That's the biggy, really. Resonance doesn't have a -set- impedance, so the SWR with a resonant antenna can be almost anything, very seldom, if ever, 50 ohms or close. There's -always- two parts/steps in tuning any antenna, obtaining resonance and getting the input impedance to where you want it. Sometimes you get lucky and the input impedance ends up close to what you want, so there's no need to adjust/change the input impedance. You can 'fudge' a little, get one of those two things close and make the other one what you want. But that means that the part you 'fudged' is then off a little (how tuning most antennas ends up). It works, but is never the 'best' it could be. But if it's "close enough" then... it's close enough, you know?
What's the big deal with resonance? Resonance means that there's no reactance present in an antenna, not that it's always a particular length (loaded antennas for instance). Reactance doesn't contribute to radiating anything, that's the biggy! If part of the signal getting to that antenna isn't radiated then it's wasted, might as well not have been produced to start with. So getting rid of reactance -IN- the antenna is good. It means that all the signal getting to that antenna is radiated.
There's another aspect about antenna lengths that's important. The shape of the radiation pattern is determined by antenna length. You may be radiating everything coming out of that transmitter, but if it ain't going where you want it to go, then 'they' won't hear you (or you them). That's why particular length antennas are desirable, they produce a radiation pattern that put's the signal where you want it and hears where you want to hear. A very rough way of thinking about antenna lengths is that the shorter they are the less range when compared to a 'full sized' antenna. There are limits to that, antennas can be too long to.
Ain't very 'simple', is it...
- 'Doc
