Made with Xara Web Designer 1954- Rocky Mountain Modeleers In the early 1990's when narrow band Radio Control (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 caused Radio Frequency (RF) noise on all RC channels. In the pre-91 era, the FCC assigned Model Airplane Radio Frequencies to even numbered channels with 40 kHz bandwidths. In 1991 RC channels were increased to 50 channels using both the odd and even RC channels. The bandwidth of these channels was reduced to 10 kHz. With newer technology, RC transmitters and receivers were redesigned to meet these specifications. These new specifications solved model airplane flyers concerns during the early 1990's. Let us briefly discuss the internal workings of radio receivers to understand the 20/21 RC channel problem. 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 received transmitter's frequency is detected by the receiver's amplifier mixer, which 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. From there, an audio amplifier detector processes the signal into signals the servos can use. 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 at the flying field. If a RC transmitter on channel 20 is in use, its frequency (72.190 MHz) will also be received by all of the RC receivers. These two additional frequencies are mixed by the amplifier-mixer just like the original two frequencies. Channel 4's audio carrier frequency acts like another local oscillator. It will mix the Channel 20’s audio and generate a difference frequency of 440 kHz. 440 kHz was in the range of the old wide band 455 kHz IF amplifier, so the IF amplifiers 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 transmitter channel 20 at RC flying fields where there was a strong TV channel 4 signal. With demand increasing for more channels in the 72-Megahertz radio frequency band by Model Airplane users, these radio frequencies became crowded. Also industrial use of remote control hoists and pagers were becoming widely used, so the FCC responded by requiring existing users 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. The 1991 FCC standard allowed 100 channels to fit into the 72 to 73 MHz band. The FCC assigned odd 10 kHz carrier (such as 72.010, 72.030 MHz) frequencies to the RC community. 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 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 carrier 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'vve got the problem at every flying field when we use 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 second order 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. Since then the Front Range clubs have complied with FCC regulations by requiring all transmitters to be narrow band and highly recommend that that RC pilots switch to Dual Conversion narrow band receivers. This makes RC channels 20 and 21 useable at the field. You can be safe 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? I can think of one issue. That is, if your club allows members to use channel 20 and/or 21, you may have trouble using that channel at fields where 20/21 is still forbidden. Information for this discussion is based on articles printed in 1) Model Aviation, June 1989, Radio Technique by Myers and 2) LAMA-RC newsletter, date and author unknown. (Updated August 2003)