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Re: Satellite Builder Question



Bob

I am going from memory here so don't take this as gospel.  There is a specification and a test regime that NASA uses for this.  There is a test for conducted emissions of radiation (CE03) and there is one for radiated emissions (I forget the test number).

These are the ones that use the model that you refer to.  The way that you find this is to go to the NASA requirements and then follow their references back to the original source documentation.  It is a MAJOR pain in the rear but eventually you can find the information that you need.

There is a guy at the Marshall Space Flight Center who can help you.  He is a great test engineer and he is the one that did our testing during our SEDSAT days.  Drop me an email and I will give you his name and phone number.

If you are doing Shuttle or Station work there is a religious issue that you are going to have to deal with.  There are two churches.  The church of the single point ground and the church of the common ground.  For a payload that flies on the Shuttle you will be forced into the church of the single point ground.  If it is a free flying satellite you can do either but they will look askance at you as most NASA engineers will assume that you will have higher emitted and conducted radiation if you have a common system ground.

The problem that you face is that the people who wrote those specs are long gone and the ones that are there now don't do the research to go back to the original source documentation.  

You are going to have a big problem at 432 MHz because it is below the knee of the curve for radiated emissions on the frequency vs allowed radiation chart.  

This probably does not help much but if you mail me I can get you to the guy and NASA who can help you.

Dennis
KD4ETA

On Thu, 8 May 2003 11:54:53   
 Bob Bruninga wrote:
>RF Field Strength Experts?
>
>I still cannot get a consistent answer from NASA and other "experts" about
>Field Strengths or how they calculate them.  THey specify limits in
>terms of X Volts/Meter.   My question is what "meters" are they referring
>to and where?  Distance from the source? or "field Strength" at the
>source?
>
>One bureaucrat said they mean "Field strength" at a normalized distance of
>1 meter.  But I cannot find this anywhere in writing.  We have weekly
>meetings with NASA folks and after 3 months, they still do not have
>an answer as to what our safe limits are or how they calculate them (other
>than saying we are over them)...
>
>I can calculate "Volts/meter" at the source by simply taking my 435 MHz 2
>Watt transmitter into a 50 Ohm monopole 1/4 wave antnena (18 cm), and I
>compute a voltage on the antenna of 50v/m over the 18cm long whip.  But
>how to normalize that to a field strength 1 meter away or any other
>distance?
>
>Or do they simply use the 1/r^2 isotropic approach?  There I would say the
>surface area of a 1m radius sphere is 4Pi square meters over which my 2
>watts is spread.  THus I get 0.16 Watts per meter squared.  The most power
>that could induce into a wire would be a 1/4 wave wire (.18 m) so I
>multiply and get a field strength of 28 milliwatts per meter.  Convert
>to volts (assuming a 50 ohm system) and get about 1.2 volts per meter at 1
>meter from the antenna.  This would go down as 1/r^2 in the far
>field beyond about 3 wavelengths..
>
>What is the RIGHT way to do these calculations?
>
>Bob, WB4APR
>
>
>----
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