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The signal from the R/C transmitter drops off rapidly with distance from the model and needs to be amplified many thousand times by the model receiver.

Amplifying the 72 MHz high frequency signal is tricky because it is so high that the coils, capacitors, wire lengths, transistors, etc. are critical. To make things worse, noise is generally endemic at high frequency. On the other hand, low frequency signals amplify very well with little induced noise or loss of quality. For example the milliwatt signal of Madonna bawling into a microphone can be amplified to fill Qualcomm Stadium with minimal loss in Fidelity (but who would notice?).

Back in the 1930s a crazy nut, but a genius, Armstrong (father of modem radio) discovered that if the receiver injected a constant signal of its own the result was a new signal at the difference (and sum) of the two with the same modulated information. For example if a received signal of 72.5 MHz is mixed with an artificial constant signal of 72.0 MHz the result is a signal of 72.5 - 72.0 = 0.5 MHz containing the identical pulse train as if the transmitter had used 0.5 MHz to start with. You didn't want to know, but the affect is called Superheterodyne mixing, or Superhet for short (us old timers may recall the advertising hype for Superhets some 40-50 years ago). 0.5 MHz is far more easily amplified than 72.5 MHz and almost all modem receivers, including R/C receivers are Superhets. The difference, or "Intermediate Frequency" (IF) for R/C receivers is around 455 Khz (0.455 MHz) which is the same as that for most broadcast receivers (surprise!) Actually it meant that the original R/C receivers could use the same cheap IF amplifier components as most radios.

Virtually all broadcast receivers have a single IF frequency i.e., they are "single frequency conversion" (or simply Single Conversion). This is fine because broadcast stations are typically separated by 200 Khz (e.g., FM stations at 95.1, 95.3, 95.5 ....) so 455 Khz can easily sort out adjacent stations (provided one is not in LA and the other in San Diego). It was also OK for R/C until 1991 when there were only 7 channels in the 72 MHz band with a 100 MHz separation. Imagine only a handful of channels total at Torrey Pines with the glider traffic of today! So in 1991 the AMA reduced channel separation to 20 Khz allowing 1.0/0.002 = 50 channels between 72 and 73 MHz. Now adjacent channels produce IFs of 435, 455 and 475 Khz which could cause a neighboring channel transmitter to glitch an older receiver if the IF component tuning is sloppy. Current production techniques produce acceptably tight tuning and AM receivers (which are limited to signal ON/OFF, remember) typically limit their IF span to less than 5 Khz each way i.e., anything out of about 450 460 Khz is lopped off drastically thereby eliminating the 435 and 475 interference. So AM is happy as a clam with 20 Khz channel separation. The problem is with FM.

Remember FM, or more precisely FSK (Frequency Shift Keying), uses the carrier frequency for OFF and a shift of 5 Khz for ON (+5 Khz for Airtronics camp, -5 Khz for Futaba camp). Consider an Airtronics transmitter on channel 14 (72.070 MHz carrier) sending 72.070 MHz during OFF and 72.075 MHz for ON. So a Single Conversion FM receiver would generate an internal signal of 72.070 + 0.455 = 72.525 MHz. This results in an IF of 455 Khz for OFF and 455 + 5 = 460Khz for ON. Now someone shows up with a Futaba transmitter on channel 60 (72.990 MHz) and happily bleeps out 72.985 MHz for ON. The channel 14 receiver, not knowing any better, mixes the 72.985 with its internal 72.525 and comes up with 72.985 - 72.525 = 460 Khz which is its ON!! So ... crash. The Superhet mixer produces the difference of the internal and external signals, irrespective of which is which; by adding special filters and other wizardry it is possible to sort out the components, but this adds to the direct cost and production labor for tuning. A more common technique is to 2 stage the IF, ala a rocket booster. First a 10.7 MHz IF is produced and amplified, then a 455 Khz IF. The 10.7 MHz IF knocks out "in-band" interference from your pal's transmitter. This "Dual Conversion" process uses a 2nd crystal for the 10.7 MHz to 455 MHz conversion. So for example a channel 14 Dual Conversion receiver has a 1st crystal of 72.070 - 10.7 = 61.370 MHz and a 2nd crystal of 10.7 - 0.455 = 10.245 MHz.

Dual Conversion is no panacea. It adds cost, weight, more things to go wrong and an extra crystal to crack in a crash. JR has taken note of this and their receivers stick with Single Conversion FM, using clever engineering design in their patented ABCBW circuit (Anti-Blocking Cross-modulation and Windowing).

Bottom line:
Dual Conversion is needed for FM to eliminate in-band interference from other transmitter.
AM receivers don't need Dual Conversion which explains why they are generally cheaper, lighter and more robust.

The frequency of the receiver crystal is NOT the channel frequency e.g., the receiver crystal for channel 14 (72.070 MHz) is NOT 72.070 MHz but displaced by the IF frequency. The exact IF frequency is vendor dependant so DON'T MIX CRYSTALS FOR DIFFERENT VENDORS. (e.g., don't plug a Futaba crystal into a Hitec receiver and expect it to work.)
Transmitters have no Superhet mixing so transmitter and receiver crystals CANNOT BE INTERCHANGED (transmitter xtals also use "5th overtone" so they are a totally different animal).

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