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

> -----Original Message-----
> From: nec-list-admin_at_gweep.ca
> [mailto:nec-list-admin_at_gweep.ca] On Behalf Of D. B. Miron
> Sent: Friday, November 29, 2002 10:49 PM
> To: NEC-LIST
> Subject: 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.

Hi Doug:

In most cases, this is exactly how an electrically small antenna is
defined. An electrically small antenna is defined as a function of KA,
where A is the radius of a sphere encompassing the maximum dimension of
the antenna.

In most cases, engineers presume that an antenna is electrically small
if it fits within a sphere of radius KA < 1. In generalizing, Wheeler
said that an electrically small antenna is one whose maximum dimension
is much less than a radianshere. More specifically, Wheeler and Chu
both defined an electrically small dipole to be one whose overall length
was less than the radianlength, or KA < 0.5.

KA being less than 0.5 is a more appropriate electrically small limit.
In fact, if a monopole antenna (fractal, meander line, normal mode
helix, etc) is self-resonant sufficiently above KA = 0.5, its radiation
resistance will converge, very near KA = 0.5, to that of a straight
monopole of the same height, independent of geometry or total wire
length.

>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.
>

I think you need to specify the size limit of a small antenna in terms
of occupied volume in order to fairly compare different small antenna
designs. Otherwise, you might end up in a situation where you have an
electrically short antenna (small h) that occupies a large plane area
(like a PIFA), but it is not really electrically small.

The sphere is a convenient reference, particularly because Chu and
others chose to describe the omnidirectional antenna radiation in terms
of spherical modes.

Steve Best

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