Choosing a zener for dynamic Rload variations
If you want something that just works, use a linear voltage regulator. They're available for pennies. If you want less power dissipation, use a switching regulator. 

If you want to use your circuit, the things to ensure are that the value of Rs is small enough to provide enough current at 3.3V, and physically large enough to dissipate all the heat that it generates, ~450mW by my calculation (12v-3.3v/50mA). Your zener also needs to be able to dissipate the required power, which will be ~160mW (3.3v*50mA). Your voltage regulation probably wont be great, but your radio can probably deal with it (although if you post what the radio actually is that would help). You should put a capacitor across the output of your voltage regulator.

Overall, 600-700mW is quite a lot of power to be continuously dissipating in a small space. You need to account for the temperature rise that this will cause. The issue with this circuit, compared to a linear (series) regulator, is that it always consumes the same amount of power, which is the maximum amount. A series linear regulator would be a much better fit for this application, as presumably the 50mA draw doesn't happen much of the time (although even if it happens 99% of the time, a series regulator is better).

Try your circuit out here.