Offset Antenna Question

[Eastcoast]

I have noticed where I live that Star Choice subscribers have oval dishes that are tipped on an angle is that becaues they are picking up 2 satelleites and the antenna has to be skewed as if it were on a motor?If this is the case when you aim a fixed offset dish with a 24 deg offset at a satellite west of your true south you skew your lnbf but what happens with the 24 deg offset as it will still be plumb?

Dave

 

[PSB]

With a Starchoice, Dish 500 or Phase III dish the whole dish is skewed but on a fixed dish its just the LNBF, nothing needs adjusted with the offset on any of the above dish.

 

[Mr Tony]

I have noticed where I live that Star Choice subscribers have oval dishes that are tipped on an angle is that becaues they are picking up 2 satelleites and the antenna has to be skewed as if it were on a motor?

correct. It has to pick up 2 satellites (107.3 & 111.1) so it has to be skewed depending on your part of the country. Where I am in MN, the skew is something like 7 where in Halifax it would be much more (because I am farther west)

If this is the case when you aim a fixed offset dish with a 24 deg offset at a satellite west of your true south you skew your lnbf but what happens with the 24 deg offset as it will still be plumb?

Dave

yes it will. The 22 degree offset comes down,. bounces off dish and hits LNBF. It has to be skewed depending on part of country you are in and what satellite you are hitting. If its your Teue south, there is no skew. But if you are trying to get a far eastern satellite or western it has to majorly be skewed

 

[Eastcoast]

Yea I figured as much so I guess the offset angle remains the same for 40 or 50 deg either side of the dish and of course a circular lnbf doesn't care.

 

[Eastcoast]

I ask these questions as grasping at straws because I spent part of the day up on the roof where I have clear LOS to the western horizon.I was trying a 33in dish and a standard lnbf to see if I could see G10R, I had the receiver a little tv and a HP spectrum analyzer.
I thought I could see some signal but no luck, had no trouble to pick up Echo 9 at 121 but I guess its much more powerful.
I will probably give up on that idea.

Dave

 

[Larry1]

The offset dish will be slightly higher than it is wide. The view of the offset dish to the satellite (and therefore the signal), will be a round surface to reflect the signal. Also, if you view the dish from the point of the LNBF, the dish will appear round as well. If the LNBF were at the centre of the dish, like a prime focus dish, the dish would be round, and appear round to the LNBF. Because the LNBF is mounted at an angle to the dish, the height of the dish has to increase to make the dish appear to be round to the LNBF. The greater the offset angle of the LNBF, the greater the height of the dish has to increase to effectively be the same size dish, or appear to be the same size to the signal and LNBF. The important thing to remember is that to the signal, the dish appears to be round. It is because of this that the feed horn on the LNBF can be round, and that it is possible to rotate either the LNBF or the dish to adjust the skew.
Now, with an eliptical dish, such as with the StarChoice dish, the feedhorn is not round, but looks oval for the feedhorn looks at the entire dish. This is why you need to turn the dish and not the LNBF feedhorn. If you see some of the earlier StarChoice eliptical dish's they had a single LNBF (for only reception of the one satellite) with a odd looking feedhorn for the LNB had a view of the entire dish, and only the dish. With that dish you adjusted the skew by rotating the dish and not the feedhorn.
The offset of the dish has nothing to do with the skew. You set the skew by turning the LNBF or the dish itself. A 22 degree offset dish, or a 45 degree offset dish will still have the same skew setting.

 

[Mr Tony]

Dave if you can get Dish 121 G10 is only 2 degrees more west...you're close

 

[Eastcoast]

Yea Iceberg
121 is booming in and I did see some signal on the analyzer when I moved a little further west.
If I have a chance to pick it up at 5 deg then poor iafire will do it.

Dave