Re[2]: Toroidal Antennas and other Remarks

      Regarding electrically small antennas:
   
   I think most readers will also benefit from "Bandwidth and Q of
   Antennas Radiating TE and TM Modes" by Grimes and Grimes, IEEE
   Trans. on Electromagnetic Compatibility, Vol.37 No.2, May 95.
   
   While this paper is complicated and generalizes some previous
   worker's assumptions, the authors conclude that there is *some*
   potential to overcome previously believed limits. But as I read
   it, not by much--not an Earthshaking result--but another small
   step in the right direction. Still, the message remains clear:
   Small antennas having directivity or gain incommensurate with their
   electrical size are subject to narrowband response and (usually)
   high I**2R loss as well.
   
   Grimes and Grimes give a nice list of references including Chu
   (1948) Harrington (1960) Fante (1969) Wheeler (1975) Hansen (1982)
   and others all of whom have worked this question.
   
   One qualitative view of what's happening is that reactive energy in
   the near-near field (within Wheeler's "radiansphere") acts as a
   kind of complex Poynting vector "cloud" with most energy shuttling to
   and fro and with only a fraction--the real part--escaping. As the
   surrounding sphere or hemisphere shrinks the reactive part relative
   to the real part grows. (I imagine the real part [the good guys]
   struggling to punch through the cloud and escape). Grimes and Grimes
   suggest this can be manipulated to some degree better than previously
   thought possible, by a considered combination of electric and magnetic
   dipole and quadripole sources.
   
   Still, on the workbench, one wonders just how to *do it*. As I
   read Wheeler, lowest Q will result from "filling the radiansphere"
   that is, using all the available volume that just surrounds the
   antenna (and I would add, "filling that volume with *smart* stuff").
   But, I don't know what that "stuff" is nor how to deploy it; I don't
   know how to realize or even approach the "filling" principle. Multiple
   sources or drivepoints sound like attendant hassles, and might be
   bandwidth-restricting if separated by much of a wavelength. Anyone
   out there care (or dare) to suggest how? How do we practically realize
   and arrange Grimes' suggested electric and magnetic dipoles and
   quadripoles?
   
   Finally I would offer an illustrative number on how crudely we
   presently do things in practice:
   
   I was once very fortunate in that some excellent impedance
   measurements were gathered over a bandwidth on 4 large towers near
   400 kHz. The towers were near 8% of a wavelength high and separated
   by many miles. The systems were very efficient, all parameters
   were well-understood and I had the benefit of a sample of 4. I was
   careful to separate the effects of the tuner from each antenna in order
   to get the antenna-only Q (a fully conjugate-matched antenna-tuner
   combination has half the antenna-only Q, or conversely twice the
   bandwidth--be careful with this sometimes-overlooked detail). Comparing
   those measurements with theory (had the tower lived up to it's full
   [hemispherical size] capability according to Chu's theory), I would
   have gotten about 8-10 times more bandwidth. Thus, a simple monopole
   over a groundplane does not substantially approach what *was* thought
   possible (Chu) and *now* (with Grimes & Grimes) even somewhat less. A
   monopole definitely does not fill and utilize it's associated
   hemispherical volume "smartly". (One might imagine some clever use
   of the otherwise non-participating guywires, for example).

   It seems to me this is a good, convenient and informal forum for
   exchanging views on this important topic, and I for one would enjoy
   hearing from others interested in understanding and doing good antennas.
                                              
                                              
                                              
        --------------------------------------
         Dan Bathker
         Spectrum Planning and Engineering
         Jet Propulsion Laboratory 303-401
         Pasadena, CA 91109 USA
        --------------------------------------
         (818)-354-3436, FAX (818)-393-1692
        --------------------------------------
            ** dab_at_jpl.nasa.gov **
        --------------------------------------
        --------------------------------------
          Opinions (if any) are mine alone
        --------------------------------------
           -.- -.... -... .-.. --.
        --------------------------------------

   Dan Bathker
   

______________________________ Reply Separator _________________________________
Subject: RE: Toroidal Antennas and other Remarks
Author: bobh_at_america.com at Internet
Date: 11/14/95 10:46 PM

For a simplified discussion of possibilities and problems of small "i.e.,
supergain" antennas, see my article in Communication Quarterly, Summer
1992, also republished in IEEE Antennas and Propagation Magazine, August,
1995.
R. P. Haviland, W4MB
Received on Thu Nov 16 1995 - 01:01:00 EST