don't want anything in the way. Just think of it like a searchlight. A laser level is a useful tool if you have one. My method involved a step ladder of known height a tape measure and sighting the local tree from about 1ft off the ground.

Basil
Posts: 71
Joined: Tue Sep 10, 2013 7:28 pm

Re: Dishes and obstructions in their field of capture

Post by Basil » Tue Sep 14, 2021 12:38 pm

Ok, thanks very much Mike, I may invest in a laser pointer, I assume mounted 90 degrees to the perpherary of the dish, top bottom and sides, it should ideally not shine on any local objects?

g0mjw
Posts: 1684
Joined: Sat Sep 20, 2014 9:15 am

Re: Dishes and obstructions in their field of capture

Post by g0mjw » Tue Sep 14, 2021 6:54 pm

Not quite. Pointing towards the satellite from the edges of the dish. That's not 90 degrees.

User avatar
keng8vdp
Posts: 111
Joined: Tue Mar 15, 2016 8:51 am

Re: Dishes and obstructions in their field of capture

Post by keng8vdp » Tue Sep 14, 2021 7:01 pm

Basil wrote:
Sun Sep 12, 2021 8:43 pm
Oscar 100 dish. X meters diameter, looking at the satellite. How do I calculate if a structure such as a bungalow or workshop roof of height Y will degrade the signal? I am thinking in terms of how far away from the structure must a given diameter dish be for the structure to have negligible effect on the received or transmitted signal? I Googled but haven't found how to calculate this. Thanks. (I picked up a big Andrew dish from Northamptonshire today and want to see where it might go).
Hi Basil,
If you are in the UK the satellite is roughly 26° up. ( If your in middle England) Get a stick , point it up towards the satellite (a bit east of south) at 26°. Look up the length of the stick like a rifle. Thats where the satellite is.
Or, download an Inclinometer app onto your phone. use that to find the angle.
You can get a chair or something outside. Prop a sweeping brush against it pointing UP towards the satellite. Put your phone on the brush handle to check the ANGLE its pointing up at and adjust the tilt until you get 26°. Look up the HANDLE to see if it clears any buildings.
NOTE: Your dish won't point up at that angle IF ... its an offset type.
Look at SKY dishes near to you. They are "roughly" pointing the right way. However they "appear" to point much lower towards the horizon. Thats because theyre mostly off-set types where the signal bounces off the dish at an angle & is reflected into the LNB.

Use this website .. move pointer to your location. The angles etc are then displayed on the left of the page.
https://eshail.batc.org.uk/point/

Good luck , have fun

Ken

G4EWJ
Posts: 1023
Joined: Wed Feb 17, 2010 10:11 am

Re: Dishes and obstructions in their field of capture

Post by G4EWJ » Wed Sep 15, 2021 2:25 pm

https://www.dishpointer.com/

is handy, as it has a draggable marker that shows how high off the ground the line of sight is, at any distance from the dish. Typically, the original rotatable dish to the right is partially obstructed to QO-100.

Brian
.
Image 673a.jpg
Image 673a.jpg (228.83 KiB) Viewed 104 times

Post Reply

Return to “DATV - Digital ATV”