"Roy Lewallen" wrote in message
...
christofire wrote:

... and when you get back, let's see some proof that all this wacky
'new-age' theory is of any practical use to mankind.

I'd be willing to bet, say, 100 UK pounds that Professor Unwin can't
create an antenna in hardware that radiates isotropically, that is, over
a whole sphere within, let's say +/-1 dB with respect to any chosen (but
constant) polarisation. Anyone wish to up the ante?

Chris

I'd be glad to, but there's no way to prove it. Measurement accuracy and
repeatability just aren't that good, especially if you're trying to do a
full 3D measurement. The closest I've seen to a 3D measurement system
actually measured just one hemisphere. It was at what used to be NRAD
(Naval Research and Development center) and before that NOSC in San Diego,
consisting of a large (100 foot diameter if I recall correctly) rotating
circular platform with a semicircular arch overhead. By rotating the
platform and moving the detector along the arch, a full hemispherical
measurement could be made. The models were physical scale models of Navy
ships having appropriately scaled antennas. Even then, though, engineers
there told me that when the measured results differed from NEC computer
model results, they tended to believe the computer results. It's extremely
difficult to make highly, or even moderately, accurate field strength
measurements.

A while back I designed an antenna for a consulting job which was simply a
two-sloping-radial ground plane made with fairly wide traces on a low-loss
PC board. George Brown, the inventor of the ground plane antenna, had
observed an omnidirectional pattern in the horizontal plane with only two
radials, but the marketing department at RCA insisted on adding two more
to make it look symmetrical before selling them. Hence the ubiquitous
4-radial design. (The pattern of the 4 radial version is more circular
above and below the horizontal plane, but not by a whole lot.) Anyway, I
was concerned that maybe the PCB or the relatively wide, flat conductors
might have a detrimental impact on the pattern circularity, so I took it
to a local lab that has a high quality anechoic chamber and ran the
pattern. When the plot was finished, the lab technician muttered "Holy
$/!%", hit the print button, grabbed a camera, and ran into the chamber to
take a picture of the antenna. Then he went around to the other folks at
the lab with the picture and plot. Seems that it was circular within about
a dB, better than their $10k reference antenna. The prototype, by the way,
was made with adhesive copper tape and an X-Acto knife and looked as crude
as it was. I can't claim that the pattern was really better than their
reference antenna because small differences in positioning of the feedline
(even though decoupled), the antenna, and anything else in the chamber can
easily cause a couple of dB of pattern deviation.

Roy Lewallen, W7EL


Interesting stuff. I think one can get a good idea what's going on by
measuring in the cardinal planes and some 45 degree cuts. It would soon
become apparent whether the antenna warrants closer inspection.

I recall seeing what you describe at NRAA and NOSC also at ASWE in
Funtington, near Portsmouth GB. There they also had an aluminium 'sea' and
copper scale models of most of the British fleet for checking the ELF to HF
patterns (scaled conductivities). For the case in hand, though, there's no
requirement for a ground plane (viz. 'spherical' in the title) so the
resulting hardware antenna could be oriented in a number of different ways
and rotated about a single axis for measurement. I've had experience of
doing this with a Lindenblad array, and measuring axial ratio at the same
time. I've also been aware of someone else's model, made using flexible PCB
material formed into a cylinder, which outperformed the brass-tube-and-rod
one we were working on!

Feed-line radiation can easily be overcome by using a small oscillator and
battery: tiny, stable ones are available nowadays. I'd be convinced if the
protagonist managed a truly isotropic pattern at just one frequency.

Chris