Only from diameter and depth it is not possible to see if the dish is shaped.
360 and 50 give a focal length of 162, that agrees with 0.45 f/d.
You would have to measure several points, from the vertex towards the rim, and
then try to fit a parabola through them, to see if your reflector deviates
from a parabola. (at least four, because you can always fit a parabola
through 3 points)
The fact that you had to move in (closer), I think would be consistent with a
shaped Gregorian (concave subreflector), these are usually shaped "inwards"
on the main dish.
So if it was originally a Cassegrain, what you observe is probably not a
consequence of shaping, which usually goes towards flatter in this case (if I
For this size (cca 150 lambda on C band), single shaped designs are also
possible (shaped subreflector, parabolic primary), because the shaping
deviation is quite small, and it is possible to find a "best fit" parabola
through the shaped shape of the main reflector, which has low enough phase
If the surface of the dish is shiny, and you can see your dish from some
distant point (a km or so), you can put a small filament bulb (car
headlight..) at the focus and send a friend with binoculars, to see if you
can get the whole surface light up from the same point.
I used this method for smaller dishes (sprinkled them with water for shine) to
check surface accuracy, but it could be clumsy for such a big dish.
Maybe you could put a few small mirrors on the surface etc....
Another way to see if you have phase errors is to measure the pattern around
the main lobe, and see if you can get deep nulls between the mainlobe and
sidelobes. ANY phase errors (like defocusing) will "fill up" the nulls - you
must try several focus points, to find the best. As long as you can at least see a
dip between main and sidelobe, you are within cca 1dB of optimum gain.
NOTE - you need a point source for this (TV satellite...) - the Sun is NOT a
point for this size dish - and your signal must be at least 25..30 dB above
noise, so that you can clearly see the sidelobe.
73, Marko S57UUU
BTW next wednesday I am off for a 3 week vacation, won't be able to answer
On Wednesday 02 May 2007 09:06, KL6M wrote:
Very interesting! Perhaps my dish is
shaped. I have a 9.2M C-band solid
reflector that used to have a
subreflector. The spec says it is .45 f/D
and it physically measures 360 inches in
diameter and 50 inches deep. But
I find that my focus with direct feed is 8-9
inches closer in toward the
dish than where it is supposed to be
Could some kind of shaping cause this?
With the feed in the "new" focal
point I get terrific results. 0.9dB moon
noise on 23cm (VE4MA CP), and
1.2dB moon noise on 13cm, with plain old
septum feed with no flare or
scalar ring. This new focal point would
suggest that the dish is actually
.43 f/D, but the physical measurement does not
I sure would appreciate your take on this.
----- Original Message -----
From: Marko Cebokli <[email protected]
Date: Tuesday, May 1, 2007 10:15 pm
Subject: [SPAM] Re: [Moon-net] [SPAM]
Jamesburg Dish Surface Mappng
the "maximum gain" dish shaping
tries to produce quasi uniform
illumination by shaping the subreflector (and
then corrects the phase by the main
dish shaping), so the pattern will be closer
to the "uniform circular
aperture" pattern, which would have -17
dB first sidelobes. But the shaping
has limits (diffraction at the finite size
subreflector), so there will still
be some amplitude taper at the edge of the
main dish, and the first
sidelobe will be less than -17.
The far-out sidelobes will actually be
REDUCED by the shaping
(more gain =
more energy in the main beam).
The main consequence of the shaping will be,
that you will be
forced to use the original subreflector,
otherwise you will have phase errors.
Depending how much your main dish deviates
from an best-fit
parabola, you might get away with prime focus
feeding at lower frequencies,
where main-dish deviations become a small
part of the wavelength.
Today you usually do not need such a big dish
So if they are designed to work with smaller
dishes, your pattern
will still fit the prescribed envelope,
simply because of dish oversize.
Just use a signal from an satellite to
measure your pattern, and
compare it to the prescribed envelope for the
sat you intend to use.
BTW, the old big "Intelsat A"
dishes (cca 30m size) were shaped too...
73 & GL with the big dish,
On Wednesday 02 May 2007 04:12, Pat Barthelow
Folks: Mike Brenner called me with a
few notes from our weekend
Jamesburg Dish surface mapping project.
He mapped the surface, using his
special software and hardware system, using a
Surveyor's Total Station theodolite,
containing a laser distance meter. I am
really happy he was successful in
gathering the desired information, (not
easy, physically) and look forward
to finding out how good the dish surface is.
Mike informed me that the dish was a
"special" one with enhanced
gain in the main lobe, of maybe a couple dB,
that modified the secondary
reflector and primary reflector surfaces from
their standard Parabloa-hyperbola
configuration. A side effect of the
special dish is poorer sidelobe performance.
the time of design (late-mid 60s) that was not an issue, but it may be now,that
spacing of geosynchrounous sats are closer together. Any big dish professionals out there,
that can give me more detailed information on the consequences of the shaped dish
characteristics? Can this dish ever be used for geosynchronous satellite
communications? Does the enhanced shaping add any limits to usable bandwidth?
I am working with some success at finding
suitable buyers for the dish and
the property it is on, and have some prospects
that are "Ham Friendly", that is will allow/encourage continued ham EME
use when the dish is not otherwise used.
Hope to see you soon from the moon...
73, DX, de Pat AA6EG [email protected];
Moon or Bust!--Jamesburg Gang Rides