[Antennas] Impedance questions...

[Harvey&Bessie]

The theoretical radiation resistance of a quarter wave vertical over a
perfect groundplane is around 37 ohms, this would give only a 1.3 to one
VSWR. The reason these mobile antennas seem to do ok with 50 ohm co-ax
feed is that there is always some loss resistance to be added to the 37
ohm radiation resistance. So that instead of seeing 1.3 to one VSWR, we
see about 1.2 to one. This of course is well within the acceptable range
of all but the "nit-pickiest" of perfectionists.
The 5/8 wave mobile vertical has a slight (theoretical) gain over a
quarter wave one. However, the 5/8 wave antenna is not resonant and
requires a matching coil at the base. Usually the losses in that
arrangement cancel out any theoretical gain and we are back to square
one.
I might point out also that, while 1/4 wave verticals do very well on
the UHF and VHF bands, where their very low take-off angles are ideal,
the use of a vertical mobile antenna on the lower bands, particularly 80
m and 160 m, is counter productive for the main reason that we are
interested in near vertical incidence (NVI) propagation on those bands
and the vertical produces almost zero at these high angles.
Nobody that I know of has found a real good solution to that particular
problem. Listen carefully to mobiles trying to check into local or
regional nets on these bands to see what I mean!
Harvey/W4TG