MAAA Wingspan

30 31 WING SPAN NOVEMBER NOVEMBER WING SPAN MAAALogo. JOE ‘WISE’ WANGID: 2.4Ghz Systems G’day All. I thought I’d do a series on some research I did to learn a little more about how and where our 2.4Ghz systems developed. When Spread spectrum radio systems for RC airplanes went mainstream, our RC planes controlled by 2.4GHz radio systems presented the end for onerous frequency control at our model clubs. The end of being worried that a fellow member would turn his radio on by mistake, or that I might leave my frequency key out of the board in the Transmitter pound. It also (for me anyway), was the beginning of no more worrying about interference frommy engine, motor, ESC’s or being shot down by a person down the street from the club. In addition to the freedom from interference, these systems have a much faster response to make you feel connected with your model airplane with instant response to your control inputs. Further to this, our 2.4Ghz transmitters now use less power than they used to which means extended flight times or number of flights at the field. Last but not least, is that the we finally get rid of those annoying extendable antennas that often get bent, occasionally forget to be extended or end up having a light sabre fight in the pilots box. So What Exactly IS Spread Spectrum? Here is a fun fact I found: The most primitive form of this type of spread spectrum technology was developed during WW2 for encrypting signals that controlled torpedoes without the enemy detecting or jamming the signals. 2.4Ghz technology allows our transmitters and receiver to be “mated” so that the receiver only listens to that particular transmitter. There are specific ways of achieving this. The two main methods that were developed 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 and any device that transmits in this frequency range first scans the frequency band and uses only the narrow band frequencies within the 2.4 GHz frequency band which are not in use by other devices. This makes it possible for many devices to utilize the 2.4GHz frequency band at the same time. Now lets break it down a little more shall we... In looking into the technology behind the 2.4Ghz, I will refer to two brands of radios for familiarity and as example being Spektrum and Futaba. However, there are many other brands on the market like Jeti, Hitec, Powerbox, Frysky etc. 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. Now in simple terms, the data to our receivers are sent through a central frequency and the frequencies either side are for want of a better word, blocked out. 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 so it doesn’t select that area band of frequencies. 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. The widely used Spektrum system uses a proprietary coding scheme called Digital SpectrumModulation (DSM). Spectrum utilised DSM technology to develop the DX range of radios which were the first spread spectrum radio system designed for aircraft use. An original Spektrum DX7 transmitter Frequency Hopping Spread Spectrum With an FHSS system, the transmitter utilizes a narrow band frequency within the 2.4 GHz range and 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 and 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 utilised FASST technology to develop the Futaba 2.4GHz Spread Spectrum System, which is used in both park flyers and larger airplanes. Why Two Receivers/Antennas? Because the wavelength of the 2.4 GHz frequency is so small it can’t go around objects very well and the 2.4 GHz signal can be easily blocked by metal, carbon or large onboard fuel volumes on the airplane. 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 at all time. All TX/RX system use at least dual receivers. In the first versions of Futaba’s system, they don’t actually have two receivers unless you install a second RX like I do on my planes. Futaba uses their Dual Antenna Diversity (DAD) technology. If the signal is blocked coming into one antenna, the receiver would quickly switch to the other antenna and use the strongest of the two signals. These two antennas need to be pointed 130 degrees from one another. Advantages of 2.4GHz Systems I touched on some of the most obvious benefits of 2.4GHz radio systems, but lets go a little deeper... Frequency Control Without a doubt the most obvious, if not the greatest, advantage of using 2.4Ghz radio systems is that multiple users can use the same 2.4 GHz frequency band at the same time. While its been many years since we have all stood around waiting for our frequency to be free at the club, its still a welcomed (sometimes forgotten) relief.