Prime Focus help!!

[trev1978]

Hi all,


I have a 1.85m prime focus dish. I bought it second hand 4 years ago- it was previously at a bookmakers shop. I have been using it to pull in the Nordic beam on 4.8’E. I always felt I wasn’t maximising the signal as I didn’t think that I got the uplift in signal that I should have when I upgraded from a 1.2m offset dish. My feeling was that I never truly found the true focal point of the dish.


I decided I would try to get to the bottom of it. I made a little device with a laser sight (see photograph- I know it is not a work of art but it does work!). I have fine-tuned this so that it throws the laser perpendicular to the surface the three screws are resting on (the three screws basically act like a tripod). I first drew a circle in the middle of the dish to match the spacing of the screws and placed the device. From this it would appear that my lnb is located too high.


This is where I am getting confused! I am correct in saying that a perpendicular point taken from any place on the dish should land on the focal point? I drew a larger circle on the dish close to the rim. My thinking was that if I placed two of the ‘tripod’ screws of the device on this radius line it should also throw a point at the focal point. However my lnb wold probably need to be two foot further out to pick up a point from here. Even just looking at the dish you would know that a perpendicular point from the surface wouldn’t be close to the existing lnb location. However the lnb legs wouldn’t be designed to go out this far.


Is my understanding of the reflection correct or have I got this terribly wrong?


Thanks Trev

 

[Captain Jack]

@RimaNTSS, this one is yours.

 

[davemurgtroyd]

Hi all,

This is where I am getting confused! I am correct in saying that a perpendicular point taken from any place on the dish should land on the focal point?
Thanks Trev

No the dish is parabolic NOT spherical. If it was spherical then that would be true.

 

[archive10]

This is where I am getting confused! I am correct in saying that a perpendicular point taken from any place on the dish should land on the focal point?

Yes you are wrong.

As dave points out, the dish is parabolic, meaning that it reflects the incoming beam onto a single point. Any point of the incoming beam is paralel to the main axis of the dish (as identified by your laser beam), and is reflected to where the LNB feed horn is.

So any point on your surface should bounce the incoming signal (pointing to the satellite) to the LNB, roughly speaking.

Nice idea with the laser pointer - your dish may indeed be slightly bent. But this does not mean that the dish does not more or less focus the incoming energy into the focal point (where the feedhorn is). It doesn'ẗ matter if it's exactly in the middle or slightly to one side - the results should be very similar.

The main thing to do to ensure your dish is still in its parabolic shape is to string it, meaning put strands of fishing line, or normal line if fishing line not available, across the face in four places, to check if the dish is warped in any way. The string should divide the dish into 8 parts, and should ideally be (almost) touching in the center of thé dish.

(The strings does not signify signal path, but purely is there to identify any warpage...)

 

[RimaNTSS]

Hi Trev!
First of all, congratulations on your efforts! Not so many people today like to do such a work.
As adviced by @st1 you check geometry of the dish by stringing it. After you make sure geometry is OK, Instead of adjusting your tripod to the surface of the dish and trying to make it perpendicular to the surface you better fix your laser instead of LNB and observe where beam is pointing. It should be pointing directly to the very center of the dish (lets call it point A). The focal point of the dish will be located on the line between point A and chosen satellite. Position if the focal point can be calculated here BTW, what kind of LNB are you using?
Good luck and enjoy your time.