f/d ratio for Ku dish & what does it really mean.

[brentb636]

Well,
With some time on my hands, I thought I'd confuse myself, and I've succeeded.
I thought I would check out my lnbf alignment , and thought that I'd start by looking at the focal length for my 1.2M dish, and setting that right.
That pretty much can't happen.
The focal length is spec'd at 28.3 " and my lnb is out at 36" from the center of the dish( which is where it's pointing ). I suppose I COULD drill a new hold in the support arm 8" closer and mount the lnbf there, but it seems kinda drastic. I wonder if I have somehow become confused ( imagine that !).
Coincidentally, the distance from the base of the dish , at the lnb support arm to the face of the lnbf is about 28.3 " . Could THAT be where the magic focal length number in the specs came from ?

All the explanations that I've run across are for Prime Focus dishes, and they don't directly relate to offset dishes.
????

 

[phlatwound]

I've oft wondered about this, Brent, but hadn't asked the forum yet.

I have a future project where it may come into play, hopefully somebody has the answer.

 

[MikeinBaja]

As I understand it (and others I'm sure understand this MUCH better than I do) to compute this you use the following: (width X width) / (16 X depth). To get the depth secure a string tightly across the face of the dish and measure depth from the center. And (unlike an embarassed NASA) use either all standard or all metric measurements.

Where I recently went wrong on a project was with the LNB itself. Depending on the LNB, the actual location of the "eye" is recessed - so the measurement is down the throat to the "eye", not to the outside of the case.

This becomes a very large difference if you are using a C-band or combo LNB as the difference from the case to the "eye" is quite large.

Others - PLEASE feel free to correct me or add.

 

[brentb636]

I'm wondering if the formula is even valid for an offset dish. The advertised focal length on my 1.2m offset dish is 28.3 inches, while my lnbf probes are just about 10" beyond that .
Surely the dish engineering wouldn't be THAT screwed up .

 

[pendragon]

One can imagine an offset dish as just a slice out of a prime focus. Normally the bottom of an offset is equivalent to the center of a prime focus dish, so it is not a coincidence that measuring from the feed to the bottom of the dish is the focal length. As a general rule of thumb, the focal length should never be greater than the closest distance from the feed to the dish. It is possible for it to be less, but that is not terribly likely as this would require more dish material than having the focal length to the bottom. The feed is pointed to the geometric center of the dish as viewed from the feed, and the point of the intercept on the dish will definitely NOT be the focal length. It sounds like your feed is in the correct place.

 

[pendragon]

As I understand it (and others I'm sure understand this MUCH better than I do) to compute this you use the following: (width X width) / (16 X depth). To get the depth secure a string tightly across the face of the dish and measure depth from the center.

This calculation will work for a prime focus dish. It would also work for an offset dish, but you would have to measure the equivalent depth of the virtual equivalent prime focus dish. That could be a tricky measurement. The most accurate method I can think of would be to stretch a wire from bottom to top, measure the distance from the wire to the dish at some regular interval and fit the numbers to a parabola. I've done it and it yields very good results, but it is a bother to set up.

 

[melgarga]

One can imagine an offset dish as just a slice out of a prime focus. Normally the bottom of an offset is equivalent to the center of a prime focus dish, so it is not a coincidence that measuring from the feed to the bottom of the dish is the focal length. As a general rule of thumb, the focal length should never be greater than the closest distance from the feed to the dish. It is possible for it to be less, but that is not terribly likely as this would require more dish material than having the focal length to the bottom. The feed is pointed to the geometric center of the dish as viewed from the feed, and the point of the intercept on the dish will definitely NOT be the focal length. It sounds like your feed is in the correct place.

Cool! Now that's what I call an answer to a question.
So PD, if I take upper 1/2 of a 10' mesh establish the focal point with respect to the "geometric center of the dish as viewed from the feed" I should be able to put multiple Ku LNBs across the front of the 'peacock tail', offset and compensate in inclination, and get equal or greater performance than a 6' offset dish (surface area of 1/2 of a 10 dia (39.25) is larger than a 6' diameter dish (28.26))....Right?
And be able to catch a wider beamwidth, although 61.5 to 148 is probably pushing it....
Anole, was the question how many stationary dishes does it take to cover the arc?

 

[pendragon]

So PD, if I take upper 1/2 of a 10' mesh establish the focal point with respect to the "geometric center of the dish as viewed from the feed" I should be able to put multiple Ku LNBs across the front of the 'peacock tail', offset and compensate in inclination, and get equal or greater performance than a 6' offset dish (surface area of 1/2 of a 10 dia (39.25) is larger than a 6' diameter dish (28.26))....Right?

Sorry, the focal point will still be the only focal point. However you could saw the 10' mesh into several offset dishes and accomplish what you want by mounting each carcass on a separate pole Each would have to point in a different direction to hit a different bird. I suppose you could put them all on one pole at the correct elevation and have a rotation axis for each. This is getting sick...

 

[brentb636]

They should put this thread in the manual for the lnbf, so everybody could be as puzzled as I .

Thanks Guys , I was thinking that the lnb should be at the focal point, and that interestingly enough , is not the case.

 

[B.J.]

They should put this thread in the manual for the lnbf, so everybody could be as puzzled as I .

Thanks Guys , I was thinking that the lnb should be at the focal point, and that interestingly enough , is not the case.

I'm not sure how you came to that conclusion from the above discussion?
Assuming by lnb, you mean the entrance to the feedhorn, then it should be at the focal point. In your previous post, you said:

The advertised focal length on my 1.2m offset dish is 28.3 inches, while my lnbf probes are just about 10" beyond that .

and in your first post, I think you said that you were measuring this from the "center" of the dish. If it's an offset dish, you don't measure FL from the center of the dish, you measure it from the center of the paraboloid, which should be somewhere close to the bottom edge of the dish. I'll bet that if you measure from the bottom edge of the dish to the opening of your feedhorn, that it will be pretty close to the published FL.

I put together a little on line calculator to relate the position of the feed to the published FL and height of dish.... it's only approximate, but it's still useful. Let me see if I can find it, and I'll edit this post with the url.


EDIT: http://www.eskerridge.com/bj/sat/offsetang.htm

 

[brentb636]

You're right , BJ. My issue is that the only information that I could find online led me to Prime Focus dishes , and setting that up. Nowhere could I find any information that could tell me why my dish's focal point was advertised as 28.3" , yet the lnbf was 38" from the center of the dish ( using the Prime focus diagram ). The coincidental fact that the lnbf support arm is about 28.3" long led me to believe there might be a relationship, but Nowhere could I find an explanation, or that confirmation. [edit] Applying your formula tells me that I may be able to bring my lnbf about another 1" closer to the top of the dish. So I'll work on that and see if there is any improvement on my current situation.
Thanks again.

 

[melgarga]

Sorry, the focal point will still be the only focal point. However you could saw the 10' mesh into several offset dishes and accomplish what you want by mounting each carcass on a separate pole Each would have to point in a different direction to hit a different bird. I suppose you could put them all on one pole at the correct elevation and have a rotation axis for each. This is getting sick...

LOL.
Well, it was just one of those well if this and that then why not try this 'out there' trains of thought. When reading your explanation, I harked back to something I read years ago (in PS or maybe PE) about a low budget "dish" that consisted of a fixed reflector fasioned from screen mesh and the guy moved an LNA on a tripod around in front of it to change satellites. I would think the TI would make it pretty much unusable but who knows.
I've never really taken the time to seriously experiment with any PF multi LNB setups. I stay behind on the stuff I NEED to do much less the other stuff I'd like to do.....lol
Besides, if it were pheasable, I'm sure one of our resident 'mad scientist' would have already tried it....lol
Speaking of mad scientist types.....now where did that Anole guy get off to........lol