ORIGINAL: laryboy
dual conversion has nothing to do with noise rejection. it is not a consideration in range calculation. single conversion has a vulnerablity to another transmitter at just the right frequency such that a 455 harmonic is generated and is passed thru your receiver. not good . dual conversion recievers with 2 xtals were invented to obviate this vulnerability. jr has a way of taking care of this with single conversion. i believe that when you buy a single conversion receiver you have opened yourself up to that vulnerability. park flyers seem to get away with it but i dont know why.
The image frequency is not a harmonic. A transmitted frequency(fundamental or harmonic) on the image frequency will be received just as well as the desired frequency. It must be rejected in the RF stage and cannot be rejected in the IF or compensated for in the decoder.
Any coded system will work better if it has a double conversion RF stage. Its probably not needed for short range receivers but would make them better if supplied. Thats why all major suppliers use it.(even JR & Hitec)