On 07/15/2010 04:01 PM, Owen Duffy wrote:

"Radiation resistance (antenna). The radio of the power radiated by an
antenna to the square of the rms antenna current referred to a specified
point. Note: This term is of limited utility in lossy media."


Hmmm. The last statement suggests that, as defined, it is not clear and
unambiguous in the real world because the real world involves "lossy
media".


Lossy media is that which absorbs radiation passing through it. IOW it
heats up. This is different than say the outside air being warmed
through conduction from the earth's surface being in turn heated up by
radiation from the sun.

The "reference to a specified point" suggests that if one gives a value for
Rr, it is necessary to also state the reference point. Is that what it
means?

Hello, and yes, you would have to specify where the quantity applies. Rr
is being calculated as I^2 * Rr = Power radiated. The usual reference
point is the feedpoint of the antenna. Note that the antenna feedpoint
could also be defined to include matching networks and even transmission
line. Of course if these other components also radiate they contribute
to the antenna's radiated power.



This is exactly the lack of clarity that is troubling me.

So if we're looking at free (in vacuo) space the radiation resistance is
simply a "load" resistance component that accounts for where the
radiated power goes. The radiation resistance doesn't include any other
resistive losses in the antenna structure/proximity operating
environment that may also be dissipating source power introduced at the
feedpoint of the antenna.

This does not address the issue of ground reflection that I mentioned.


It doesn't matter to the definition of Rr what other agencies may modify
an antenna's characteristics. For example, we measure (at a particular
frequency) the real (resistive) part of its feedpoint impedance. A
portion of that resistance is due to ohmic losses in the earth, antenna
structure, and any other items forward of the feedpoint. The remainder
of the resistance is Rr. In this example the "antenna" consists of the
monopole and its near-field operating environment.

An aerodynamic analogy would be the
distinction between "induced" drag (the price paid for "lift") and
"parasite" drag, which are both components of the total drag.
Sincerely, and 73s from N4GGO,

I am not an aerodynamics type, so drawing that analolgy only helps to
confuse. You might as well use optics!

I know you are trying to be helpful John, but the IREE definition doesn't
seem to clarify the issue.

Well, I've spent a great deal my professional career as an EE dealing
with USN shipboard antennas and just happen to have ham radio as an
"office" related hobby. As I said in my previous post I don't have a
problem with what Rr means. It seems like a rather straightforward and
simple concept. I think you're trying to read more into it then is there.


To put some numbers on my first example, if I have an NEC model of a centre
fed half wave dipole with zero conductor losses, mounted over real (ie
lossy) ground, and feedpoint R at resonance is say, 60 ohms, and total
power in the *far field* divided by I^2 is say, 50 ohms, is Rr 50 ohms? Is
the power "radiated" from such a dipole ONLY the power that makes it to
'distant space', or is radiated power input power less dipole conductor
losses?


The radiated (far field) power is what is relevant to Rr. The radiated
power is the power accepted by the antenna designated feedpoint less the
other ohmic (items that are dissipating heat) losses forward of the
antenna feed and in its (near field) vicinity. Also, by "accepted"
power I mean the actual power into the antenna terminals (incident power
less reflected power).

The IREE definition suggests that I need also to state that Rr is 50 ohms
at the centre, and the term is is of "limited utility" (not unambiguously
clear?) because of the lossy ground reflections.


No it doesn't.


If indeed the term Radiation Resistance is only applicable in lossless
scenarios as suggested by the IREE dictionary, what it a clear and
unambiguous language for the real world?

Cheers
Owen

The definition doesn't say that (cf the word "limited"). Again I think
you're trying to read items, that while possibility contributing to the
measured/calculated Rr value are irrelevant to the basic definition.
IOW those other items such as earth grounds if present really ARE part
of the antenna. The power radiated by the antenna could propagate as
ground wave, sky wave or in combination - it doesn't matter. Sincerely,
and 73s from N4GGO,

--
John Wood (Code 5520) e-mail:

Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337