How critical is a dish focal length

[patmos]

I refurbished a 12 ft C band dish and calculate its focal length to be 49.85 inches. After assembling all three LNB supporting struts to the "horse shoe", I find the distance from centre of dish to be 50.25 inches. In its original design there were 3 plastic screws( 1 inch) to adjust the focal point,i think. While installing it I removed all three screws because two were broken. I get a decent signal with it but I am not satisfied.
my question is(1) should the scalar ring be at the specified focal point( just as it is now) or(2) is it that the LNB probes should coincide with this point.
I am making my mind to thread the screws so that the LNB is more inside the dish. Will it make a difference by moving it inside by 3/4-1 inch?

 

[voomvoom]

Nice pictures patmos! I don't know the correct answer, but it's my observation from looking at the pictures that the Dish was made for it's time. Meaning, the Dish is designed for an adjustable feedhorn assembly. The old style assembly's could be adjusted inside the Scaler Ring to achieve the correct Focal Point. A modern LNBF with built-on scaler ring can not be adjusted like this, so if you are not satisfied, you will need to modify it. I would suggest trying to put some washers between the feedhorn and bracket it mounts to, to see if your signal will improve.
Actually, I would really suggest a feedhorn assembly that will adjust, but first try a work around for the LNBF you already have. Good Luck!

Al

 

[patmos]

The old style assembly's could be adjusted inside the Scaler Ring to achieve the correct Focal Point. A modern LNBF with built-on scaler ring can not be adjusted like this, so if you are not satisfied, you will need to modify it. I would suggest trying to put some washers between the feedhorn and bracket it mounts to, to see if your signal will improve.
Actually, I would really suggest a feedhorn assembly that will adjust, but first try a work around for the LNBF you already have. Good Luck!

Al

I don't know if you understand me. If you look at the first pic you'll see the original adjustable screws which are not present in the LNB pic. I've already made three new plastic screws which I intend to put on. The dish is old , may be 20 years or more but is not the scaler ring adjustment same for both old and new lnb. I believe it only adjust my f/d which is 0.35.This I think should be unchanged.
Your idea of putting washers will amount to the same result as threading the screws a bit more. f/d will remain same, only f will change. Thank you for sugestions.

 

[Inno]

I would have to say yes, it is important, that small amount of movement can improve your signal. Focal length is very very important as well as making sure the feedhorn is perfectly centred etc.

 

[gabshere]

ok i'm not the expert but i can see what Al is saying and i think you are saying the same thing. The scaler needs to be adjustable (a little bit ) to achive the correct focal length (from 50.25 inches down to 49.85 inches) to set the proper F/D 0.35 ( and this is measured 1/8 inch inside the feed on most feedhorn) so weather it be with washers to lower the scaler to proper focal length or longer screws they both get you there.
Also i can't tell from your dish but with the special collar mounted to hold the feedhorn ( and i can see that that end of the feedhorn arm are not adjustable) but where the arms mount to the dish are those slotted so you can make adjustments. On my dish each end of the feedhorn arm is slotted for slight adjustment . this adjustment allows you to push the feedhorn out further or closer to the dish ( on my dish movement of these arms might make a 2 inch difference) but each arm would have to be adjusted equally.

also i noticed that this looked like a mesh dish now covered with something is that correct, or was it a solid dish to begin with

 

[linuxman]

If you look closely at picture 1, you can see the little plastic screws sticking through the horse-shoe. Those fit onto the scalar ring to allow for movement in and out of the whole feed-horn assembly after the F/D ratio has been set to .35.

And yes, it is my opinion that the Focal length measured from the back plate to 1/8-1/4" inside the mouth of the wave-guide can make a huge difference in signal strength.

Fred

 

[patmos]

Thank you guys. I'll adjust the screws and report back.

also i noticed that this looked like a mesh dish now covered with something is that correct, or was it a solid dish to begin with

Yes it's true.I got the dish without mesh and I put thin alu sheet (those used in printing). In fact I copied it from a friend who used the same material.

And yes, it is my opinion that the Focal length measured from the back plate to 1/8-1/4" inside the mouth of the wave-guide can make a huge difference in signal strength.

So focal length is around 1/4" inside ( from the mouth of the guide). So it is not where the probes are ? Is it not true that the waves has to be concentrated to a point. And I said to myself that it must be near the probes.
Don t be annoyed with me I like to discuss technical matters.

 

[gpat]

Well on C band analog its not a big deal.
Just for laughs I set a small TV up next to my dish
used the Ku side of coax to feed the signal back to the dish
and its an extreme challenge for that as its about 200' each way.
I took the feedhorn loose in the scalar ring and began to move it in and out.
I pushed it all the way in until it bottomed out,still a good signal.
Then I began pulling it out and got it all the way out of the scalar ring and still had a signal.
The only way it would lose signal was to pull it out about an inch past the scalar ring or to move it about 50% off center with the ring partially blocking it.
Just can beat those old analog C bands.
I didnt try this on Ku or C band digital tho.

 

[linuxman]

So focal length is around 1/4" inside ( from the mouth of the guide). So it is not where the probes are ? Is it not true that the waves has to be concentrated to a point. And I said to myself that it must be near the probes.

It can be from 1/8" to 1/4", it varies depending on the wave-guide.

The waves do concentrate to a point, and as Caddata describes it as an onion with about 1" which is the size of a small onion. You will hit best signal in the middle of the onion, and will lose signal if too far in front, (not concentrated enough) or too far behind the onion (signal starts to spread back out).

Fred

 

[patmos]

Merci. Already remove everything. Three plastic screws threaded. wait tomorrow to get exact length metallic screw. Night has fallen here and mosquitoes have risen

 

[bubby-joe50]

Setting focal point

This may have nothing to do with anything, my newly received Orbitron manual places the focal point for my SX-12 dish at 50 and 1/8th inch, measured from the dish surface to the scaler ring face and F/D ratio of .36. The dish formula places the focal point of this same dish at 49.75 inch, with the lnbf placed at .36 f/d ratio, the 49.75 inch focal point comes to within manual specs, coming 1/8 inch inside the LNBF throat. Hope this helps.

 

[patmos]

plastic screws threaded 3/4 inch. f/d on scaler ring adjusted to around 0.37. I get some 6% quality improvement on a a known transponder.I will do further testing. May be with depth of screw. I discovered something that needs adjust. A satellite half way on the left needed a lower elevation to maximize whereas another one to the right ( of true north) needed a bit raising. I guess I am not on a perfect arc. Thank you all for help.

 

[bubby-joe50]

Your true south setting is off very slightly, 1/16 inch of pipe diameter is a very large move. Very little steps to fine tune. Best tune for me was to align the true south satellite and tune in a Ku digital channel at .3 degree's off of true south and set dish at .3 degrees off with actuator. My Clark belt align gives very strong Ku signals at center and far west as well as far east. Lots of messing around to finally get it set but worth it in the end. C-band shows 100 across the belt on analog signals.

 

[patmos]

Thank you I'll move it slightly.