LNB Performance vs Temperature

cyberham

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I know a mini-BUD user in Toronto who can receive the Bahamas mux. But I've never locked it here. Lyngsat shows CBS feeds on 89W on 3920 V. I no longer can receive BYU on 91W on 3925 V which always used to come in before my most recent LNBF tweaking. I may need to re-tweak my LNBF once again. It is tuned perfectly to receive everything else I receive with strong stable signals but no more BYU on C now.
 

Cham

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That CBS mux on 89W is 9/10 FEC, S2 8psk. Likely won't work on the 1.2m antenna... Maybe a 1.8m if very well tuned.
 

cyberham

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That CBS mux on 89W is 9/10 FEC, S2 8psk. Likely won't work on the 1.2m antenna... Maybe a 1.8m if very well tuned.
For sure, my receiver won't lock using a 1.2m. But I wondered if it might still cause adjacent satellite interference being almost on the same frequency/polarity. Although I can't lock a tp doesn't mean I'm not receiving some signal from it.

I'm deviating from topic. More likely than above issue is I need to reinstall my LNBF/scalar following some rules. I will start by installing scalar at 0.6 f/D. Then I will try to get the LNBF positioned as lose to the dish focus of 72 cm. This is from spec sheet for Fortec Star 1.2m. This could be a challenge since the LNBF holder may not allow both of above points. Then I will try optimizing signals across multiple frequencies, polarities, transponders. I think this procedure may improve results overall. I know if I can't get BYU on 91W then something is wrong.
 

a33

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I will start by installing scalar at 0.6 f/D. Then I will try to get the LNBF positioned as lose to the dish focus of 72 cm. This is from spec sheet for Fortec Star 1.2m.

Dish focal length 72 cm likely is the focal length for the mother (primefocus) parabola. Would be about the distance bottom of dish to focal point (depending on still unknown position of parabola vertex).
Needed f/D setting can be calculated from LNB viewing angle (f/D-value from an offset dish has a rather indirect relation to f/D setting of a feedhorn!).

Why not measure some more dish measures, and thus be able to calculate more precise parabolic specs?

Needed are exact height and width measure of the parabolic working area of the dish (assuming the dish has a flat face, and passes the string test).

And depth measure, very precise: easiest at the center
(but taken at another height of the dish is also possible, but the measureing height must then also be given (as height along dishface, measured from the bottom; or (better, I think) as straight-line-distance from bottom to measuring point of the dish surface) ).
Please do mention which of these measures you've taken.

Greetz,
A33
 

cyberham

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....Please do mention which of these measures you've taken.

Greetz,
A33
Thanks for the response. I know this is your field!

I was just out in the rain adjusting as I said I would. I didn't think those would work from previous hands-on testing. I was correct. Even in the rain, I can see results are terrible. You explained why.

The 72 cm is approximately the LNBF arm distance from dish edge to LNBF. The 0.6 cannot be set since the scalar falls off the mouth of the feedhorn before 0.6 can be reached.

Specifications for the dish face are 132 cm x 120 cm. I will confirm by measuring once rain stops. Offset angle of the dish according to specs is 24.62 degrees. The face is not new having survived a couple of falls but it has better performance on Ku when compared against a couple of commercial 1m dishes. I've done string tests in past and used "pass the basketball" and "shaken like a baby" procedures to correct dish shape as well as possible.

I will get the depth measurement at centre as soon as the rain allows.

Added: Measured depth is 11.3 cm. Measured height is 132.5 cm. Measured width is 120.5 cm.
 
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a33

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Offset angle of the dish according to specs is 24.62 degrees.

That would fit exactly to height 132.0 and width 120.0 cm! Is the x.0-precision right, indeed?

Depth please measured to half a mm precise, if possible.

Depth of 11.29 cm would give focal length = 72.0; topstring = 127.1 and bottomstring = 72.1.
f/D of the dish would be 0.600, indeed, but f/D-equivalent for the feedhorn would be (ouch!) 0.71 (equivalent to 77.6 degrees).


Greetz,
A33
 

cyberham

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That would fit exactly to height 132.0 and width 120.0 cm! Is the x.0-precision right, indeed?

Depth please measured to half a mm precise, if possible.

Depth of 11.29 cm would give focal length = 72.0; topstring = 127.1 and bottomstring = 72.1.
f/D of the dish would be 0.600, indeed, but f/D-equivalent for the feedhorn would be (ouch!) 0.71 (equivalent to 77.6 degrees).


Greetz,
A33
I measured more like 132.5 x 120.5 cm but it's hard alone to get these kind of measurements precisely. Half a mm measurement? No way. I would say my measuring error is + or - 1 mm for dish depth.

So this afternoon I repositioned the LNBF and scalar according to real time measurements of received signals at the dish. I monitored multiple tps on a couple of satellites. In the end, best performance resulted with the scalar near the mouth of the LNBF and the LNBF pushed to almost as far back as it can be positioned in its bracket. This places the mouth of the LNBF at about 87 mm from dish centre. Skew has been peaked checking a couple of satellites.

I am now getting very good results for satellites and tps received. I will likely leave everything as it is.
 
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a33

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.... but it's hard alone to get these kind of measurements precisely. Half a mm measurement? No way. I would say my measuring error is + or - 1 mm for dish depth.

I guess that when the dish is on the pole, the exact measuring is indeed more difficult than when the dish is lying on its back.

Trouble is, many people don't take dish measurements before putting the dish on the pole, but do that only after some problems arise.....

Greetz,
A33
 

cyberham

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Based on my measurements, can you calculate the focus position of LNBF from centre of dish? If so, would that be "1/4 inch inside feed horn" as is often stated? It would be fun to compare the calculation vs where I physically have it located now. Same for the conical scalar. The LNBF must be close to where it should be since last night in the clear cold, I received some of the strongest tps ever since I started playing with the mini-BUD.
 
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a33

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Based on my measurements, can you calculate the focus position of LNBF from centre of dish?
Yes, of course.

I measured more like 132.5 x 120.5 cm
That is without the rim, so only the parabolic surface?
And I need depth at the center (easiest), also without the height of the dish rim (!) .

All parabolic specs are then calculated: focal distance, focal position, G-spot (aiming point for the LNB, at half the illuminated angle), clearance from vertex, illumination angle (f/D-equivalent), etc etc.

If so, would that be "1/4 inch inside feed horn" as is often stated?
I usually take 1 cm inside, for Ku. But I believe its different for Ku and C, and for various forms of feedhorns?

I have another calculator, that calculates the distances 3.5 cm behind the focal point (along the line focus to G-spot), where the center of an LNBholder for a Ku LNBf should be about.

Greetz,
A33
 

a33

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So, it looks like the specs (offset angle, and focal length) that you already had, are not bad at all!
That would fit exactly to height 132.0 and width 120.0 cm! Is the x.0-precision right, indeed?
...

Depth of 11.29 cm would give focal length = 72.0; topstring = 127.1 and bottomstring = 72.1.

Here are the calculation results, and a picture with the indicated points:

Dish specs calaculation Dish cyberham 31oct2021.png


A33 Offset satellite dish specs calculator Picture of the relevant points.png


Greetz,
A33
 

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  • A33 Offset satellite dish specs calculator Picture of the relevant points.png
    A33 Offset satellite dish specs calculator Picture of the relevant points.png
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cyberham

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Interesting, for sure. I will file this information in my master file for my Fortec Star dish.

Just for fun, once the rain/wind storm ends, I will go out and try to measure dimensions as accurately as possible and compare against this data. For example, I can check top string and bottom string lengths. If I physically install the top and bottom strings, I can approximately measure the bisector line distance from the focus to the G-spot to see how close it is to 84.16 cm. These measurements are made difficult with the scalar in place. So there will be some guessing. I don't want to remove the scalar now since it is tuned well.

Your data shows f/D of mother prime focus dish as 0.29. I can compare this to where my scalar is installed since the scale is calibrated for a prime focus dish.
 

a33

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Your data shows f/D of mother prime focus dish as 0.29. I can compare this to where my scalar is installed since the scale is calibrated for a prime focus dish.

Don't know what you mean by this?
The f/D=0.29 would be the feedhorn setting for the dish with diameter 253 cm.
For your dish, with viewing angle 77 degrees, the f/D-equivalent (so f/D-setting) would be 0.71.

By the way. Aiming a bit above the G-spot, but below the deepest point, should also be OK, I think. Compromise between signal density, and least spill-over. But I never tested that.

Greetz,
A33
 

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