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Re: Satellite Builder Question



See:
http://www.astron.nl/craf/conv.htm


John  WA4WDL

At 11:54 AM 5/8/2003 -0400, you 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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