I've been out of town since this was posted. There are a couple of
details I'd like to check in some texts before responding, so I'll
postpone my response until after I return home. In the meantime, I see
that Ian and Tom have made contributions, both to their usual high
standards. Both these gentlemen know what they're talking about, and I'm
glad to have them help out.

Roy Lewallen, W7EL

Dr. Slick wrote:
Roy Lewallen wrote in message ...

Here's the problem with that transformer concept again. A field is not a
voltage. So you can't measure it with a voltmeter. You can convert the
fields to voltages and currents by use of a transducer -- an antenna --
then you can measure the voltage and current from the antenna with
ordinary meters.



I agree with you that the field is first converted by the antenna
before it can be measured.

But by definition, the E field is definitely related to voltage
potential.

Hugh Skillings' Fund. of Electric Waves: "Voltage from point 1 to
point 2 is the line integral of the electric field along any path from
point 1 to point 2. This is the amount by which point 1 is at a
higher potential than point 2."

Say PD = E^2/Z0 = H^2 * Z0. If you say the Power Density =
V^2/(R*m^2), and the R=Zo, then these will cancel, giving you E =
V/meter, which are the correct units. So here we are equating the
impedance of free space will a resistive impedance or load.

Roy, what do you think 1uV/meter really means in terms of how you
measure it? I mean, under what conditions must you have to measure
this 1uV/meter?

I'm starting to think that what this really means, is that an
exploring particle with a unit positive charge, when placed in a
electric field of 1uV/meter, will experience a change of voltage
potential of +1uV when it is moved directly towards an isotropic
radiator ("the potential of a point in space is the work required to
move to that point a unit positive charge, starting an infinite
distance away...potential increases as one positive charge is moved
closer to another positive charge" - Skilling).





Good thing, too. Otherwise we'd all get electrocuted by the Earth's 100
volt/meter field. (And that's on a day with no storm nearby.)



But that's a static field, so we don't have to worry about
touching metallic objects that aren't grounded.




Perhaps the far-field measurements would require too sensitive a
field-strength meter? Or maybe it's just more convenient to measure
up close.

No, it's far field measurements that are more common. One problem with
making near field measurements is that the near field varies all over
the map with the type of antenna and the exact spot where you're making
the measurement. And it's of no importance at all to anything very far
away at all. I've only seen near field probing done to locate the source
of a problem emission. Compliance measurements are usually done with
far-field techniques, in or at least at the fringes of the far field.
The "within the near field" measurements I'm referring to are HF
measurements done at distances that aren't firmly in the far field. (The
far field boundary depends on the nature of the radiating structure, and
is nebulous anyway.) The FCC addresses this issue for Part 15 somewhat
in section 15.31(f).

Roy Lewallen, W7EL



I was going to ask you to define "far-field", and i thought maybe
people defined this as a number of wave-legnths away, but if it's
nebulous like a lot of RF topics, then i would certainly understand.


Slick