Problems with focal length

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steve515

SatelliteGuys Family
Original poster
After searching and reading different things here on satelliteguys, i decided to go down to the basement and measure my dish. It is currently in halves, so it is real easy for me to measure.

The diameter of my dish is 91 inches. The depth is 14 inches. I found and used a focal length forumula in this forum. I cam up with a focal length of 36.97 inches. Well, okay....Fine....

A co worker had this dish and gave it to me. I took some pics when i went to look at it the first time. I got a picture measuring the distance from the center of the dish to the feedhorn. It measured 32.750. I looked at the feedhorn and checked what the scaler was set on. It is set to 38.

The owner said they never had any issues and that they really liked having cband.

How can these measurement be different?

There is no way i can get 4 more inches from those struts. Surely i measure wrong, calculated wrong or something........

Can someone set me straight?

Pic attached

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• Focal Length.jpg
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i hope i don't get in trouble for c/p
but here goes
To calculate the focal distance, you need to measure the diameter (D) and the depth (d) of the dish. Measurements should be in like units (you can't use feet for the diameter and inches for depth). For this example, let's say we have a dish that is 120 inches in diameter (D) and 18 inches deep (d). Focal distance (f) equals the diameter squared (D x D) divided by 16 times the depth (16 x d) or:
D x D = 120 x 120 = 14400
16 x d = 16 x 18 = 288
D x D/16 x d = 14400/288 = 50
Therefore focal distance f = 50 inches
After you have calculated the focal distance (f), you can use that figure to calculate the f/D ratio of your dish. In this case, using the same diameter of (D) = 120; and the calculated focal distance (f) = 50
f / D = 50 / 120 = .416
f /D = .416
And round up to give a setting of .42.

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That is excactly what i did. And i still came up with a focal length of 36.97. When the dish was installed years ago, the feedhorn was set to about 32.750.

That is what i dont understand.

thanks

Your calculations seem correct, but I think your measurements are off. Your measurements would give an F/D of a bit above 0.40, not .38. I'm guessing that since the dish is in halfs, that it is sagging a bit, and the depth is smaller than you think. I tried a depth of 15" and that gives an F/D of closer to .38, and a FL of around 34". The sag of the dish would probably also cause the diameter to be off a bit.

Anyway, I'm guessing that when the dish is together, your measurements will be different.

my 10' sami mesh dish uses .38

Focal length calculations assume a reflector that's true parabolic. It's possible that when the dish is assembled and you find the "sweet spot", you my find that the calculated focal point in not correct. Use the calculation as a starting point only, always adjust it in, out, and sideways to find the "sweet spot". Any warpage or variance from a true parabolic will move the focal point (sweet spot).

Harold

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I am using the right forumula and the math is correct. Its just that my answer is several inches away from the dish then where the feedhorn was originally installed years ago. Could the dish be warped? Yeah, i guess it could be. But by measuring it....It isnt.

I am not installing right now, and wont until march or april. I am just getting some things together at this point. I am stuck on this focal length though. I just dont understand why the original feedhorn is in a different location than what i calculated.

thanks
steve

I just dont understand why the original feedhorn is in a different location than what i calculated.

Just because the feedhorn is mounted in this position and previous owner was receiving C-Band signals doesn't mean that the system was optimized. Signals can be received even if the feedhorn is mounted off center, crooked, incorrect skew or with an incorrect focal distance. The system will have better performance when the mechanicals are properly set.

I wouldn't be too concerned about this discrepancy at this point. It will be obvious if the feedhorn adjustment increases the SQ during the peaking of the installation.

I know that this thread is a bit old but I thought of a possible answer to your question, the effective aperture size that you input for the calculation was likely incorrect. If you measured 91 inches but the actual effective aperture size according to the manufacturer is a little less such as maybe 87 inches it will then calculate properly. The full diameter of the dish may not always be effective due to certain formed edges, etc.

I use the following and with your posted measurements I came up with the same length which contradicts the physical F/D as you noted.

Parabolic Reflector Analysis

I know that this thread is a bit old but I thought of a possible answer to your question, the effective aperture size that you input for the calculation was likely incorrect. If you measured 91 inches but the actual effective aperture size according to the manufacturer is a little less such as maybe 87 inches it will then calculate properly. The full diameter of the dish may not always be effective due to certain formed edges, etc.
.....
http://kb9mwr.dyndns.org/n9zia/parabolic.main.cgi

I see your point, and I agree that there are problems when taking measurements of dishes. But if you measure at a point inside the edges, then you need to also measure the depth of the dish consistent with where you measure the diameter. Ie the depth would be less too, so usually, it's not going to make much difference unless the edges are significantly different from where the surface would be. It's usually easier to measure the depth by sighting across the edges, even if the edges aren't part of the actual reflector surface, and if you do that, you sort of have to measure the diameter by estimating where the surface would be.
I can usually estimate the diameter at where the surface would be, but my biggest problem is measuring the depth. My dish has a flat circular plate at the center of the dish, ie the reflective surface doesn't continue at the center, so I also have to estimate where the surface would be when measuring the depth and when measuring the focal length.
But yes, you have to try to measure to both the diameter and depth either at the actual surface, or by estimating where the surface would be, and the likelihood of measurement errors causing the problems here are pretty high. However I still think that the biggest error is because the dish is in halves, that there is nothing holding the dish in it's proper shape, and it is probably sagging significantly. Tie the 2 halves together, and the measurements will be different, and I'm guessing the new measurements will fit the equations.

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