[amsat-bb] Satellite Builder Question

[Bob Bruninga <bruninga@xxxxxxxx>]

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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