The usable band-width of any antenna is limited, it's never going to be very wide. That depends on what you use to determine that band-width and the limits you put on it. The 'limits' are usually determined by what a transmitter will find 'acceptable' so that it will put out full power, or at least a 'typical' amount of power. That amounts to how much variation in the 'load' that transmitter can have before it get's 'cranky' and starts doing less. That's going to be a sort of 'wide' range depending on exactly what that 'load' consists of. If that 'load' has a lot of the 'right' stuff and none of the 'wrong' stuff then the transmitter is going to be really happy about it and 'eat' it all up, make the 'cook' happy cuz it ate it all, you know?
Now it starts getting a little complicated because RF is AC, not DC. That means that there's a few things added to the recipe when that 'cook' starts cooking. The new 'ingredient' in that recipe is reactance. Consider reactance as a spice or flavoring. That doesn't always make something taste delicious, sometimes it can get to be too much and ruin an otherwise nice meal.
What's the big deal about reactance electrically? The biggest thingy is that it doesn't contribute to producing power/radiation. Resistance is the only ingredient in the recipe that does that, that makes a signal radiate. So, getting rid of any reactance ->in<- that antenna is the idea. (Another fact about reactance is that it isn't always 'bad', it depends on where you find it and what it's purpose is. Typically, it isn't something you want in an antenna.) One definition of resonance is the absence of reactance, leaving only resistance. There are two kinds of reactance, inductive and capacitive, '+' or '-'. That means if you got some inductive reactance present then add just the right amount of capacitive reactance, they 'cancel' each other and you got 'no reactance' (or it's neutralized). Another fact is that reactance is frequency 'sensitive'. The right amount (of either kind) changes with the frequency you want the antenna to work on. Complicated enough for you yet?? Hey, it get's more complicated so hang on.
Reactance, inductive and capacitive is present in any/all conductors when you're dealing with AC/RF. That means a straight piece of wire has them too. So the right length of wire can be made resonant on any frequency all by it's self, no inductance or capacitance need be added. That means that if that wire's length is too long for whatever reason, you can make it appear electrically shorter. Or, if it's too short you can make it appear electrically longer. (Did that light bulb just turn on??
) Every thing is right with the world, right? Nope, sorry, that's not quite complicated enough so here's some more complication.
Impedance consists of both resistance and reactance in an almost infinitely variable amount of each. The 'ideal' input impedance of an antenna is usually considered to be 50 ohms, which means 50 ohms of resistance 'R' and no reactance 'X' which means the thing is resonant and has a 50 ohms input impedance. The problem is that the 'R' in that equation is almost never 50 ohms or even close to it. That means you have to make it 50 ohms so that your transmitter is happy. How the #3\\ do you do that!? There are a number of ways, they all mean adding just enough reactance to make the thing -appear- to be the resistance the transmitter wants. But to keep the antenna resonant that reactance needs to be outside of the antenna, which means an impedance matching device/scheme at the input to that antenna. Geez, can it get any more complicated? Yes it can, and does.
I'm going to stop here. Time to go to work. There's enough here that once you get your head around it things will make more sense. Sort of...
- 'Doc
This doesn't pertain only to CB antennas, but to all of them!
Now it starts getting a little complicated because RF is AC, not DC. That means that there's a few things added to the recipe when that 'cook' starts cooking. The new 'ingredient' in that recipe is reactance. Consider reactance as a spice or flavoring. That doesn't always make something taste delicious, sometimes it can get to be too much and ruin an otherwise nice meal.
What's the big deal about reactance electrically? The biggest thingy is that it doesn't contribute to producing power/radiation. Resistance is the only ingredient in the recipe that does that, that makes a signal radiate. So, getting rid of any reactance ->in<- that antenna is the idea. (Another fact about reactance is that it isn't always 'bad', it depends on where you find it and what it's purpose is. Typically, it isn't something you want in an antenna.) One definition of resonance is the absence of reactance, leaving only resistance. There are two kinds of reactance, inductive and capacitive, '+' or '-'. That means if you got some inductive reactance present then add just the right amount of capacitive reactance, they 'cancel' each other and you got 'no reactance' (or it's neutralized). Another fact is that reactance is frequency 'sensitive'. The right amount (of either kind) changes with the frequency you want the antenna to work on. Complicated enough for you yet?? Hey, it get's more complicated so hang on.
Reactance, inductive and capacitive is present in any/all conductors when you're dealing with AC/RF. That means a straight piece of wire has them too. So the right length of wire can be made resonant on any frequency all by it's self, no inductance or capacitance need be added. That means that if that wire's length is too long for whatever reason, you can make it appear electrically shorter. Or, if it's too short you can make it appear electrically longer. (Did that light bulb just turn on??
) Every thing is right with the world, right? Nope, sorry, that's not quite complicated enough so here's some more complication.Impedance consists of both resistance and reactance in an almost infinitely variable amount of each. The 'ideal' input impedance of an antenna is usually considered to be 50 ohms, which means 50 ohms of resistance 'R' and no reactance 'X' which means the thing is resonant and has a 50 ohms input impedance. The problem is that the 'R' in that equation is almost never 50 ohms or even close to it. That means you have to make it 50 ohms so that your transmitter is happy. How the #3\\ do you do that!? There are a number of ways, they all mean adding just enough reactance to make the thing -appear- to be the resistance the transmitter wants. But to keep the antenna resonant that reactance needs to be outside of the antenna, which means an impedance matching device/scheme at the input to that antenna. Geez, can it get any more complicated? Yes it can, and does.
I'm going to stop here. Time to go to work. There's enough here that once you get your head around it things will make more sense. Sort of...
- 'Doc
This doesn't pertain only to CB antennas, but to all of them!
