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Permalink Reply by Biggles on November 3, 2016 at 21:44

Because the LNB is not situated at the focal point of the Dish. It is offset by about 20 degrees and so doesn't point directly at the Satellite. If you spot some of the larger dishes around Town ( C Band, not Ku Band) with the LNB at the focal point you'll see they have a different elevation and are pointing directly at their appropriate satellites. Having an offset (or also elliptical) Dish is also why it's possible to mount several LNBs on one Dish aligned to different Satellites.

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Permalink Reply by Simon Gilmour on November 6, 2016 at 19:24

Ah thanks. That site is potentially very dangrous, then, if you don't understand what you are doing.

I susect that my problem may actually be that the LNB is not pointing directly at the centre of the dish. It should be, right?

Another problem I had while up measuring the dish is that I couldn't get an accurate azumith. I put a straight piece of wood across the dish and tried to measure the orientation with an app on my phone, but it measured different angles depending on whether I measured in the middle, or the left, or the right side of the dish. I flipped the wood but had same measurements. With the dish being made of metal I imagine it nterferes with any magnetic field...?

thanks,

Simon

Biggles said:

Because the LNB is not situated at the focal point of the Dish. It is offset by about 20 degrees and so doesn't point directly at the Satellite. If you spot some of the larger dishes around Town ( C Band, not Ku Band) with the LNB at the focal point you'll see they have a different elevation and are pointing directly at their appropriate satellites. Having an offset (or also elliptical) Dish is also why it's possible to mount several LNBs on one Dish aligned to different Satellites.

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Permalink Reply by Biggles on November 6, 2016 at 21:47

I susect that my problem may actually be that the LNB is not pointing directly at the centre of the dish. It should be, right?

Yes, but the mounting arm is pretty robust so it shouldn't shift off centre very easily.

Another problem I had while up measuring the dish is that I couldn't get an accurate azumith. I put a straight piece of wood across the dish and tried to measure the orientation with an app on my phone, but it measured different angles depending on whether I measured in the middle, or the left, or the right side of the dish. I flipped the wood but had same measurements. With the dish being made of metal I imagine it nterferes with any magnetic field...?

That's why installers use a good quality satellite finder signal meter. A compass will give a rough initial direction which can then be fine tuned by peaking the satellite signal on a signal meter while adjusting both Azimuth and Elevation. Alignment needs to be very precise for highest signal strength. Phone Apps are useless for accuracy and are also affected by external interference as they use a magnetometer chip in the phone which is sensitive to light, air pressure and nearby objects.

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Permalink Reply by Simon Gilmour on November 8, 2016 at 18:21

Thanks. About the arm. I've noticed that the other dishes in my neighbourhood, the arm comes from underneath the dish and then a straight arm comes up, but mine has an arm that appears to come through the dish and then bends(!) about 10cm before the LNB. I'm not sure whether the arm was designed like that or if an installer did it, but it certainly looks like it was supposed to be straight and was then bent. but it looks professionally done - what I mean is, the bend is not damage. Thanks. (I can't get a decent image from the ground and I don't have a ladder at the moment, sorrry)

Biggles said:

Yes, but the mounting arm is pretty robust so it shouldn't shift off centre very easily.

 

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Permalink Reply by willis on November 10, 2016 at 1:44

where the arm comes through the dish is known as a feedthru dish

as for angles there are different marking on each side

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