Joel Koltner wrote:
Hi Tim,

"Tim Shoppa" wrote in message
...
On Mar 22, 9:24 pm, "Joel Koltner"
wrote:
I think his method, especially for physically compact antennas and
feed systems which tend to have very low radiation resistance at HF
frequencies, is a great check on theoretical calculations. There has
to be a meeting point between mathematical models/NEC and reality and
he is working at one such point.

Agreed -- the controversy comes into play in that he ends up computing
electrically-small loop antennas as being upwards of 70-90% efficient,
when everyone "knows" that such antennas are typically 10% efficient.
He even goes after Chu/Wheeler/McLean/etc. in suggesting that the
fundamental limits for the Q of an ESA are orders of magnitude off
(slide 47), and that's pretty sacrosanct terriority (see, e.g.,
www.slyusar.kiev.ua/Slyusar_077.pdf -- even the Ruskies buy into the
traditional results :-) ).


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.
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.

"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?

---Joel

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.