KingCobra,
Dipoles are directional given a few considerations. One of those considerations is how high above ground it is. That height is measured in wave lengths, not just feet. That means that an antenna made for a lower frequency would have to be higher than one made for a higher frequency for it to be very directional at all. After about 1/4 wave length, a dipole starts getting to be directional till it gets to something like a full wave length, where it sort of averages out, kind of. Don't count on it being as directional as some kind of beam, it never will be, but will tend to put a signal out more in a couple of directions than all of them.
A dipole, a horizontal one, tends to be directional 'broadside' to it's self, off to the sides, sort of. There's still signal off the ends of it, but not as much as the sides. A very general sort of pattern is like... " O|O " with the dipole being the "|" thingy and the "O" thingys being the radiation pattern. Don't take that very literally, it's only a generalization, and a really 'crappy' one at that.
It'll give you a general idea anyway.
So how long should a dipole be? It's frequency dependent, right? So the 'magic' number to use is "468". You divide that 'magic' number by the frequency in Mhz, and you get an answer in feet. So, 468 / 27.185Mhz = 17,21 feet, or about 17 feet and 2.5 inches. That's going to be slightly too long, which is better than it being slightly too short, cuz it's easier to shorten an antenna than to make it longer. You probably also want to allow a few extra inches so you can make wraps, and connections. And extra foot or two isn't going to hurt anything, just wrap that 'extra' back onto it's self at the ends and don't worry about it (electrically, it disappears, sort of).
There's always two steps to tuning any antenna. Making the thing resonant by adjusting it's length, and then matching it's input impedance to that of the feed line and radio. That first part, the resonance thingy, you can get close to with that formula. Closeness does count.
The second part of the tuning is to get the thing at least close to 50 ohms input impedance. That can be as 'tricky' as you want to make it. Some facts first. A typical 1/2 wave dipole at about 1/2 wave length above ground, has an input impedance of around 75 ohms (that's not exact, just an approximation). At best, that'll give you an SWR of about 1.5:1. Not really all that bad at all. But, there's an easy way of reducing that 75 ohms to something very nearly 50 ohms. That's by reducing the angle between the 'legs' of that dipole less than 180 degrees, letting them 'dangle' a bit, slope down some. When the "angle of the dangle" gets close to 50 ohms impedance, or the SWR gets closer to 1:1... quit. That's good enough. It doesn't take a lot of 'dangle' to get that 75 ohms closer to 50 ohms, but it'll take some. If that angle between the dipole's legs ever gets close to 90 degrees you've gone way too far, something else isn't 'right'.
That's a very, very genral way of doing things with a dipole. There are others. Using the length of feed line as a tuning tool is just not a very good idea except in very specialized circumstances. A typical dipole type thingy just doesn't fall into one of those special categories. Use whatever length of feed line it takes to get from the antenna to the radio, with a few feet 'extra' to make things easier to deal with. Coax doesn't 'stretch', so more is good to some ridiculous point. Lots of variations in all that, do it how is 'best' for you.
- 'Doc
