On Thu, 15 Jul 2010 20:01:57 GMT, 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".

Hi Owen,

All of this neatly fits into Broadcast Band transmission where the
current pulse (we are now into the shadow zone of SWR) occurs at the
feedpoint of an antenna that is conventionally a quarter wave tall,
the current can be measured, and the far field power is known. The
matter of "lossy media" has been studied (BL&E) and that variable
reduced by good engineering practices (which brings us back to the
known far field power).

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?

The "reference" is typically the current node. It gets messier with
more complex antenna design.

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

This implies the more complex designs following (or not following)
what you reject as "rules of thumb." Or at least their appearance.
I'm sure there are long and elaborate academic treatises that explain
the current node current measurement in relation to the known radiated
power. I haven't read any of them that I can glibly quote here.

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

I will return to your original and comment to that:
Some implications of the above are that:
- Rr of a horizontal half wave dipole with zero conductor loss, above
real ground, would have Rr less than R at the feedpoint by virtue of some
loss in waves reflected from real ground;

There are two mechanisms hiding in one description. The ground is
lossy - period. The ground is reflective - period. These are two
different issues in regard to radiation resistance. The Rr is not
ground loss although the measure of Rr may be corrupted by Rground.
That is a problem of separating out the variables. Others have
described that. The reflection from ground may upset the measure of
Rr as well, but if that does not upset the total power, and the
current node can be measured, then you still have a way to quantify
Rr.

- Rr of a half wave folded dipole of equal conductor diameters would be
around 300 ohms.

I thought someone else preceded this discussion with Tom's
explanation. Maybe it went unread, or unrealized. So, in other
words: A folded dipole/monopole is a current transformer. That
transformation ratio is driven, in large part, by the ratio of the
diamters of the conductors. You have acknowledged as much in your own
specification of equal sized conductors. Having said that, the
transformer is also transforming the Z of the load (Rr + Rground) by a
square law. The usual sense of current node has been lost in a more
elaborate design, but the transformation of it returns us to the usual
Rr.

73's
Richard Clark, KB7QHC