What are the basic components of an RF transmitter, and there respective roles ? (I'm learning electronics so excuse the newbie-language!). Any help would be great.
Transistor(s) - used to amplify the signal ? Possibly what else? Resistors - to limit current entering transistor base, collector and biasing ? Capacitors - ??? filter out higher frequencies than needed? Why are caps sometimes connected to the collector of a transistor? Crystal - I understand the concept of this, i.e. say 27 Mhz - does it filter the inductor frequency to precisely what the crystal is labelled at? Inductor (which antenna eventually links to) - does this generate radio frequency (which must then be amplified by transistors)? Signal Diode - ?
Sorry again if I sound like a newbie, I'd like to learn as much as I can bout RF circuits.
Get yourself a copy of the Amateur Radio Relay League (ARRL) Handbook. It's published every year or so, and has been since about when radio was first invented, so you shouldn't have trouble finding old editions for not too much money.
The ARRL Handbook is chock full of very good introductory material on basic electronics, RF electronics, and practical applications. It is precisely aimed at enabling people like you - intelligent people with an interest but not much academic background in electronics - to learn and do interesting things with radio electronics.
But it is just not possible to answer your questions in a brief Usenet post, in a way that will actually give you useful understanding. Asking "what does a capacitor do in an RF circuit" is a lot like asking "what does a + sign do in a mathemetical equation". The nature of a capacitor is simple (at least theoretically - at RF, things don't always behave like perfect components), but it can perform many different functions depending on the circuit that it's in, limited only by the creativity of the designer. The magic happens in the relationship between the components, not in the components themselves.
The components you listed could be considered the basics of practically ANY electronic product. It would be better to ask about the basic "blocks," or fundamental circuits which would be used together to create a practical transmitter.
For RF communications, the most common "basic building blocks" would be:
Oscillator: This is a circuit which generates a sinusoidal output (a "sine wave signal"), or more broadly SOME form of periodic waveform, with no input other than power. At a minimum, it is a transistor or some other active element which is basically being used as an amplifier, plus some form of frequency-selective positive feedback path; that latter item may be either a combination of components such as capacitors and inductors which act as a filter, or a quartz crystal - which ALSO acts as a filter, just through a slightly different mechanism. There are various named classes of oscillators, which are most often distinguished by the form this filter and feedback path take.
Modulator: A circuit which somehow varies the signal produced by the oscillator, in accordance with the variation of an input signal (i.e., the signal carrying the information you want to transmit). There are various forms of modulation, and so quite a few different forms of modulators. This is too complex a subject to even begin to cover here.
Output amplifier: A circuit which increase the power of the modulated signal for transmission, and which drives the antenna. It is possible in some types to combine this "output amplifier" function with the "modulator" function, but you'll learn more about that as you get into the specifics of transmitters.
There can certainly be other circuits and components involved; there can be additional amplification between these stages, tunable oscillators (for varying the transmit frequency), additional filtering here and there, and of course all of this is going to require an appropriate power supplier. I would strongly second the recommendation that you find a good book covering the basics of RF and aimed at the amateur or hobbyist - as noted, the ARRL Radio Amateur's Handbook is certainly among the best choices here.
Early simple transmitters used only coils, capacitors, a spark gap and battery. Do a Google search for "spark gap transmitter" for more information. Here's one example:
Thanks for everyones post. Yes, I realised after posting that the questions were extremely vague - I know a capacitors have hundreds of uses, it was stupid to put the question like that.
I had meant that in a basic 27 Mhz transmitter, like that used for radio controlled cars (late 70s), what the BASIC foundations were - Bob Myers answered this beautifully. I had thought that the xxx pF capacitors etc that were connected to the transistors were used for a particular purpose that was common to alot of RF circuits.
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