[Antennas] Broadbanding an antenna...

[Rick Karlquist]

Another way of thinking about this problem is that the antenna
can be modeled as a series resonant LC tank.  Together with the
shunt resonant LC tank, this topology looks just like a 2 element
bandpass filter.  You cannot choose the series tank, but you
can choose the shunt tank.  Given these constraints, there will
be a Butterworth filter of a particular bandwidth that is
compatible with the series tank.  You can then calculate what
the shunt tank needs to be.  It's that simple.  Play with your
favorite filter software and you can figure it out.  You can
also choose a Chebyshev filter with some ripple parameter.
Remember that the higher the ripple in the Chebyshev, the
higher the SWR.  This technique can be extended to a 3 pole
filter by preceeding the shunt tank with a series tank.  IIRC,
its usually not worth going to a 4th or higher order filter.
The ultimate limits of what you can do were described by MIT
professor Robert Fano in his 1947 MIT thesis, which was also published
in the Journal of the Franklin Institute in 1950.  It doesn't
appear to be available online.

Rick N6RK