Upside down offset dish

7mdish

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I noted that some forum member installed offset dishes upside down. Could anyone explain what is the reason? Which advantage with such installation? I really have no ideas. Thanks.
 

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I'm so far north my dish was difficult to install as the mount wouldn't let it point low enough. It was easier to install upside down but it filled with snow. Ended up inverting the rear part of the mount.
 

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You asked similar questions some seven years ago.

 

7mdish

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Oh I really did not remember. Probably I made those questions when we scheduled another installation for a friend, then not completed. Now I'm asking some questions in these days because I'm planning a new system for myself scheduled in September or October, so I'm really more interesting in such details yet. Thanks.
 

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... I'm planning a new system for myself scheduled in September or October, so I'm really more interesting in such details yet. Thanks.


I thought your planned setup was for a small TV station or was that a separate question on behalf of your friend re NS4 from 10.0E?
 

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Well, I'm very interesting in satellite TV. I have friends working in a small TV station and we often work together (oh, it is only a great hobby for me), and I want satellite TV for myself, too. I do not know which dish they will install for 10 East yet. At my home I must replace a dish that was damaged in a recent storm, so I want to make a professional-like installation. I do not want little consumer dishes anymore, but I'm stuck in a few point. The first is prime focus vs offset dish. I really do not understand if the same polar mount with elevation correction can be used for both dishes, or if offset dishes need a specific mount. The second point is finding a motorised feed for the best LNB skew, because each dish has its own feed, and I'm afraid that a standard feed could cause signal loss (and it would be very expensive, too). So, this is the reason for which I posted many questions in this forum, it seems there are many advanced users here from which I can get very useful suggestions I believe. Thank you so much for all your advices.
 

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You can use the same polar mount if there is sufficient declination adjustment, or you make an additional bracket to add the required declination for the offset dish. However you may now have issue with the offset dish hitting the pole or the mount. Adding a spacer to move the dish away from the mount may overcome this issue.

As for skew, s-band showed you his motorised skew adjustment setup in your other thread:
 

7mdish

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Thank you Ozumo for your suggestion. Oh I did not evaluate the chance to hit the pole..... Yes you are right! So I continue to believe that prime focus dishes are the best antennas for a motorised system having no limitations to reach extreme positions in the Clarke belt.
Yes I saw S-band solution for LNB skewing, very interesting. Not sure if I'm able to build one, I will see.
Back to the dish, the unique company building prime focus dishes seems DH, but no less than 2.4m. Surely it is a very good dish but I must provide it from the USA with a very high shipment cost. Other manufacturers like Skyware or CPI have only offset dishes, prime focus are only for larger diameters. Thanks.
 

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Thank you Ozumo for your suggestion. Oh I did not evaluate the chance to hit the pole..... Yes you are right! So I continue to believe that prime focus dishes are the best antennas for a motorised system having no limitations to reach extreme positions in the Clarke belt.
Yes I saw S-band solution for LNB skewing, very interesting. Not sure if I'm able to build one, I will see.
Back to the dish, the unique company building prime focus dishes seems DH, but no less than 2.4m. Surely it is a very good dish but I must provide it from the USA with a very high shipment cost. Other manufacturers like Skyware or CPI have only offset dishes, prime focus are only for larger diameters. Thanks.


Buy a second user dish, if the parabolic shape is good it should perform almost as good as a brand new one and should be easier and cheaper to source.
A second user dish might actually perform better than a brand new one if the second user dish is from a good manufacturer and the brand new one is from a lesser quality manufacturer.

You are severely limiting your options by only considering a brand new DH prime focus dish in the size range you are looking for and could end up with large costs for transport and import fees from the USA.
 
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ozumo

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If the polar mount is appropriately designed there is no reason an offset dish cannot be used to cover the whole arc. For example the 'Big Polar' design.

Invacom has an LNB changer with optional motorised polarisation. It won't be cheap.
 

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Thank you Ozumo for your suggestion. Oh I did not evaluate the chance to hit the pole..... Yes you are right! So I continue to believe that prime focus dishes are the best antennas for a motorised system having no limitations to reach extreme positions in the Clarke belt.
Yes I saw S-band solution for LNB skewing, very interesting. Not sure if I'm able to build one, I will see.
Back to the dish, the unique company building prime focus dishes seems DH, but no less than 2.4m. Surely it is a very good dish but I must provide it from the USA with a very high shipment cost. Other manufacturers like Skyware or CPI have only offset dishes, prime focus are only for larger diameters. Thanks.


There is plenty of choice for a brand new prime focus dish available from China, have a look at the Suncom / Probecom range of dishes.
They are available with dual axis or triple axis motorisation options in a range of sizes.
Shipping from China might be cheaper than from the USA?

Here are a couple of pictures of a Suncom prime focus dish that I use in Australia.

.
Suncom p0.jpg Suncom p1.jpg
 

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If the polar mount is appropriately designed there is no reason an offset dish cannot be used to cover the whole arc. For example the 'Big Polar' design.

Invacom has an LNB changer with optional motorised polarisation. It won't be cheap.
Thank you again Ozumo for your very useful suggestion! Very important thread for me, you know. Reading this post I'm convincing that maybe my initial idea to use only prime focus dishes must be revised a little bit. I did not know this Global Invacom product, and I saw in the web site they have other motorised feeds. I know they are so expensive, but it could be a very good solution. I must evaluate all details. As I said in my previous thread, I could choose to purchase the complete system (dish and polar mount) sold by SSS. As that 2.4m (or 1.8m) dish is Raven branded, I think this motorised feed could be easily matched (Raven, Andrew, Channel Master, now Skyware/Global Invacom is exactly the same I believe, or am I wrong?). If so, I could get that complete system and change the feed and LNBs. My doubt: Global has a specific controller called APS700, but I would prefer to use good old RC2000a device. Do you think it is possible? I do not know how these feeds work, I hope they have no proprietary electronic protocol so only its own controller could be used.... Any idea? Thank you again.
 

7mdish

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If the polar mount is appropriately designed there is no reason an offset dish cannot be used to cover the whole arc.

Could you please explain better this topic? I never used large, motorised offset dishes. An offset dish has an elevation angle of about degrees lower compared to a prime focus one. So, for example, to get Hotbird satellite here in Milan, a prime focus dish must be pointed at an elevation of about 40 degrees, as an offset dish has an elevation of about 20 degrees or less. But to get Hispasat at 30 West, a prime focus has an elevation of less than 20 degrees (always in Milan) and an offset one would be pointed below the horizon! And going forward on the arc to the West (or East) the offset dish goes lower and lower, and when the prime focus dish reaches 0 degrees the offset would be at -20! How can a dish receive satellite signal below the horizon? I'm a little bit confused on this topic, could you please explain? Thanks.
 

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Well, I'm very interesting in satellite TV. I have friends working in a small TV station and we often work together (oh, it is only a great hobby for me), and I want satellite TV for myself, too. I do not know which dish they will install for 10 East yet. At my home I must replace a dish that was damaged in a recent storm, so I want to make a professional-like installation. I do not want little consumer dishes anymore, but I'm stuck in a few point. The first is prime focus vs offset dish. I really do not understand if the same polar mount with elevation correction can be used for both dishes, or if offset dishes need a specific mount. The second point is finding a motorised feed for the best LNB skew, because each dish has its own feed, and I'm afraid that a standard feed could cause signal loss (and it would be very expensive, too). So, this is the reason for which I posted many questions in this forum, it seems there are many advanced users here from which I can get very useful suggestions I believe. Thank you so much for all your advices.
Your friends at the small TV station should be able to answer many of the repeated questions posted.

You have as yet to mention a maximum budget which, if provided might save time and effort from valued members of this forum.
 

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Could you please explain better this topic? I never used large, motorised offset dishes. An offset dish has an elevation angle of about degrees lower compared to a prime focus one. So, for example, to get Hotbird satellite here in Milan, a prime focus dish must be pointed at an elevation of about 40 degrees, as an offset dish has an elevation of about 20 degrees or less. But to get Hispasat at 30 West, a prime focus has an elevation of less than 20 degrees (always in Milan) and an offset one would be pointed below the horizon! And going forward on the arc to the West (or East) the offset dish goes lower and lower, and when the prime focus dish reaches 0 degrees the offset would be at -20! How can a dish receive satellite signal below the horizon? I'm a little bit confused on this topic, could you please explain? Thanks.
Offset dishes reflect the signal down towards the LNB. For example the offset dish in this post receives signal from 68.5°E even though the the dish looks to be pointing down - the signal is coming from (just) above the horizon. The second picture shows a prime focus dish pointed at the same satellite:
As for the Invacom polarisation adjustment motors, you would have to consult with Invacom but I would assume you need their controller. The system s-band has that I linked to previously might be controllable with an RC2000a.
 

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The system s-band has that I linked to previously might be controllable with an RC2000a.
I'm not sure of the pulse width range of the RC2KA. Mine are the X variant which don't have pulse output. To get the full range from the servos you need ~250-2000us pulse width.

@7mdish Attached is a table of offset angles for GD/Prodelin dishes. The diagram shows the RF axis (i.e. where it is pointing)

If you want full H-H from an offset, the dish has to be a long way forward of the king post. It becomes very unbalanced at the extremes and needs heavy counterweights to reduce the load on the polar mount, motors and king post. See RimaNTSS post:
 

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a33

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How can a dish receive satellite signal below the horizon? I'm a little bit confused on this topic, could you please explain? Thanks.

So for a bit of terminology:
A (correctly mounted) dish is always pointing towards (the elevation angle of) the satellite. That is called the boresight, or boresight angle. It is along the symmetry axis of the parabolic form of the (offset or prime focus) dish.

The (flat) dish face of a PF dish is exactly perpendicular to that boresight angle.

The (flat) dish face of an offset dish differs exactly the offset angle to that 'perpendicular to the boresight angle' plane.

So the offset dish itself is still pointed towards the satellite, but the dish face is pointed lower (or higher, when you mount it upside down).


BTW. When the LNB is not mounted at the correct offset angle, you get suboptimal reception by the parabola, as then the offset angle of the LNB location doesn't match with the offset angle of the dish. To get reception, the dish then needs to be set at an angle that doesn't match the boresight; hence the suboptimal reception.


Greetz,
A33
 

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'Big Polar' design.

Has this got a link on the forum ?
 

ozumo

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'Big Polar' design.

Has this got a link on the forum ?
 

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Laminas 180 offset
88 est 63 west no problem
 

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