[Moon-net] Big Dish Optics/Drive Systems

[Marko Cebokli]

Hello,

With reflectors you usually go for -10dB illumination on the rim, and some
cassegrains use -20 at the subreflectors.
However, with a 30m dish, you can afford to loose a couple of dB gain - 
and the increased spillover wont be a noise problem, because it looks
upwards.

With 500W even with suboptimal illumination, you will rock.

73, Marko S57UUU 



On Thursday 26 October 2006 08:55, Pat Barthelow wrote:
> Folks:
>
> I am walking through a planned operation of the Jamesburg 30 meter Dish in
> Monterey County for 1296 EME.  My first EME experience.
> See:
> http://maps.google.com/maps?f=d&hl=en&saddr=San+Jose+CA&daddr=Comsat+Rd+Car
>mel+Valley+CA&ie=UTF8&z=17&ll=36.403116,-121.646966&spn=0.003782,0.007231&t=
>h&om=0
>
> http://groups.msn.com/towertrailers/shoebox.msnw?action=ShowPhoto&PhotoID=5
>0
>
> http://groups.msn.com/towertrailers/shoebox.msnw?action=ShowPhoto&PhotoID=6
>5
>
> http://groups.msn.com/towertrailers/shoebox.msnw?action=ShowPhoto&PhotoID=6
>3
>
> We have 500 watts at the PA output connector to feed a quad or yagi
> antenna, which will illuminate the dish via a 12 ft dia hyperbolic
> secondary reflector, from the cassegrain feed location at the center bottom
> of the dish.  The secondary reflector is about 60 ft above the bottom dish
> surface, and subtends about 12 degrees from that location.  My
> inexperienced logic says that I should illuminate that secondary reflector
> with a beam pattern that theoretically does not spill past the edges of the
> secodary reflector, or ideally a 12 degree  beam width.  So, I build a yagi
> or quad with sufficient elements/boom length to keep a tight beam width,
> say 12 degrees pattern at the -3db points.  Something like 16 db gain or
> so.   My question is, how much do I need to be efficient here with the
> feed, considering that the dish gain alone at 1296 is, say near 50 db? 
> What is the likely EIRP of the antenna system, assuming a feed power of 500
> watts, say 13 db gain feed yagi, low efficiency illumination, and 50 db for
> the 30 meter dish?   Is this sufficient to do SSB EME contacts with other
> average 1296 EME stations?
>
> Also, I am looking to communicate off line with any professionals with
> operations experience with Big dishes.  The contractor who in the past
> serviced this dish, was used to COMSAT/AT&T  cutting a Work Order, to have
> a couple of techs come from, say, Texas, and service the various systems
> that control dish movement.  This service call entailed paying
> transportation, and per diem for a couple of techs for a several days, 
> with specialized experience in the various dish drive systems, including
> the various motor drives, position encoders, gearboxes, limit switches,
> lubrication systems, control computers, etc.  In the Satcom industry, this
> cost is just absorbed as overhead, and cost of maintaining a large cash
> flow through the earth station, and could probably buy a complete big gun 
> EME station for ham 1296, for only a few days of contract service.  Us
> local hams have to figure out how to operate safely, do the maintenace or
> safety checks, and have fun with the hobby.  We have also learned that ANY
> professional work on the dish is EXTREMELY expensive.  This was confirmed 
> when we explored what changes would have to be made, and what the costs
> would be, to change the dish function from Satcom to Radio Astronomy.   We
> are looking for institutions who may be interested and able to use the dish
> for Radio Astronomy, or perhaps Deep space communications.  Maybe we could
> be a  marketing tool for Cisco, HRO, M squared, or somebody, by painting an
> appropriate logo across the face of the dish (;>)
> We just want to have fun, and not see this dish be reduced to scrap metal.
>
> 73, DX, de Pat AA6EG aa6eg at hotmail.com;
> Skype:  Sparky599
> http://www.youtube.com/watch?v=_lfWQybXHeM&NR
> http://www.youtube.com/watch?v=bbCcGQXuqXw&NR
>
>
>
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