Roy Lewallen wrote in message ...
I think I've done about as much as I can here, and the time spent is
getting out of proportion to the communication achieved. It's time for
someone else to take a crack at it, or for it to be taken to another
newsgroup.

Roy Lewallen, W7EL


Thanks for your input Roy.


Slick

Roy Lewallen wrote in message ...

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

Well, yes, speed (meters per second) is related to distance. Force
(Newtons) is related to work (Newton-meters). But speed isn't distance,
and force isn't work. The mass of the Earth is related to its orbital
velocity, and mass certainly isn't velocity. Worse yet, the impedance of
free space isn't a measure of the same thing as the characteristic
impedance of a transmission line. What I'm trying to illustrate is that
because two things are related doesn't make them the same thing, or
necessarily even close to the same thing.


I never said that the E field IS a voltage, only that they are
related, and that you can measure the strength of the E field by
measuring voltages, which is what you really do with a field strength
meter anyways.




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.

No, all you're doing is showing that they have the same dimensions. It
just doesn't seem to be sinking in that having the same dimensions
doesn't make two quantities the same thing. I've tried with the example
of torque and work, but that doesn't seem to be having any effect. Maybe
someone else can present some other examples, and maybe, just maybe,
with enough examples the concept will sink in.



Are you saying that Newton*Meters are different depending on the
situation? I don't think so. A Newton is a Newton. A meter should
always be a meter. And an Ohm should always be an Ohm.

I'm not saying that the impedance of free space is relating
voltage to current, because there is no current flow in free space.
But why do they use the same units (Ohms) to describe the relation of
the E field to the H field?

Hugh Skilling - "Intrinsic impedance is somewhat analogous to the
characteristic impedance of a transmission line. It has the
dimensions of Ohms, for the E is Volts/Meter and H is Amperes/meter."

Perhaps I'm asking the wrong NG.




But electric field is actually defined in terms of the force on a
charge. You'll find an explanation in any basic physics text, as well as
many places on the Web. In Weidner and Sells, _Elementary Classical
Physics_, Vol. 2, the authors define electric field E as F/q, or the
force that would be exerted on a (sufficiently small) charge at the
point at which the field is being measured. They explain that the units
of electric field are newtons per coulomb which, it turns out, has the
same dimensions as volts per meter. So to your argument that electric
field is "related" to voltage, it's equally related to distance, force,
and charge. You can, in fact, find a bundle of other equivalent products
and quotients of units that are equivalent.



The E field is certainly related to all these things. If you
could somehow accurately measure the repelling force on a unit
positive charge, that a static positive charge exerts upon it, then
you would know what the E field is in Newtons/Coulomb, which would
also be the Volts/meter.

If you could also measure the voltage potential of this unit
positive charge in the same positive field, at two different points
that are 1 meter apart (normal to the gradient of the E field), then
you should get the same answer as above.

This second situation is what i'm asking about when i say where
would the 1uV be measured. But in the real world, it would seem that
most field strength meters have amplifiers (LNAs) either before or
after rectification by Schottky diodes.

A field strength meter seems to relate the received power with
the field strength, but not actually measuring the uV/meter directly.
Certainly they must calibrate these instruments with a known amount of
power into an antenna of known characteristics, in an anechoic
chamber, at a fixed distance away, etc.




Here we are again. Potential and voltage have the same dimensions, but
aren't necessarily equal. And as far as I can tell, "voltage potential"
is meaningless. To quote from Holt, _Electromagnetic Fields and Waves_,
"When the electromagnetic fields are static, as we shall see, the
voltage drop along a path equals the potential drop between the end
points of the path. Furthermore, these quantities [voltage and electric
potential] are also equal in *idealized* electric circuit diagrams, and
they are approximately equal in physical circuits, provided voltmeter
leads do not encircle appreciable time-changing magnetic flux." Pay
particular attention to the last qualification. When a time-changing
magnetic field is present, the voltage drop between two points depends
on the path taken, while the potential drop is simply the difference in
potential between the two points. So the voltage between two points in
an electromagnetic field can be just about anything you'd like it to be.


I was simplifying the situation with the static E field case, but
the change of 1uV for every meter moved normal to the gradient of the
field can also apply to the AC situation, except that the +/- 1uV
would apply the RMS values.





RF isn't any more nebulous than any other aspect of engineering.
Engineering is a practical discipline, so compromises and trade-offs are
universally necessary. Because we deal with real, physical objects and
are stuck with real measurements, the absolute precision of mathematics
and the pure sciences is never attainable. This is as true for using an
I-beam as it is for RF design. But the principles of RF are at least as
well known as the properties of I-beams. In fact, a good argument could
be made that RF is better known.

Roy Lewallen, W7EL


You wrote: "(The far field boundary depends on the nature of the
radiating structure, and is nebulous anyway.)"

So i used the word "nebulous" after you did.

You're talking to a B.S. in Mechanical, so i know a bit about
I-beams, but i've done only RF since college, so I'm more of a EE now.

Among the Top Ten blunders of my life: not doing Electrical.


Slick