On Mar 31, 8:52 pm, Jim Lux wrote:
wrote:


Uhhh. actually there ARE laws of physics putting some pretty severe
constraints on it, if not actually forbidding it, if you also accept the
constraint that the material of which you make the antenna has finite
resistance.


Where ease might be defined in terms of being able to be made of
actually realizable materials?


The term 'actually realizable materials' seems to shift it's
definition every time something new is discovered




Chu and, later, Harrington said nothing about bandwidth, by the way.
They were more concerned with directivity and size and stored energy
(the latter of which ties to efficiency and bandwidth).

True, I didn't imply that.


Also, even if you created a very small antenna with high efficiency
(e.g. with superconductors), the fields around such an antenna will be
quite intense, so while the antenna may be small, its near field will be
pretty much the same size as the dipole it replaces, so you'll need to
put that tiny antenna way up in the air with a non-conductive, non-lossy
support to get it away from everything else. Finding a feedline might be
a bit of a challenge. One has to be careful when one draws "the
boundary" of the antenna.

Ok, it was my mistake to not clarify 'high efficiency'. By that I
meant 'at the same order of efficiency as normal scale designs'.
I am currenty interested by what I have seen claimed as 'compacted
antennas', which behave similar to normal ones, except their
dimensions are smaller, X-axis wise at least. That those designs do
not perform as well or better than their counterparts is no problem
to me, as long as the figures are in the same ballpark. That would
mean they still are more efficient than previous designs which
attempted to solve the problem of physical dimensions, which is an
advancement in my book. That some other unexpected features as the
broadband factor may appear is only a bonus, because we can achieve
that with full scale antennas too.
To be more specific, I was reffering to such designs that reduce the
scale of antennas in at least one axis:
http://adsabs.harvard.edu/abs/2004ITAP...52.1945P
http://ctd.grc.nasa.gov/organization...i-antennas.htm
http://ntrs.nasa.gov/details.jsp?R=362773
http://ntrs.nasa.gov/details.jsp?R=470415
I have seen some of them described as fractal trees, but the
information is relatively scarce. I know research is continuing on
this subject and even found some info at a website somewhere but I
can't remember where. Since you probably know more about them than me,
I would appreciate some guidance here too


In practical terms, the size of an antenna isn't just the dimensions of
the metal, but the "keepout" area within which you can't tolerate any
intrusions and still keep the same antenna performance (i.e. a 40m
dipole laying on the ground doesn't work nearly as well as a dipole
suspended 10 feet off the ground)

For that matter, avoiding the breakdown of air might be a problem.
Consider a tesla coil, which is basically a fairly inefficient (in terms
of radiated power for RF input power) small antenna for 100 kHz or so.
The limit on performance for the tesla coil isn't thermal heating of the
coil, but HV breakdown. Even a few hundred watts into a "shoebox" sized
coil will have breakdown problems (and this is fully predicted by Chu's
analysis... it's that "energy stored in the field" problem)


I do not dispute that, however I get a feeling we're talking about
different things.

Best,
Robert