RF transmitter circuit explanation
Firstly, the circuit shown is just a carrier wave generator - it's an oscillator that produces a constant sine wave; there is no provision to be able to modulate that carrier because there is no defined input to do that. So, rather than a transmitter, I'd call it a beacon - it generates a fixed carrier wave with no modulation.

Generally, you can put a small AC voltage on the biased base with respect to the emitter of a BJT circuit and you will likely get an amplified version of this at the collector. This is usually called a common emitter amplifier but if you think about it, you can also hold the biased base voltage steady and wobble the emitter a bit and you will get the same voltage amplification. This is a common base amplifier.

Why choose the emitter for an input - if you held the emitter constant and applied a small positive pulse to the base, the BJT would turn on a bit more and the collector voltage would produce an amplified negative going pulse. Whereas, if you held the base constant and applied a small positive pulse to the emitter you'd be slightly shutting-off the BJT and this would produce a positive pulse on the collector. In other words, the voltage at the collector is an in-phase amplified version of the voltage applied to the emitter in common-base configuration. 

It's the opposite on the base and hence positive feedback cannot be achieved with a simple capacitor like C5 but, it's positive feedback on the emitter and thus will help the tuned-circuit sustain its sine wave.