NEC-LIST: Re: NEC-LIST:Simulations of Q and the Chu Limit

Hello All,

I tried the link Chip gave to the Gianvittorio-Rahmat-Samii presentation,
but my browser (IE 6) wouldn't go there. I shortened the address to en at
/fractals, and that worked. It was more useful,actually, because I found a
.pdf version there. I don't have PowerPoint installed, so the Acrobat
version is more useful. There were a few characters not translated
correctly, but it wasn't hard to figure out what they should be. The file
is 20 pages, mostly pictures,figures and graphs, as you would expect for a
presentation.

I used the same formula for Q as the authors' final expression, which I
think is loaded Q. It shows the importance of designing a small antenna to
be resonant at the working frequency. I personally dislike the radiansphere
as a measure of antenna size. It is a holdover from H. A. Wheeler's
writings on small antennas, which I also didn't like because I couldn't see
where he got his equations. I think the maximum dimension in wavelengths of
the antenna is a more practical measure. If one wants to be volumetric
about it, how about the fractional wavelength cube? That is, the size of
the smallest upright cube that will enclose the antenna. We aren't talking
small if the cube side is bigger than 0.1 lambda.

The nub of the presentation is the graph on page 18 that plots Q for a large
selection of antennas. Unfortunately, either some of them overlap entirely,
or were omitted, because the legend shows 8 lines, but the graph only has 6.
I saw some antennas new to me. These were the 45-degree spherical monopole
and the multi-armed loops.

While a comparison among antennas in freespace is a useful beginning, mostly
the applications for small antennas are in some other environment. If one
models an antenna for the HF band close to some kind of real ground, the
results can be surprising. A small antenna on a handheld metal box can
drive the box and the two become a much larger antenna. I wonder how one
would set up a multi-arm loop for such an application, and how it would
behave. The 3D fractal loops would also be a puzzle in such an application.

Doug Miron

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