| T.R | Title | User | Personal
Name | Date | Lines |
|---|
| 1689.1 | By construction. | CADSYS::COOPER | Topher Cooper | Wed Nov 04 1992 13:05 | 14 |
| Here's a "construction" (rather than measurement) method that occurs
to me.
Find the center of the dish. Use a piece of clay and a thin dowel
to mark the perpendicular to that point. Now take a point on the
rim of the dish. Find the angle-of-incidence = angle-of-reflection
relative to the normal, for an incoming beam parallel to your first
point. Extend the reflection until it intersects with the dowel.
If the dish were perfectly formed and all your operations were exact
then that would be the focal point. In practice, you should repeat
and find the center of all the intersections.
Topher
|
| 1689.2 | | SSAG::LARY | Laughter & hope & a sock in the eye | Wed Nov 04 1992 14:47 | 25 |
| Another method, which does the same as .1 but without dowels:
- Put pieces of tinfoil, reflective side out, over sections of the dish,
including the center, some sections halfway up, and sections near the edge.
Make the tinfoil as smooth and as conformant to the dish surface as you can.
- Point the dish at the sun when it isn't obscured by clouds.
- Wave a piece of paper (on a long handle!) around near where you think the
focus ought to be, until the paper shows as small a light spot as possible.
If you have too little tinfoil this may be hard to make out, if you have too
much tinfoil the paper may catch fire at the spot (which is why the long
handle...). In any case, that spot is the focus.
- When you are installing your electronics or whatever at the focus, keep the
tinfoil in place and use this to check your placement (with a piece of paper
substituting for the electronics, of course!)
The nice thing about this is that even if you don't point the dish exactly at
the sun, you will get a focus point which will work if you point the dish as
far off from the radio source as you had pointed it from the sun; whereas if
the dowels in method 1 were not quite at the correct angle you could wind up
with a focus that was never optimal. If you want to correct the pointing error
you can use the fact that when the dish does point exactly at the sun the
shadow of an object at the focal point should fall on the center of the disk.
|
| 1689.3 | | 3D::ROTH | Geometry is the real life! | Tue Nov 10 1992 12:14 | 15 |
| If x is the radial distance from the axis of symmetry of the dish
and y is the height above the base of the bowl of the dish, then
x^2 = 4*f*y
Measure the diameter of the dish (x = d/2) and the depth of the bowl
(y) and plug in to get the focal distance. That's where the phase
center of your feedhorn should be.
This doesn't have to be dead on since you'll usually end up tweaking
the feedpoint anyway, but you should try and keep the feed equidistant
from the rim of the dish to minimize sidelobes. Unless you're at Ku
band, that's a pretty small dish and tolerances won't be too tight.
- Jim [former satellite TV hacker, circa 1979-82]
|
| 1689.4 | off center on purpose? | MAST::GRUNDMANN | Bill | Fri Nov 13 1992 13:02 | 2 |
| I've noticed several dishes where the sensor appears to be quite a bit
off-center. It's got to be intentional; any ideas why this is done?
|
| 1689.5 | | STAR::ABBASI | Nobel price winner, expected 2035 | Fri Nov 13 1992 13:07 | 9 |
| .-1
may be the dish it self is not perfectly spherical shaped? may be
it has more curvature on one side than the other? or something like
that?
just guessing offcourse..
|
| 1689.6 | looks symmetrical | MAST::GRUNDMANN | Bill | Fri Nov 13 1992 13:12 | 4 |
| I'm pretty sure the dishes I've seen are symmetrical. The sensor
appears to be about a foot below the point where I guess the focal
point would be. There's one on the Tuck's Trucks building in Hudson,
Mass.
|
| 1689.7 | | 3D::ROTH | Geometry is the real life! | Fri Nov 13 1992 18:27 | 14 |
| > I've noticed several dishes where the sensor appears to be quite a bit
> off-center. It's got to be intentional; any ideas why this is done?
Offset feeds are used to reduce feedhorn shadowing, and can also
slightly reduce the noise temperature of the dish by making sidelobes
aim up into the sky instead of down to the ground (which is very
noisy compared to the sky.) Actually this latter is not that big an
issue at Ku band since the preamps are so noisy already, but it's
possible to make C band LNA's with noise temps around 40 degrees K.
That's a lot quieter than the temperature of the ground.
Ideally, the reflector should be shaped to make this work best.
- Jim
|