View message header detailMarko Cebokli <[email protected] 


Friday, May 4, 2007 10:01 am


KL6M   [email protected] 


Re: [SPAM] Re: [Moon-net] [SPAM] Jamesburg Dish Surface Mappng Preliminary Results


Hello Mike,

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
remember right...)

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 (162").

 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 support this.

 I sure would appreciate your take on this.


 Mike, KL6M

 ----- 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
 Preliminary Results

  Hello Pat,
  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 for satellites!
  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,
  Marko S57UUU
  On Wednesday 02 May 2007 04:12, Pat Barthelow wrote:
   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. 

  At 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];
   Skype: Sparky599
   Moon or Bust!--Jamesburg Gang Rides Again!