On Mar 31, 7:52 pm, Jim Lux wrote:
wrote:
I find this topic very interesting, including the mandrill part 
We all want to have small, broadband, eficient antennas. I believe Art
is right in his original post, today we can have all these
characteristics in the same package. There is no law of physics
forbidding that.
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.
Through advances in computation power we can achieve
today in months what took decades in the past and there is much
research directed at these kinds of new antennas. Eventually
everyone will be able to choose and model his own antenna based on the
characteristics one wants, but without the cumbersome dimensions,
without significant bandwith limitations, without major efficiency
compromises. I believe the tradeoff (for it has to exist one) will be
ease of manufacturing.
Where ease might be defined in terms of being able to be made of
actually realizable materials?
Incidentally these new antennas have a lot to do with what Art
defines as equilibrium although I don't think he has a clear enough
definition. But it's all related to patterns, patterns which can be
found everywhere in nature an which can be expressed almost entirely
through matemathical formulas. Scaling of antennas is clearly
possible, despite of what the Chu-Harrington limit states ( or to be
fair, by applying them in a new way ).
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).
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.
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 eagerly await the day when the 80 meter dipole will be replace by a
small device the size of a shoe box ( although it might be a bit
larger in the beginning ).
Regards,
Robert
Jim,
With all due respect a discussion is futile if you stray from the
concept of
a small FULL WAVE antenna and use the ELECTRICALLY SMALL antenna
as a straw man. The electrically small antenna is a fractional wave
antenna
which is represented by a series circuit. This is totally different
to a parallel
tank circuit. This correlates to a pendulum being cast as a weight
that comes
to a abrupt stop and instead of swinging up goes back from the bottom
to the
top from whence it came! A electrical small antenna assumes an
awefull lot
as to the mechanics of action involved in a full period. The tank
circuit is a
good example that shows that all segments of a period in terms of area
are exactly the same
where the tank circuit clearly shows that radiation occurres only in
the last
quarter of a period! The idea or concept of a fractional wave antenna
came
from the assumption that a sino soidal pattern can be seen as four
areas
under a line which can be considered the same as four times a quarter
segment,
a concept around which the NEC programs were formed. You NEVER get
radiation
at every quarter segment of a period. The concept implicit in Maxwells
laws is
that equilibrium is a given which means that the root C L portion is
that of a
full wave antenna as a minimum.
All the laws of the masters are based on a stable boundary at the
beginning and
at the time for a period of time., Time has removed a lot of memory of
the human race.
I suspect that the NEC programs around the current flow OUTSIDE the
arbitary boundary
that allowed them the successes they have gained without having to
consider
the mechanics of the innards within the boundary.
Regards
Art Unwin