Joel Koltner wrote:
"Jim Lux" wrote in message
...
One wants to be careful about "Q" and Chu, etc. If you haven't
actually read the paper, you might think that Chu is talking about Q
as in filter bandwidth (e.g. center frequency/3dB bandwidth), but it's
not.

I read it well over a decade ago. I like to think I've learned a fair
amount since then, so I should probably go back and do it again some
time...

I had McLean as a professor as an undergraduate -- he was already
ruminating about Chu not having the full story back in the early '90s,
several years prior to his (apparently pretty regularly referenced)
paper on the topic on '96
(http://www.physics.princeton.edu/~mc...44_672_96.pdf).
(He was also a fan of Goubau antennas and wanted me to help him figure
out just how they worked... I never managed to contribute anything of
use towards that end and graduated and moved, but I did visit him a few
years later at which point he told me it'd really been rather more
difficult to figure out then he'd first thought. Harumph! I do think
it's cool that it eventually ended up on a cover of a book:
http://www.amazon.com/Electrically-S.../dp/0471782556
)

It's the ratio of energy stored in the system to that radiated/lost.
For some systems, the two are the same, but not for all.

Something like... it's exactly true of a simple RLC network (2*pi*total
stored energy/energy lost per cycle)... but one can concoct fancy,
higher-order networks where it isn't exactly correct?



or, an antenna, for which the approximation of an RLC is only true in a
limited frequency range.

There's a fairly good literature out there about the limitations of Chu
(after all, he was only the first shot, and modeled it as a single
spherical mode). Harrington was the next bite at the apple, and then
there's a whole raft, particularly when you get into superdirective
arrays or antennas/systems which have non-reciprocal devices in them.
R.C. Hansen and McLean (as you note) are others. When you start talking
about antennas directly coupled to active devices, that's another thing..

Consider that the low impedance of a small loop is a good "match" to the
low output impedance of semiconductor devices in RF applications.. Now
you've got a reactive load hooked to a reactive source.