Spread Spectrum RadiosThe introduction of Spread Spectrum radio systems for RC aircraft is the beginning of the end for frequency control at the airfield. With spread spectrum systems there is no more waiting for the frequency pin! No more worrying about interference from your engine, motor, or ESC's! The kid down the street from club can't shoot you down while flying his park flyer!
What is Spread Spectrum?The most primitive form of spread spectrum technology was developed during WW2 for encrypting signals that controlled torpedoes without the enemy detecting or jamming the signals. Today this technology is used in just about every aspect of military communications. This is the most secure radio link that has ever existed. Spread spectrum technology is also responsible for the success of many modern household items such as Wi-Fi systems, blue tooth communication, cordless phones, and Global positioning system (GPS).This technology allows a transmitter and receiver to be “mated” so that the receiver only listens to that particular transmitter. There are two ways of achieving this. The two methods are Direct Sequence Spread Spectrum (DSSS), and Frequency Hopping Spread Spectrum (FHSS). Both DSSS and FHSS radio systems transmit in the 2.4 GHz frequency band. The FCC requires that any device that transmits in this frequency range must first scan the frequency band and use only the narrow band frequencies within the 2.4 GHz frequency band which are not in use by another device. This makes it possible for many devices to utilize the 2.4 GHz frequency band at the same. Direct Sequence Spread Spectrum (DSSS)DSSS systems transmit on a single selected frequency with a much wider bandwidth than the original data signal. The original narrow band data signal is sent through a “spreading code generator” within the transmitter. The spreading code generator multiplies the original narrow band data signal buy a much higher frequency. The spreading code generator also multiplies the signal by a coding scheme which is unique to each radio. This coding scheme could be as simple as multiplying the signal by 1 or -1 in a systematic way that appears to be random.The 2.4 GH transmission will appear to be random white noise for any receiver other than the mated receiver. The mated receiver picks up this “noise” signal. From the mating process the receiver knows the coding scheme used by the transmitter. It uses this to reconstruct the signal back to its original form. Spektrum is the industry's leader in designing and manufacturing SS radio systems for RC aircraft. The first such system was available only for park flyers. When developing this system Spektrum developed a proprietary coding scheme called Digital Spectrum Modulation (DSM). Spectrum utilized DSM technology to develop the DX6 radio system which was the first spread spectrum radio system designed for aircraft use. Spektrum had to go back to the drawing board and come up with an entirely new coding scheme which is DSM2. Spektrum utilized DSM2 technology to develop the Spektrum DX7 radio system The term DSM is often mistakenly used to refer to all spread spectrum radio systems for RC aircraft. While this used to be the case, Futaba now has SS radio systems available that uses their own technology. Frequency Hopping Spread Spectrum (FHSS)With an FHSS system the transmitter utilizes a narrow band frequency within the 2.4 GHz range. It continuously changes its frequency several times a second. From the mating process the receiver knows the code or pattern of frequencies that the transmitter will utilize. It then listens on the appropriate frequency as the transmitter changes from one frequency to the next.Futaba developed its own Futaba Advanced Spread Spectrum Technology (FASST) that utilizes FHSS spread spectrum technology. Futaba utilized FASST technology to develop the
Futaba 6EX 2.4GHz System
which can be uses with both park flyers and larger airplanes. Why Two Receivers?Because the wavelength of the 2.4 GHz frequency is so small it can't go around objects very well. The 2.4 GHz signal can be easily blocked by metal objects on the airplane such as the engine or landing gear. A second receiver is added to the system so that at least one of the receivers will have a clear unblocked signal from the transmitter.Spectrum's systems use their proprietary DuaLink technology. With this system the transmitter transmits on two different frequencies simultaneously. Each receiver receives the same information on each of their respective frequencies. If one receiver's signal is blocked or corrupted the second receiver will take over. The main receiver resembles standard 72Mhz receiver and is installed in the same manner. The second receiver is satellite receiver connected to the main receiver via an umbilical cord. The two receivers need to be mounted at least 2" apart and  preferably 3-4 inches apart. It is important to mount the antennas pointing 90 degrees from one another so that one of them is always receiving a signal at any given time. The satellite receivers are typically mounted to a former in the fuselage where it is easily accessible. This is much easier than running a standard 72Mhz antenna wire!
Spread Spectrum Conversion ModulesSpektrum provides conversion modules for a variety of radio brands including Futaba. That's right, you can get a Spektrum module for your Futaba radio system. Don't be mislead by the picture, these modules do come with antennas! Futaba also makes conversion modules utilizing their own (FASST)technology to convert 72 Mhz Futaba radio systems to 2.4 GHz SS radio systems. The latency of a converted system will be more than that of a true integrated 2.4 GHz system but will be less than a conventional 72 MHz system. A converted system will also not have the model protection feature.
Advantages of Spectrum RadiosNo more frequency pins:Gone are the days of checking the frequency board hoping that someone else isn't using your frequency. Gone are the days of waiting for the frequency pin so you can fiddle with your radio in the pits. Gone of the days of worrying about getting "shot down" by the idiots who can't follow directions :) With a spectrum radio you can turn on your radio and fly whenever your heart desires! Virtually zero interference: No more worries of glitches due to interference. No more worries about metal to metal contact in push rod connectors causing interference; this is a great plus for gasoline engines with electronic ignitions. With a spectrum radio, there is virtually zero interference. Short Antennas: No more worrying about forgetting to extend your antenna, or replacing it when you accidentally bend or break the awkward thing! Shorten Latency Times: The time it takes between moving the sticks and the airplane responding is referred to as latency time. Spectrum radios respond instantaneously. The difference is especially noticeable when performing maneuvers that require timing, such as rolling circles. Spectrum radios really do make you fee more "connected" with your airplane. Extended battery life: These radio systems use less power which means extended flight times and longer battery life.
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