Dish illumination

[2cvbloke]

I vote for the 8 leds

It is soon X-mas so use red ones

Red & Blue for me...

 

[koansrc]

So in order to actually physically see this "illumination", would there be any point in attempting to actually shine light thru the feed and onto the dish ? ]

Well folks I actually went ahead and tried it (it was a slow kinda Sunday). I think it works! Well sort of. I removed the lnb and left just the feedhorn attached to the dish. I then got a flash light and centered it on the back the feedhorn and guess what ... a nice circle of light appeared centered on the dish. It didn't show me how much of the dish is illuminated but it did show whether the feedhorn was centered.

 

[skull]

now u r using ur brain bro but even if the feed horn is not pointedtoward centre u can get a signal by moving the dish.

thnx

 

[koansrc]

now u r using ur brain bro but even if the feed horn is not pointedtoward centre u can get a signal by moving the dish.

thnx

I believe the point is to get it centered, so assuming the focal distance is correct, you are "illuminating" = using the entire surface of the dish. If it's off center, then your lnb will be looking over the edge on one side and not all the way to the edge on the other, which in my mind is less signal and more noise.

 

[koansrc]

This may help you out Koansrc.

-http://gjullien.fr/mirrors/mirrors.htm

cheers crossbones, I think the mirror trick is one way to check the dish geometry and see if it's bouncing evrything back at the lnb and therefore checking if you've got the focal distance right. I think it should work and give some good indications if something is wrong
My problem ( and that's why i started this silly thread ) is that i now realize that it is very difficult to work with a 3m dish. I simply can't get to the upper half and tie strings and stick mirrors etc. , so I was looking for a way from working only from the lnb.

Now, about the LEDs and the colour scheme. How about a christmas tree light rope, all the way around the perimeter which will rapidly flash in all sorts of patterns and have it's own 4-note xmas carol player feeding of the 18v.

 

[TJExcalibur]

I suppose those that want to tinker with their feedhorns in the dark and put LED's around it must have an unusual way with s_x. Somehow I don't think it will turn the missus on but I'm game for anything.

I believe the point is to get it centered, so assuming the focal distance is correct, you are "illuminating" = using the entire surface of the dish. If it's off center, then your lnb will be looking over the edge on one side and not all the way to the edge on the other, which in my mind is less signal and more noise.

 

[Steamgauge]

Hi guys just thought that you might like to know, I've been doing a lot of web searches recently on a related subject, Offset dish focus calculations, but during the searches I came across a paper from some leaned microwave organization that concluded that aiming the feedhorn/LNB directly at the center of an offset dish, or not (within reason that is) , doesn't actually make a great deal of difference to the signal strength and quality.

 

[Steamgauge]

Guys can anyone shed any light (sorry about the pun- not intended) on the thread "Feedhorn Illuminaton Angles" by Steamgauge

 

[koansrc]

aiming the feedhorn/LNB directly at the center of an offset dish, or not (within reason that is) , doesn't actually make a great deal of difference to the signal strength and quality.

I suppose it depends on how desperately you need every last bit of signal as it must make "some" difference. On my 3m prime focus dish I find that signal level drops when the lnb is moved off center by just a few degrees. However moving the hole dish just a fraction of those degrees drops the signal signifficantly more.

 

[Steamgauge]

Hi Koansrc - The report was only intended for offset dishes. As you say Prime Focus antennas are very sensitive to getting the centre point right. Steamgauge