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Center of Box, AMC-8

Satellite dishes have been a part of radio station technical equipment for years. I am surprised at the number of broadcast engineers that do not consider center of box when aiming dishes. As dishes get larger and focal points get smaller, center of box aiming is not a nice thing to do, it is a necessary thing to do.  The latest generation of satellite receivers, (AKA XDS) have a somewhat less than lively RF front end, they require higher E/B than the previous generation Starguide receivers to stay locked.

For years, the majority of commercial radio networks were carried on AMC-8 or its predecessors living at 139° W.  On the east coast, particularly in the Northeast, that makes aiming points relatively low to the horizon, anywhere between 8-10° elevation.

3.2 meter comtech dish

3.2 meter COMTECH satellite dish

This all means that precise aiming the satellite receive dish is critical for satisfactory performance. SES Americom owns AMC-8 and thus they have a web page about all of there satellites and important operating information. SES Center of box for AMC-8 is available in one month blocks, which makes scheduling the aiming chore fairly easy.

Large satellite dish aiming diagram

Large satellite dish aiming diagram

I have always used a spectrum analyzer though a 3 dB splitter to look at the 950 MHz  LNB output.  This aiming setup allows the best combination of Azimuth/Elevation/polarization.  Using the satellite receiver to confirm and maintain signal lock, peak the wave form that the  receiver is locked to.  It is pretty crowded up there, so there will be lots of signals on the spectrum analyzer trace.

It is a pain in the rear end to lug all that equipment out to the satellite dish, especially if it is on the roof.  That is why it only need be done once; the right way the first time.

Any shortcuts will likely lead to those annoying chirps and drop outs or complete loss programming, particularly when the weather turns bad.

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2 comments to Center of Box, AMC-8

  • Paul, do you also have to check that the feed is at the focus of the dish?

  • Paul Thurst

    The dish manufacture usually sets the focus point of the dish by designing the feed horn mount. Since these are single band, e.g. C band or KU band dishes, there is not much variation. The use of a Chaparral feed horn makes any minor variations moot.

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