Interesting question. There are a lot of different designs, from the 1/4-wave
(unity gain) up to stacked 2-element ones (with the phasing coil between them).
Gain ranges from 0dB for 3" whip to about 5dB for 5/8-over-5/8.
There is no practical 1-wavelength antenna by itself, but there are 1/2-wave
over 1/2-wave designs. The "optimal" impedence is 50 ohms, and impedence
matching is done at the base for all designs if needed (the 1/4-wave doesn't).
There is always a trade-off between higher gain and narrower bandwidth,
plus extra cost; you don't get "something for nothing". Really high gain has
another disadvantage. The vertical angle over which this gain occurs gets
progressively smaller. In some cases you can end up with lower gain than
for a simpler antenna, such as going up and down hills etc.
The ones I use are typically older cellular antennas (mag mounts) with
3dB gain, in one case a 5/8-5/8 with 5dB. They all seem to work OK.
Avoid ones pre-wired with "spaghetti" coax due to high loss.
Dave
