I plan on installing my Yaesau FT 867 in my car. The car was a former state police vehicle and so holes were already cut in both the middle of the trunk and the middle of the roof. I have a bunch of
NMO type antenna bases around here that cover these frequencies and I would like to use these antennas for this. I have used these antennas before but only for one specific frequency. It has been my
experience that these antennas tune rather sharply. Since both of these bands are 4 MHZ wide what frequency should I cut the antennas for? Center frequency would be easy but wouldn't an antenna cut
for 52 MHZ have very poor SWR when used at 50 MHZ? The rig has both low and high band outputs. I'm not sure what I should do. Thanks for any input on this. Lenny

"Insufficient data." NMO is a connector type, and can be used to
support all sorts of different monopole antenna types. How sharply
these antennas tune, will depend a lot on just what sort of design
they have... some are simple quarter-wave whips, some are shortened
antennas with inductive loading (sometimes a lumped inductance right
at the base, sometimes a spiral-wound distributed inductor).

As a very coarse rule of thumb, inductively-loaded monopoles will
probably tune more sharply than a full-length whip... but a _really_
short whip with a lot of loading is likely to be quite lossy, and
these losses may mask the sharpness of the tuning.

The other rule of thumb is "When you need to tune or trim an antenna,
you probably want to trim it for the frequencies that you're going to
use most."

So, the only real answer to your question is probably "try it and
see."

Most two-meter mobile antennas are broadbanded enough that they can
cover most or all of the band with an acceptable SWR (some may not
tune well down at the low end of the band, which is where SSB and CW
are done). You may not need to be concerned about this - put a
commercial 2-meter NMO-mount antenna up on the roof and be happy.

For 6 meters... FM tends to concentrate at the top end of the band,
and by convention this is vertically polarized. Some magic-banders
prefer to have a second antenna which is tuned to the lower end of the
band and is horizontally polarized, for CW and SSB operations in those
frequencies.

Some antennas for the lower bands have a fixed lower section, and an
adjustable upper whip... you can change the tuning "in the field" in a
minute or two, and can pre-mark the whip with tuning marks for various
frequencies. This might be a nice, inexpensive compromise for 6-meter
operation.

Or (as somebody else suggested) install a remote antenna tuner in your
vehicle, or a manualy-tuned matching box (might be as simple as a
two-way switch with a couple of LC components, to select the upper and
lower parts of the band), or go whole-hog and install a small
screwdriver antenna for HF.