[ Reply | Next | Previous | Up ]
From: Alan G3NOQ
Date: 08 Nov 2002
Time: 03:16:19
Remote Name: 20.138.254.2
Bill - More lateral thinking here and maybe you have something, I won't comment directly but thought a summary of Chu's idea would help. Chu's analysis is based on a general solution of Maxwell's equations as a set of spherical harmonics. Many people will be familiar with Fourier series, which allow a waveform to be expressed as a set of harmonics, and spherical harmonics use the same idea in 3D, and they don't use sines and cosines but other orthogonal polynomials. As the solution is a general solution, that means that ANY electromagnetic field distribution in a region of space can be represented as a set of spherical harmonic modes, in principle. Chu used the series to calculate exactly how much radiative and reactive power there is in each mode, and from that he found out the minimum Q that can be achieved by a source contained inside a sphere of a given radius. It's directly relevant to small antennas because he found that the minimum Q is equal to (kr)**(-3) where r is the radius of the minimum sphere containing the source (antenna) and k is 2pi/lambda (provided r<<lambda). From that, the maximum achievable bandwidth can be calculated. This theory is quite general and takes no account of any particular antenna layout, and since it was published in 1948 in the Journal of Applied Physics there have been no credible examples of antennas that come close to the limit. That's not to say it's impossible, but any prospective Chu-buster had better have impeccable credentials. . . Of course, any particular antenna can have its bandwidth increased by adding resistance and reducing the gain in direct proportion, but Chu's theory applies to a lossess, 100% efficient antenna. . . Best regards, Alan