ok..
Basically, it's harder to build a very narrow filter to pick out your channel the higher you go in frequency. So, at 72+ MHz, it's really difficult to make a receiver selective enough to pick out a single channel.

So - they use an internal signal in the receiver called a local oscillator to mix with the input signal, shifting it down to a different frequency. This is called a "conversion" or shift of the frequency.

A single conversion receiver typically mixes your channel down to 455,000 cycles per second, or 455KHz. At 455 KHz it's pretty easy to buy filters that are narrow enough to pick out your channel. A dual conversion receiver typically mixes your channel down to 10.7 million cycles per second, or MHz the in the first conversion, does some of the filtering there, and mixes again to 455 KHz where it's filtered again.

Receiver sensitivity has nothing to do with number of conversions, that's a matter of the design of the receiver input itself. There is this thing called an "image" frequency that can cause some trouble, though.

For example - say your channel is 72.15 MHz. If we mixed that with a crystal controlled local oscillator at 72.605 MHz, we would get two output signals - the sum (144.755 MHz) and the difference (0.455 MHz, or 455KHz) from the mixer. The high frequency is far enough away it's pretty easy to filter out. We put a narrow filter on the 455 KHz signal, and there's your channel, and nobody elses. But, if a signal 455KHz ABOVE the LO (local oscillator) comes along, it also mixes with the LO and produces 455 KHz - in this case that would be 73.06 MHz. This "image" frequency is filtered out by the input section of the receiver, but the closer it is the harder it is to filter out.

So - along comes a dual conversion receiver. Same channel, 72.15. Mix it with 82.85 MHz, we get the sum (155 MHz, easy to filter out) and the difference (10.7 MHz, which is what we're looking for). THe 10.7 is filtered, then mixed again with a second LO, (10.245 MHz or 11.155 MHz) to get the 455 KHz, where it is filtered again. The image frequency of the receiver in this case is 10.7 MHz ABOVE the LO signal of 82.85 MHz, or 93.55 MHz - which is far enough away that the receiver front end filter has an easier time of getting rid of it.

JR took a different approach - they use typically single conversion receivers (less parts) and don't use the channels below channel 15 - so the image frequency is well above our RC band. It takes either a very close transmitter or a very strong transmitter to cause an image problem with them.

I've used both - Futaba and Hitec dual conversion receivers, and JR single conversion receivers - and had good luck with either. The actual design of the receiver and filters is probably more important than the number of conversions, dual conversion just makes it a little easier to get selectivity.

Any help?