Focal length on FTA 85cm dish.

[chadg2]

I have a Samsonic 85cm ku dish with a .2 lnbf. Never knowing what the focal length was on this dish I just set it with the lnbf in the middle of the 40mm holder. I found the specs for this dish and low and be hold the focal distance in not possible to achieve I can only get within 4" of the correct focal length. Have you seen this before and what should I do? Thanks, Chad

 

[Wescopc]

If you were off 4" the dish would not work, I say the specs have a typo or are intended for another dish. Samsonic has had quality control problems in the past. I once installed one where the elevation marks were off by 11 degrees!
Bob

 

[chadg2]

That's what I was thinking. Isn't there a formula to figure it out using the measurements of the reflector?

 

[Larry Allen]

I have a Samsonic 85cm ku dish with a .2 lnbf. Never knowing what the focal length was on this dish I just set it with the lnbf in the middle of the 40mm holder. I found the specs for this dish and low and be hold the focal distance in not possible to achieve I can only get within 4" of the correct focal length. Have you seen this before and what should I do? Thanks, Chad

Determining the focal length of a parabolic dish

 

[chadg2]

Thanks I got it from here.

 

[FaT Air]

That: Determining the focal length of a parabolic dish , only works for a prime focus dish. Not for Offset dishes. What FL do you have? I've found a spec for: Description
Samsonic 36" Ku Dish (85Cm)
Offset Angle 24.62
Diameter 85*93.5
Focus Length (mm) 510mm ((51Cm)works out to be 20.0787 inches)
Ku-Band Gain @12.5GHz 39.44dB
Elevation Angle Range 20-60/23-90
Apeture Efficiency 75%
F/D Ratio 0.6
I agree, if you were truly off by 4 inches, don't think you'd get anything. Is that what's happening. I.E. No signal? If you are getting some TP's, adjust it in or out for max Q.
EDIT: Found this PDF (attached) a while ago, wish I could give them credit, but don't know where I found it.

 

[chadg2]

These are the exact specs I found for my dish and all the measurements are correct for my dish except the focal length. From the center of my satellite dish straight out just to my feedhorn end is exactly 24" I can loosen my lnbf clamp and push my lnbf in towards the dish, but at most only 1 more inch or so. That would leave me somewhere close to about 22 3/4inches, not possible to get to 20.07 inches. I wonder if the 510mm could be a mistake? It's the only measurement on these specs that are not for my offset dish. Thanks, Chad

 

[FaT Air]

From the center of my satellite dish straight out just to my feedhorn end is exactly 24"

What is the measurement from the bottom lip? I think that's where you measure the focal length on an offset, as an offset is just a portion of a prime focus. (or close to the lip, depending on design)

focal length= xx.xalso the bottom string length)

Located in the appendix of attached pdf

 

[chadg2]

OK now we are getting somewhere! I measured and I'm very close to 20" from the bottom lip of the dish, but I think I should measure 1/4 inch inside the feed throat or something like that. I remember this from installing my c-band dish. Do you know if this pertains to the offsets or not? The 1/4" inside the end of the feed I mean. I suppose thats where the probes are located. Thanks.

 

[FaT Air]

Should be close enough to try get it on a sat. Then move the LNBF in or out for maximum Q. 1/4 inch is at C band freqs, with Ku, it's probably 1/16. & the Ku has a plastic cover, how far under that is the lip of the scalar??

 

[chadg2]

just behind the cap it starts, I have all the sat's stored. I'm just tweeking now for best picture. Only sats with problems are Galaxy 16k and AMC 21k. Q# are very low on G16k and on AMC 21k I have to bump dish west for some and east for the others.

 

[FaT Air]

Same here.