A shortened 5/8 wave mobile antenna, or a shortened 1/2 wave mobile antenna, or a shortened full wave mobile antenna. How do you shorten any 'size' antenna? By 'loading' it with the appropriate amount of inductance in the form of a coil of some 'shape' until you end up with an antenna of the height that you want. You then have to also add an impedance matching device so that it will end up close to a 50 ohm input impedance. A typical device like that can also have a capacitive component as well as an inductive one. You know, that just doesn't work. It's just a 'play on words' as to what that length of radiator is called. It no longer acts like a full sized 5/8, 1/2, or full wave antenna. It does not produce a radiation pattern the same as any of those full sized "size/type" of antennas. You can call it anything you want, but that doesn't make it so.
What you have done is produce a resonant antenna of whatever particular length you happen to want. Resonant meaning no reactances present, which means it radiates everything that gets to it, which is good. That's the whole point, right? Making it shorter so it's more convenient? Not a thing in the world wrong with that! Just don't expect the same radiation pattern you'd get from the full sized antenna, it'll never happen. Test it any way that you want to, doesn't make a bit of difference, the results will be the same as what I'm telling you. Doesn't matter what frequency range you're talking about either.
Another one of those 'myths' is that the wire used in winding that coil is the equivalent of the full length of whatever particular length of full sized antenna that it's used to make that full sized antenna shorter. No, it don't. The length of the conductor that makes up that coil has nothing to do with the "length" of antenna it's supposed to be replacing. The size and shape of that coil determines the amount of inductance it will produce at some frequency much more than the total length of that conductor affects things.
Another little thingy that people don't seem to think about is that any coil has losses. An antenna made entirely out of a coil has more losses than an antenna that uses 'less' coil. That means that a helically wound antenna is the least efficient of any 'shortened' antenna.
Lot's of meaningless/incorrect/dumb ideas when you start talking antennas and their characteristics. Is my post above one'a them sort of ideas? ... Prove it to me. I dare you. Opinions don't count, repeatable facts do.
- 'Doc
