And the PT encoders/decoders do what exactly?... they take a carrier and MODULATE it with a PWM ENCODED signal at one end, and then decode it at the other end. They DON'T try to send RAW DATA. Actually the PT encoders DON'T modulate the carrier themselves, they provide the Tx Module with an equivallent 'AUDIO' signal which is then modulated within the AM or FM Transmitter Module. At the receiver end, the receiver, after demodulation, sends this 'Audio' component to the PT Decoder which then decodes the signal.

The usual junk Tx/Rx Modules are NOT the wireless equivallents of a piece of wire linking two ends of a Serial Link (like RS232).

Any Radio Ham will tell you (and there are quite a few on this forum) that you can't send high-speed RTTY (eg your Serial Data) using CW (A1A class modulation). If it was Audio (tone) Modulated (A2A) you'd have a better chance (because you obviously have either an AM or an FM Tx/Rx pair) and at least you'd be using the integral modulator - the way the designers of the modules intended them to be used. If you want range AND reliability, then for example you would pick say an F1 or F2 Modulation type.

Some Tx/Rx Modules have integral DATA MODULATORS with integral ENCODERS/DECODERS and can guarantee DATA-IN/DATA-OUT. Most modules haven't! They rely on YOU providing a signal (usually an AUDIO Tone or Tones) which are then modulated onto a carrier by either AM or FM (with FM having superior noise rejection - which is why you pay more for those). Now the PT encoders usually provide a 1kHz signal (give or take) which then varies depending on what information it conveys... note that 1kHz signal being the equivallent of your Audio Tone which is then MODULATED onto your carrier.

If you are just keying a carrier (which is what you are doing), all you are doing at the receiver end is the equivallent of switching your squelch control (a crude anomaly but will suffice for this explaination). Great. It works. But in doing this you have just sacrificed 95% of your range.