Best Way to Centre a Feedhorn?
Harry King
is a ADL RP1 C/KU. I seem to be having a lot of trouble getting the
thing centred. A lot of the problem is that the dish is up 30' and I
have a lot of trouble spending long periods of time on the ladder.
Could anyone offer tips on the best way to centre the feed? Right now
the arms of the mount are positioned at the 10 o'clock, 12 o'clock and
2 o'clock position. I have seen similar feeds mounted at 5, 7 and 12.
I can't have an arm mounted at the 5 o'clock position as the KU LNB is
in the way. The Skyvision catalog sells a laser type device that
shines a beam on the centre of the dish but this sells for $200. We
have no dealers in this part of Colorado so it looks like I'll have to
do the job myself.
Any help and suggestions you could provide would be greatly
appreciated. Thanks in advance.
Harry King
Michael Seiy
be 12, 4, and 8 o'clock (or 1,5,9 etc.). Since I don't know the
mechanics of your mount, I can't be real specific, but I can tell you
the goals, and suggest a few things.
The first measurement is that the feedhorn should be located in the
middle of the dish. This can be checked by measuring the length of the
arms and making them all equal in length. This guarantees centering
best if the arms are at 120 degree spacing.
The second measurement is to be sure that the feedhorn axis points to
the center of the dish. You can make your own pointing aid as follows.
You need a flat, circular piece of material, preferably aluminum or
steel, which is large enough to cover the opening of the feedhorn with
the plastic cover removed. Next you need a piece of threaded stock,
6x32 or 8x32, long enough to span most of the distance between the
feedhorn and the dish. Put a nut and washer on one end of the stock.
Drill a hole in the center of the disc just large enough to put the
threaded stock thru. Place the disc, another washer, and another nut on
the threaded stock such that the disc is held firmly between the two
nuts at one end of the stock. Check the "trueness" of your pointing aid
by rotating it. Observe that the threaded stock is straight, and the
disk should not "wobble" as it rotates, indicating that the disk is at a
perfect 90 degrees to the stock. Remove the cover from the feedhorn
throat and place the disk flat against the feedhorn with the threaded
stock end pointing towards the dish. The far end of the threaded stock
should be very near the center of the dish.
The third measurement is the distance between the feedhorn throat and
the center of the dish. Best way to do this is with a tv near the
dish. Point the dish at a relatively weak signal. Ku band is more
critical and gives the best adjustment result. Do whatever you need to
do to your mechanical setup to make this measurement adjustable. Adjust
for minimum sparklies.
The fourth measurement is that some feedhorns have an adjustable scalar
ring which can be moved towards or away from the feedhorn opening. This
changes how much of the dish is "illuminated". Too small and you're not
using the full size of your dish. Too large and you pick up "earth
noise" from the ground behind the dish. Once again, adjust for minimum
sparklies, best done during daylight.
Don't forget to replace the plastic cover on the feedhorn throat.
Mike
Harry King
information and work on the dish tomorrow. The only thing I'm not sure
about is the scalar ring but I'm pretty sure the RP1 C/KU is not
adjustable. Do you know if the Corotor 2+ have adjustable scalar
rings?
Thank you very much for providing this terrific information.
Harry King
On Wed, 16 Aug 2000 07:14:00 GMT, Michael Seiy <NoS...@from.this.list>
wrote:
R. Rikoski
<dirty_...@hotmail.com> wrote:
> I have a 9' solid dish with a tripod mount for the feedhorn. The feed
> is a ADL RP1 C/KU. I seem to be having a lot of trouble getting the
> thing centred. A lot of the problem is that the dish is up 30'...
The best way is to not have to center the feed at all but to depend on
the tripod to do it for you. Figure the geometry: if each of the three
legs is exactly the same length, the feedhorn will be centered.
Optimizing the distance from the feed to the center of the reflector is
a practical impossibility from 30 feet away. The best you can do is to
use the dish manufacturer's specs for focal point of the dish and set
the feedhorn to that distance. Take into account that the point of
focus is supposed to be an eighth to a quarter of an inch inside the
throat of the feedhorn.
Given the imprecision of most home dish reflectors, you can be off an
inch or so before it matters. C and Ku, even though theory says
otherwise, will focus to different points anyway.
I don't place much stock in the suggested homebrew threaded rod and
washer pointer, simply because the rod will sag enough to point away
from the center of the reflector at about the same amount that would be
significant to setting the centering. Besides, if you are thirty feet
up, you will need one hand for the ladder and handling the rod pointer
becomes unsteady enough that you won't be sure that it is pointing to
the center anyway.
You could use a laser pointer and washers to duplicate Skyvision's
laser centering device but the pointer would have to be exactly at 90
degrees to the washer surface. If the focal length is 36" and being a
half inch off would be significant than the laser pointer( or also the
homebrew rod/washer setup) would have to be at an angle of 88.2 to 90.8
degrees to the surface of the washer for the device to be useful in
setting angle with sufficient precision. Could you make that? And be
able to precisely measure it to be sure of what you made? (I couldn't
on a device that small but a machine shop could).
(There is a guy in Brooklyn named Howie who advertises a laser
collimator for 2" telescope eyepieces. Beautiful piece of work. I think
he has a machine shop and makes each one by hand to optical specs. I
have one for my dobsonians and it cost a lot less than what Skyvision
is selling. Unfortunately the diameter of satellite c band throats seem
to be 2 3/8", if my Chaparral is typical.)
Hope this helps,
Rick
Michael Seiy
The later comments about the do-it-yourself threaded rod pointing aid
are somewhat true, it's not _real_ precise. But it's way better than
you can do by eyeball alone with no aids of any sort, and it's _WAY_
cheaper than a skyvision pointing laser! And like the other
measurements, practically speaking, some "slop" is allowed.
You're welcome. <G> Let me know how you make out.
Mike
Harry King wrote:
>
> Excellant information Mike! I will follow this very usefull
> information and work on the dish tomorrow. The only thing I'm not sure
> about is the scalar ring but I'm pretty sure the RP1 C/KU is not
> adjustable. Do you know if the Corotor 2+ have adjustable scalar
> rings?
>
> Thank you very much for providing this terrific information.
>
> Harry King
>
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