wrote:
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


Sure, but there are "laws of physics" which determine the limits, at
some level. No materials with negative conductivity, for instance.

And, by "realizable" here, I mean practical, and a bit beyond. If one's
new and novel widget requires a 30 Tesla magnetic field, that's within
the laws of physics, but somewhat challenging to produce in a practical
sense.


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.

Where you're basically defining "efficiency" as power radiated into the
far field in a desired direction vs power into the feedline. That's a
fair definition for most amateur applications, if not necessarily
defensible in a rigorous analysis. Certainly, if you had two 40m band
antennas that were 3 meters across, and one produced a far field that
was twice that produced by the other, with the same transmitter power,
you'd be justified in saying the first antenna was more efficient than
the second, even if neither were particularly efficient in an absolute
sense (i.e. 2% vs 1% is a big jump in practical terms).



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

This is an interesting paper.. I note that they use MoM to evaluate the
performance. It's a special case of a general class of antennas where
you have lots of little pieces that can be reconfigured by frequency
selective traps or rf switches to achieve wide feedpoint match bandwidth
in a small package. To a certain extent, the SteppIR is also in this
same class. The thing where you hook multiple hamstick loaded elements
onto one base is also similar. This paper addresses a moderately
systematic way to generate candidate physical layouts. Others have done
things like rectangular or hexagonal grids of wires or metallic patches.


http://ctd.grc.nasa.gov/organization...i-antennas.htm

A clever use of antenna layout to get multiband performance in a small
package. The small comes from fairly traditional approaches(i.e.
dielectric medium, bent and folded elements). I note that they don't
claim that it's comparable in efficiency to a full size antenna for the
same bands.

http://ntrs.nasa.gov/details.jsp?R=362773

That one is titled: "Ten Commandments Revisited A Ten-Year Perspective
on the Industrial Application of Formal Methods"

http://ntrs.nasa.gov/details.jsp?R=470415

That one is the executive summary of a Optical Systems technology
workshop held in 1991.


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


The antenna literature of the last 10 years is FULL of various and
sundry schemes for bending wires and brushes in one way or another
(mostly enabled by cheap computation to see how well it will work,
before fabricating a prototype). Check the IEEE Transactions on
Antennas and Propagation.