Getting accurate dish diameter

[cracklincrotch]

To calculate dish diameter, do I take the measurement from the outside to outside of the rim of the dish or the inside edge where the parabola starts?

In the photo, would I measure from the top edge of the rim or the inside edge? The outside rim of the dish does not form part of the parabola so I wouldn't think I'd measure outside to outside, but inside to inside.

On this dish it means moving the feed a bit more than an inch toward the dish if it's inside-to-inside.

 

[Anole]

one lizard's opinion:

If it ain't part of the parabola, then it's part of the handle. -

Here's a close up shot of the rim of my dish, where it flairs over and turns back toward the ground.
It ain't reflectin' no signal to the LNB! -
http://www.satelliteguys.us/attachment.php?attachmentid=34623&d=1235890534
Here's a long shot where you can see the rim better:
http://www.satelliteguys.us/attachment.php?attachmentid=34868&d=1236536884

 

[cracklincrotch]

Cool. That's what I thought. This dish was probably working as good as my 7.5 with the LNB where it is then. The feed should be a 7/8" to 1-1/8" inch closer to the dish.

Found out today that the 12 footer is really 10.5. I never measured it. I just took the guy's word for it, that it was about 12' and it was big. He was right. Nobody has ever disagreed.

Here's a pic of it laying down. Still need to get the pole up though.

 

[cracklincrotch]

The dish depth is 22". The diameter is 126". So, F = D^2/16d = 126^2/16x22 = 15876 / 352 = 45.10", F/D = 45.10/126 = 0.3579, or 0.36. Right now the dish is set for an F of 46-1/4" which is how I got it.

 

[pendragon]

A calculated focal length should be considered only a starting point for adjustment. In fact many C-band feeds will perform best if the focal point is slightly within the feed, say 1/8-1/4".

 

[cracklincrotch]

I understand that the focal point should be just inside the throat of the feed. The feed that came with the dish is set a bit more than an inch back of the calculated focal point based on the parabola diameter. So, the current feed is probably in the wrong spot.

I'm taking the polarotor feed off and replacing it with the CalAmp LNBF from my 7.5 when I get the new pole in the ground. I'll be setting the feed to the calculated location based on the parabola diameter and go from there. Just didn't want to have it start an inch out and scratching my head about why it had to move in so much closer to the dish to get a decent signal.

 

[Skyv1]

Just curious, are you measuring the focal length to the scalar or to the feed-horn?
If the scalar is at the proper(by calculated value) length, wouldn't the feed-horn, set at an f/d ratio of .3579 put the feed-horn about where it is now?

 

[cracklincrotch]

My understanding is you set the f/d ratio by inserting the feed into the scalar and lock it down at proper mark on the feed. Then the scalar/feed assembly is adjusted in or out from the dish to satisfy the focal point being just inside the throat of the feed.

If I'm wrong (and it happens) I'm sure (and hoping) to be corrected.

 

[Lak7]

My understanding is you set the f/d ratio by inserting the feed into the scalar and lock it down at proper mark on the feed.

That's the proper way to do it, because the Scalar and Feedhorn act as 1 unit.