In certain geographical regions, if you use a single conversion RC receiver you may encounter interference due to powerful VHF radio transmitters operating in the 103 - 105 MHz area.  With a single-conversion receiver the reception frequency is approximately 455 kHz (intermediate frequency - IF) below the transmitter frequency. A radio transmitter operating on 104 MHz produces interference on the 35 MHz band which can affect single conversion RC receivers. The dual-conversion process utilizes a second, internal crystal which changes the intermediate frequency in such a way that this interference can have little if no effect.

    In countries other than Germany, similar effects have been noted on the 40 MHz band. Here again dual conversion receivers reliably eliminate this type of interference.   If you regularly fly at sites whose reception conditions are not known to you, you can feel confident of avoiding this kind of interference by using a dual conversion receiver. If you always fly at the same site, simply switching channels can often help if you encounter interference of this type.

    In the early 1990's when narrow band RC channels were first introduced, RC channel 20 was off limits at many flying sites. The reasoning was that a strong TV channel four signal mixed with RC channel 20 was causing noise on all RC channels.

    Almost 15 years has passed since the RC aircraft frequencies have expanded to 50 channels. Before 1991, there were only even numbered channels spaced 40 kHz apart. RC receivers in the pre-91 era used 40 kHz bandwidths as compared to the narrow band 10 kHz receivers of present day.

Let us briefly discuss the internal workings of radio receivers. Most radio receivers use a local oscillator (LO), running at 455 kHz below (or above) the received frequency.  The LO's frequency along with the transmitter's frequency is sent to the receiver's amplifier mixer. The mixer generates two additional frequencies, the sum and the difference of the original frequencies. All four of these frequencies are passed to the Intermediate Frequency (IF) amplifiers, which selectively amplify the difference frequency (455 kHz) and reject the other three frequencies. From there, a decoder/audio amplifier processes the signal into signals the servos can use. A typical receiver block diagram is shown below (this taken from an 8 channel Heath Kit receiver my father built)

With this introduction to mixer theory, we can discuss some of the problems and the reasons for excluding RC channel 20 (72.190 MHz). Television's channel 4 audio carrier is located at 71.750 MHz. If channel 4's field strength is strong in the area, it will be received by all of the RC receivers regardless of the RC channel in use. If RC channel 20 is in use, its frequency (72.190 MHz) will also be received by all of the RC receivers. These two frequencies are mixed by the amplifier-mixer just like the original four frequencies. Channel 4's audio carrier frequency acts like another local oscillator. It produces a difference frequency of 440 kHz when mixed with channel 20.  440 kHz was in the range of the old wide band 455 kHz IF amplifier frequencies, so the IF would amplify 440 kHz as well, sending this signal as noise to the servos. Notice that all RC channels would be sensitive to this problem. The solution was not to use RC channel 20 at RC flying fields where there was a strong TV channel 4 signal.

     As the radio frequencies became crowded, demand for more channels went up. Pagers were becoming widely used, so the FCC responded by requiring existing users, including RC frequencies, to use less space around their center frequency.  To the RC world, this meant reducing the bandwidth of their transmitters from 40 kHz to 10KHz. This 1991 FCC standard     allowed 100 channels to fit into the 72 to 73 MHz band. The FCC assigned to the RC community all of the odd 10 kHz carrier frequencies. The even 10 kHz frequencies went to support the pager frequencies.

    Well, new problems! Now there was channel 21 (72.210 MHz). It mixed with TV channel 4 and produced a 460 kHz difference frequency. So even though RC had narrow band transmitters and receivers, 460 kHz was at the edge of the IF amplifier's new 10 kHz bandwidth and could cause some problems. In fact, the 1991 narrow band standard solved the RC channel 20 problem by making the IF a 10 kHz narrow band amplifier. Besides the TV channel 4 interference, which only caused a problem around strong TV transmitters, there was an even greater danger to all RC using the narrow band standard. This was known as the "23-channel second order modulation problem". For example, let us turn on RC channel 38.  Its frequency is 72.550 MHz.  No problems until another pilot turns on RC channel 15 (72.090 MHz). These two frequencies get mixed in everybody's RC receivers and produce a difference frequency of 460 kHz. Have we heard this before, it is the same problem as TV channel 4 and RC channel 20, however, now we got the problem at every flying field when we have two RC transmitters 23 channels apart. This is not good and the problem had to be solved.

    Well, the FCC along with the AMA and all of the RC manufacturers recognized this. They came up with the DUAL CONVERSION receiver design. As the name implies, another LO was added as a pre-amplifier. The first LO mixes with the received frequencies to produce a 10.7 MHz difference frequency. The bandwidth of the stage is 20 kHz, so the 23 channel and channel 4 problems are solved. The 10.7 MHz frequency is passed to the second LO where it is mixed again to convert the signal to the 455 kHz IF. The 455 kHz IF amplifiers were cleaned up to meet the 10 kHz narrow band specifications.  The closest unwanted frequencies now come from the adjacent pager frequencies located only 10 kHz away from our RC carrier frequencies. The 455 kHz IF narrow band amplifiers attenuate these frequencies, which have difference frequencies of 465 or 445 kHz, so there are no problems.

    In 1990/91, Front Range RC clubs recognized that some time would be needed to switch to the narrow band transmitters and dual-conversion receivers. Many years has elapsed and it is time to revisit the subject maybe one last time to discuss and decide if RC channel 20 and channel 21 can now be used safely. You can help by checking your receivers. Is 'dual-conversion receiver' printed on the case? Does the receiver use a crystal? Do you remember if you bought the receiver as 1991 compatible?