RF Module Not Working
The "voltage" you see should be the RMS of a modulated voltage, right? That's what the encoders are supposed to do.

What does the oscilloscope say about the output of the HT-12E and input to the HT-12D?

Also, if you hook the HT-12E directly to the HT-12D without the RF modules, do they work?

If you hook the TG-11A to the TG-11B directly without the encoders, do they work?

Finally, is there a particular reason you have to roll all of this from scratch? Using an existing module, be it a cheap NRF24L01 module ( http://imall.iteadstudio.com/wireless/im120606002.html ) or a robust Xbee module ( http://goo.gl/noMZ7Y ), will save a lot of time and leave the RF engineering to a team that has all of the right tools.