does having encoder/decoders in 433MHz rf links improve range and reduce noise?
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How do you intend to send the PWM value? You can send a 4 bit value as data between the HT12 pair.

The encoder/decoder do not help with the range but you get rid of false transmissions. The HT12 encoder sends the message three times and the address needs to be the same on both decoder and encoder before a Valid Transmission is recognized by the decoder.