Roy Lewallen wrote in message ...
Well, let's see. We can start with an isotropic antenna, which
distributes its power equally in all directions. I did that one three
days ago on this newsgroup, in the thread "Theoretical antenna
question". The result is that the power density from an isotropic source
at any distance r is

PD = P / (4 * pi * r^2)

where P is the total power radiated. Power density PD will be in
watts/square meter if P is in watts and r is in meters.



That's just the power divided by the surface area of the outwardly
traveling EM wave that is a perfect sphere in the case of an isotropic
raditator.



In the far field, the field strength E from any antenna is sqrt(PD *
Z0), where Z0 is the impedance of free space, very nearly 120 * pi ohms.
E is in volts/meter if PD is watts/meter^2 and Z0 is in ohms.
Substituting in the first equation gives

E = sqrt[(P * Z0) / (4 * pi * r^2)] ~ sqrt(30 * P) / r



This is more proof that the "transformer" action between two
antennas is highly dependant on the impedance of the medium between
them.

Roy, i don't mean to be an overly inquisitive laid-off engineer
with too much time on my hands, but how was E = sqrt (PD * Zo)
derived exactly? This is really the key equation.

Here's something else I'm wondering about. If you get an answer
of 1 uV/meter, does this mean that a perfect conductor of 1 meter
length placed in this field (polarized with the E field) will measure
1uV RMS if you measure the AC voltage on the ends? In the real world,
what sort of receiving antenna do they use to measure E fields?
Obviously, the recieve antenna will affect the measurement...perhaps
you want something broadband, so as not to favor a particular
frequency (a resonance on the receive antenna will throw off the
reading)? Perhaps something as isotropic as possible, so orientation
is not as critical. How does the FCC measure it, what equipment do
they use?



Slick