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

[Marko Cebokli]

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 aa6eg at hotmail.com;
> Skype: Sparky599
> Moon or Bust!--Jamesburg Gang Rides Again!
>
>
>
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