Have been looking around for plans for a 1/2 or 5/8 2 meter mobile
antenna to play with.
The tricky part is rigging up the matching/loading coil.
Also, you might want to read up on actual (as opposed to simple-model)
results with a 5/8-wave mobile antenna, on a typical mount.
I've read a couple of reports which state that the theoretical gain
advantage of a 5/8-wave antenna, over a quarter-wave monopole, depend
a great deal on the 5/8-wave being mounted over a large ground plane
(several wavelengths at least). Without a large ground plane, the
5/8-wave tends to "squint upwards" - its primary gain lobe takes off
at an angle significantly above the horizon, rather than directly out
forwards and backwards from the vehicle. This can result in poorer
signal "towards the horizon" than a quarter-wave would give you - not
good if you're trying to do simplex communication between vehicles,
and maybe-not-good if you're using repeaters (depending how high above
horizontal the direct line to the repeater antenna lies).
5/8-wave antennas are also more prone to "lean back" away from the
vertical when you're driving at highway speeds as a result of air
pressure, and this also can cause the pattern to squint upwards more
than is true for a quarter-wave. The thin "spaghetti noodle" 5/8-wave
antennas such as Radio Shack used to sell (and perhaps still does)
would really have a problem with this, I think.
Anyhow, here's one set of plans:
http://www.vk4adc.com/web/index.php/...58th-wave-whip
The article has some comments attached which make reference to plans
in the ARRL Handbook (doesn't say which edition) which use a tapped
loading coil for impedance matching. Most of the 5/8-wave antenna
designs I've seen use this approach... it has the advantage that the
whole antenna is DC-grounded through the tapped coil, and thus static
buildup on the whip isn't an issue.
I believe I've seen other 5/8-wave mobile plans in various of the ARRL
Antenna Compendium collections.
http://www.arrl.org/files/file/Techn...df/8009022.pdf has
some other interesting approaches.