Okay, this may be a dumb question, but: Space constraints aside is there any practical reason to use fractional wavelength antennas? Why not use a full wavelength or taller? The specific application I am looking at is a 110w mobile in the 150-160 mhz range, the best condition/easiest to mount antenna on hand is an old Motorola low band(?) unit (102" whip+4" spring) that I would rather not cut any more than necessary. It would be mounted to the side of a steel box on a relatively slow vehicle so wind deflection shouldn't be much of an issue. Thoughts?
So, gotta ask....
110 watt VHF mobile. You want a BIG antenna. Who are you trying to talk to?
As stated the 102" whip isn't resonant at 150-160MHz, so that isn't going to work well. SWR will be high, might even damage your radio.
Longer antennas tend to have more gain. More gain can be a good or a bad thing, depending on what you are looking for.
Antennas achieve gain by focusing more energy towards a specific point or points. On a mobile antenna, a higher gain antenna will focus more RF energy towards the horizon. This CAN be useful, if you are out on the flatlands. However, it can also be a drawback in the mountains. Lower gain antennas radiate more in all directions. This can be helpful in some situations, like a mobile in the mountains, or a VHF antenna on top of a sail boat mast.
Also, to make the antenna provide a proper 50Ω impedance match, which the radio --really-- wants to see, you need to match the impedance of the antenna to the rest of the system.
With a 1/4 wave length antenna (about 18 inches tall on VHF) their characteristic impedance is 50Ω, so no matching is required.
1/2 wave, 5/8 wave, full wave, collinear, etc. will all require some sort of matching.
In the end, the coverage you are trying to achieve AND the ERP limitations on your FCC license will all dictate what sort of antenna you need. Bigger isn't always better or legal.