was activated. These units which were extremely susceptible to interference were still in use into the 60's. With the exception of some 400 MHz equipment all units were on the same 27.255 MHz frequency and two people never flew at the same time.

With the popularity of R/C growing, more frequency were allocated (still on 27mhz) and more sophisticated equipment was used. This included the use of a modulated carrier frequencies and the superhetrodyne receiver. The superhetrodyne receiver (which is what we use today) had a front end which was tuned to the carrier frequency, followed by an IF (intermediate frequency) section, which was tuned to the industry standard 455 KHz.  An internal oscillator was required in the receiver to beat against the incoming carrier frequency and produce the 455 KHz IF frequency. This beating of the frequencies, called  "heterodyne" is the basic operation that we call conversion today.

The advantage of this conversion to 455 KHz is that it is easier to tune the bandwidth of this lower frequency and that one receiver, with a fine tuned IF bandwidth, can be made to operate across a wide carrier spectrum by changing the beat oscillators frequency. Applying this to our environment we see that the front end (RF section) of our receivers are tuned to a bandwidth which covers multiple channels. An internal crystal controlled oscillator, who's frequency is controlled by the Xtal plugged into the receiver, generates the beat frequency necessary to convert the incoming signal to 455 KHz. The 455 KHz IF section of our narrow band receiver is tuned to pass frequencies from 445 to 465 KHz (20 KHz).

If you are operating on channel 48, your transmitter is sending a carrier on 72.750 MHz. Your receiver Xtal would be 73.205, or 455 KHz above the carrier frequency (most receivers use an over crystal). The result is that when you frequency enters the receiver, and beats against the oscillator, it produces a 455 KHz result. If someone was flying on channel 47, 72.730, their frequency would also enter your receiver and beat against the oscillator. There frequency, 72.730, beat against the oscillator 73.205 would produce a beat frequency of 475 KHz (73.205 minus 72.730). Since your IF is tuned to pass frequencies in the range of 445 to 465 KHz the

there of. For the 72Mhz  bands used in our hobby this is defined at .75 Watts maximum.

Modulation: A method of superimposing a data signal onto a carrier frequency.

Amplitude Modulation (AM): A method of modulation where the strength of the carrier frequency  (amplitude) is varied  based on the data signal.

Frequency Modulation (FM) A method of modulation where the frequency of the carrier signal is varied  based on the data signal.

FM is generally defined as being less sensitive to interference than AM modulation.

Unlike radio or TV where constantly varying audio or video data is applied to the carrier, we apply  on/off data to the carrier. In AM mode this means that the carrier will switch from one power level (amplitude) to the other to represent a data bit. In FM the frequency is changed from a frequency below (or above) the assigned carrier frequency to a frequency above (or below) below the carrier frequency to represent a data bit. This frequency shift in our environment is usually 2.5 KHz.

Positive vs Negative Shift: We've stated that when working in FM mode,  Futaba and Hi-Tech are compatible, and that JR and Airtronics are compatible. This is because of the way that each has chosen to implement the frequency shift to represent a data bit. One has selected to send the lower frequency normally and shift to the higher frequency to represent a data pulse, while the other sends the higher frequency normally and shifts to the lower frequency to represent a data bit. Both Airtronics and Hi-Tech now offer a transmitter where you can select the shift mode which means that they can be set to work with either brand.

Single vs. Dual Conversion receivers:

As mentioned in the introduction, early R/C equipment operated on a carrier wave only which was not modulated with any data. When the button was pushed on the transmitter, the carrier frequency was turned on and when received by the receiver a relay