[TowerTalk] passive "autotuner"?????

Jim Lux jimlux at earthlink.net
Fri Jun 9 00:33:04 EDT 2006

At 11:42 AM 6/8/2006, doc wrote:
>  > discom101 at sohostar.net wrote:
> > my first thoughts on how to do the automatic "selection" is to use series
> > resonant circuits in series with the parallel resonant circuits. the
> > simplest way to test it is to use 1/4w stubs. a shorted stub behaves like a
> > parallel resonant circuit, and an open stub like a series resonant circuit.
> > the advantage of using stubs is that they behave that way at more than one
> > freq, and will lessen the "parts count". at least if it doesn't work, i'll
> > find out why. i'll also be able to share my results with the group. who
> > knows, somebody might come up with a better way to do it?
>Is this what you are proposing?
>It would present a mostly horizontal component with the stubs
>serving as a small vertical component.
>The stubs might just hang and be flexibly tensioned to prevent
>tangling in the wind, or perhaps attached to the sides of a
>non-conductive vertical center support like thick-wall PVC.
>I have no idea if this would work!  :-)

If you start allowing multiple Ls and Cs in your matching network (whether 
they are lumped or transmission line stubs) there's no reason why you 
couldn't get a 1:1 match in all bands with almost any antenna.  At one 
extreme, imagine you had narrow band filters for each band designed to 
match that band, including the parasitic effects of the other bands.  This 
isn't much different than designing a multiplexer (diplexer/triplexer, etc)

This is a fine problem for an optimizing program.  The challenge would be 
in determining the minimum number of Ls and Cs (as well as the circuit 
topology) while minimizing losses.  And, there's probably some IEEE paper 
around that gives an analytical bound on the maximum number of components 
it should take.  Certainly, there were some papers on wideband LC models 
for dipoles and monopoles, and if you can adequately model the antenna with 
a collection of Ls and Cs, then you can match it with no more than the same 
total number of Ls and Cs.

In some ways, this is really what designing a multiband trapped antenna is 
all about, because at most frequencies, the traps are in the circuit as 
"loading coils/caps" and also radiate to boot.


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