Mismatch in f/D Between Dish and Feedhorn Specs

[hops]

A recent survey of my back yard revealed that I can set up a motorized 8ft dish and see from about 22W to 121W uninhibited by trees and other clutter. I unsuccessfully searched for a used dish locally, as many that are available were damaged in a severe storm over the summer. Luckily, I'm about a 1 hour drive from Sadoun and noticed that they have a nice solid 240cm steel dish for sale, called the SD2480PM.

From what I've read here and elsewhere, orthomode feedhorns are the way to go, but I'm having a bit of trouble deciding between the Chaparral Bullseye II and the Chaparral Dual Feed feedhorns. The f/D of the Sadoun dish is reported to be 0.385 whereas the feedhorns support f/D ranges of [0.39,0.42] and [0.33,0.45], respectively. So, it appears that the focal length of the dish is about 1.2cm too short for the Bullseye feedhorn to operate optimally, meaning that (I think) the feedhorn would not "see" the entire surface area of the dish.

I did some searching and came across a thread where pendragon had a similar, but more significant mismatch in f/D requirements. He was able to modify the feedhorn assembly to improve reception. I'm no stranger to doing such modifications, but I'd rather not have to do that to a $300 feedhorn right out of the box. Assuming the SD2480PM I assemble is within spec, should I not be concerned about this difference in f/D and get a Bullseye or just play it safe and get the Dual Feed?

I should add that I have a 1m Primestar dish on an SG2100 elsewhere in the yard for Ku reception, but it's currently limited to seeing 61W to 127W. I'd like to pick up Ku on the 8 footer since it will see the Atlantic birds better, it's convenient, and I'd feed the output from it into a TBS 6925 (for playing with 16APSK and 32APSK feeds; the feasibility of doing so in C band from an 8ft dish is probably a topic for another thread) while running the Primestar off a Prof 8000.

 

[pendragon]

The f/D specs for the Bullseye are bogus. You will have no trouble adjusting it to 0.385. A single ortho feed will slightly outperform the Bullseye on C-band, and it's possible your 1m Primestar will beat the Bullseye on Ku-band. It may come down to a trade-off between Ku visibility and overall performance.

 

[Anole]

To see more of the arc, is there any chance you could locate a motorized Ku dish directly north of your prime BUD location?
That way, at true south, it would look over the rim of the BUD.
I don't think the BUD would impede the Ku anywhere else on the arc.

I'd prefer the Ku dish closer to the ground, which would mean placing it further north.
However, if you don't have much northerly room, what about putting the Ku dish higher in the air, on top of a BUD-like 3½ inch pole?

 

[hops]

The f/D specs for the Bullseye are bogus. You will have no trouble adjusting it to 0.385. A single ortho feed will slightly outperform the Bullseye on C-band, and it's possible your 1m Primestar will beat the Bullseye on Ku-band. It may come down to a trade-off between Ku visibility and overall performance.

Thanks for clearing that up, pendragon. I think I'll go with the Chaparral Dual Feed in order to maximize my C band reception, but now I know that I can use a Bullseye if needed.

 

[hops]

To see more of the arc, is there any chance you could locate a motorized Ku dish directly north of your prime BUD location?
That way, at true south, it would look over the rim of the BUD.
I don't think the BUD would impede the Ku anywhere else on the arc.

I'd prefer the Ku dish closer to the ground, which would mean placing it further north.
However, if you don't have much northerly room, what about putting the Ku dish higher in the air, on top of a BUD-like 3½ inch pole?

I hadn't thought of putting a Ku dish near the 8 footer but it would be possible. In the meantime there's some tree branches I can clear out that would help the existing motorized Ku dish see more to the east (exactly how much more...we'll see).

 

[FaT Air]

To see more of the arc, is there any chance you could locate a motorized Ku dish directly north of your prime BUD location?
That way, at true south, it would look over the rim of the BUD.

Put the Ku in front of the BUD and spacing requirement would be less as the Ku dish is a lot "less tall" than the BUD. I've got my fixed Ku's in front of the BUD. Even the 30 inch mounted to the lower lip of the BUD is looking over the 84e and 1.2m.

 

[Anole]

bingo !

Put the Ku in front of the BUD and spacing requirement would be less as the Ku dish is a lot "less tall" than the BUD.
I've got my fixed Ku's in front of the BUD.

Yea, this is potentially a much better idea! - :up
Just as long as you weren't putting the BUD up against and looking over a fence.

See? That's why we like to discuss things out in the open here on the forum, rather than asking questions in PMs.
There's always someone with a clever trick or three.!.