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

When the air breaks down around an antenna it is because the antenna
is not in a state of equilibrium. When a dipole is replaced by a quad
ala
a series circuit is replaced by a tank circuit it clearly shows that
the latter
is more efficient.This was firmly proven in Quito.Maximum radiation
efficiency requires equilibrium. Period
Art