BOOTY MONSTER,
You ready for this??
There are two types of coils used with antennas. One of them, a 'loading coil', effectively makes the antenna longer, sort of. The other coil is an 'impedance matching coil'. That thing makes the antenna's input impedance closer to 50 ohms, what most radios want to see when looking at an antenna. These two types of coils can be in different places in the antenna, or, can be combined at the input of the antenna, or, the whole dang thing may be wound into a coil to really make the antenna shorter.
At alternating frequencies there's such a thing called reactance. It basically deals with the differences in timing between current and voltage levels, current and voltage do not increase of decrease in 'synch' with one another. A plain old piece of wire, any shape including straight, has reactance at radio frequencies, how much and what kind (inductive or capacitive) depends on its length in relation to the frequency (wave length). Something else that's a characteristic of any kind of conductor is resistance (ohms). So any conductor/antenna has those three thingys, inductive reactance, capacitive reactance, and resistance.
It gets a little more complicated so hang on.
For an antenna to be resonant is has to have equal and opposite reactances, which when 'added' equal 'zero' (equal reactances of opposite polarity =0). Since reactance has 'canceled' out, the only characteristic left is resistance. That's resonance.
Antennas tend to produce the most radiation when they are resonant. Radiation is a product of current and voltage, the same as power. Voltage times current equals watts. One interesting characteristic of alternating current is that power is only developed in a resistive load, not a reactive one. So, getting rid of all the reactances is a very good thing in regards to power radiated. No reactances in an antenna is good (resonance, right?). Resonance is also not strictly necessary! Just depends on where the @#$ thing radiates, as in where you want the power to go, as in radiation pattern. (Not exactly what you asked about so forget about that part for now.)
There are formulas for determining reactances in conductors. The two that are most used are for straight conductors, and for coiled up conductors. If you know the length and diameter of a straight chunk of wire you can figure it's reactance at a particular frequency. Same for coils, diameter, number of turns, spacings, and total length of the coil. If you determine that there is so much inductive reactance needed to make some length of conductor resonant at a particular frequency, you can get very 'close' with those formulas, and since no one (except me) is perfect, you gotta do some 'trimming'/adjusting to get that length or coil just right in each circumstance. You than call that kind of coil a 'loading coil'. It can be at the bottom, in the middle, or at the top of the antenna. When that coil is the right size the antenna is resonant, no matter what it's total length.
In the case of the antenna being too long, then you don't add inductive reactance, but you do add capacitive reactance. Inductive reactance is (+), capacitive reactance is (-). Coils make antennas longer, capacitance makes antennas shorter. (Ever heard a capacitor called a 'trimmer'? That's cuz it tends to 'trim' things shorter, sort of.)
Now for 'matching coils'! (All this @#$% gets sort of complicated, doesn't it?)
There are various combinations of reactances that change the -apparent- resistance of an antenna. The actual resistance isn't changed at all, but it ,makes the transmitter 'think' it sees whatever resistance it's looking for. (At DC, it's called resistance, at AC it's called impedance and can really get to be a messy thing cuz impedance is a combination of reactances and resistance.) Remember that no radiation from reactances? There is always some losses when using matching coils (or matching capacitors), BUT, if that loss is small compared to what would be lost if the transmitter doesn't see it's desired resistance (50 ohms +/-), it doesn't amount to anything, and is worth loosing for the gain in overall radiated power.
There are always more than one way of doing things so adding a matching coil/capacitor isn't always the easiest way of doing stuff. The 'angle of the dangle' of radials, or the legs of an antenna can do it to some extent. If the overall length of the antenna is really 'different', then count on using some means of matching. None of the typical lengths of antennas 1/4 wave, 1/2 wave, 5/8 wave, full wave, etc, have an input impedance of 50 ohms. So, doing some 'matching' is always something that's done in one way or another. (Depends on where the antenna is 'fed' too!)
The next question you will ask is, "What brought all that on?".
And the answer is that thats the way all this @#$% works. If you can get sort of 'wrapped around' all that, you've got it made!
- 'Doc
Capacitors are called 'cow-pastures' for a good reason, they are full of 'cow-S_ _ _'!
Loading coils are called that cuz they're a 'load of S_ _ _"!
There's almost always a reason for what things are called, right?
This is NOT a very good way of explaining any of this, but if you want the exact reasons... buy a basic electronics book. It took me longer than I care to remember to find out this "S _ _ _" and I am certainly not a good teacher. Good luck!
