Highland Ham wrote:
... to
get a low impedance at the matching unit (tuner if you like) the length
of half the dipole + the length of the feeder should be about an odd
number of quarter wavelengths.

That rule-of-thumb is a linear approximation but the
actual function is not linear. Here is a graph (generated
using EZNEC) that is more accurate than the-rule-of thumb.
Using this graph, quite often one doesn't even need a tuner.

Of course, half-wavelengths of feedline are added to
the matching section length to achieve a reasonable
total feedline length.

http://www.w5dxp.com/majic.gif
--
73, Cecil http://www.w5dxp.com