"Jeff Liebermann" wrote in message
...
On Fri, 4 Sep 2009 00:56:39 -0700 (PDT), wrote:
Sure, you can get fairly close to isotropic with the right
system, but how are you going to do it by tipping a
vertical? The likely results do not fit my idea of isotropic.
I forgot to connect my comments to the original question. Sorry(tm).
You're correct. There's no way to get a good isotropic radiator
pattern with a simple vertical radiator. However, you can still get
fairly close if you make the antenna sufficiently small relative to
the operating wavelength. As the physical antenna size approaches a
point radiator, the pattern starts to look rather spherical.
That doesn't sound right. The directivity gain of an infinitesimal electric
doublet (i.e. a dipole with infinitesimal length) is about 0.4 dB less than
that of a half-wave dipole. Its similar lemniscate-of-rotation radiation
pattern results from the symmetry about its axis. The results for monopoles
derived from these forms of dipole won't be too different. It is rather
obvious that a receiving dipole of any polarisation won't receive much
signal from the end of a transmitting dipole or monopole, however it's
oriented - there's no apparent length over which unopposed current is
flowing so there's no 'moment' in that direction.
One solution to spherical radiation is the Lindenblad array (and variants
that others have chosen to re-name) which presents finite resolved
components of the lengths of some of its dipoles in all directions ... but
the tilt of the elements has nothing at all to do with Art Unwin's 'theory',
it's simply a matter of making sure there's a resolved component in each
direction. Of course, a Lindenblad designed for a near-omni pattern
achieves this in respect of circular polarisation so it would be
ineffiecient in a system where a linearly polarised antenna is used at the
other end of the link.
I was once told a true isotropic radiator would have to be circularly
polarised because it would be so small that it could contain nothing with a
defined axis of symmetry ... that is, the antenna would have the form of an
infinitesimal sphere. The question then is 'which sense of circular
polarisation' ... which undoubtedly has nothing at all to do with Coriolis
force! The real answer is that it doesn't matter because, as you mentioned
(below) such an antenna has an infinitesimally small radiation resistance
and cannot be made to radiate.
Chris
Unfortunately, the gain drops, efficiency drops, and feed point
impedance drops, resulting in a rather inferior antenna.
There's also a question of how close to perfection does the spherical
pattern need to become? Within 0.1dB, 1dB, 3dB, etc???? Offhand, I
would guess anything within a few dB of spherical could be considered
isotropic, as in all the patents I noted.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558