Norcim rc electronics club page 20……
SOME USEFUL LINKS <![if !vml]><![endif]> HobbyCity <![if !vml]><![endif]> GreatHobbies <![if !vml]><![endif]> BangGood <![if !vml]><![endif]> ModelFlight RC <![if !vml]><![endif]> HorizonHobby
FURTHER RADIO CONTROL ELECTRONICS NOTES.
This page is started by Georges Bery from Switzerland with some interesting DIY transmitter designs:
Bit of bragging is in order here.
The pick with 3 transmitter contains : two 2 stick units built around 1984/5 and the single stick is 1989 vintage. ne5044 encoders with Terry Platt RCM&E RF section. SLM stick for the two red ones, the woody has nondescript Chinese stick . The 3 axis was purchased from lextronix in Montfermeille .
The one called microstar contains The reworked single stick with a microstar encoder job finished 2011 and my very first transmitter 1979 vintage, Cmos encoder 5 channel and plug in RF modules according to the TF6 standard published by Francis Thobois in le Haut parleur. It allowed me to swap frequency in those troubled times of CB pirates and go from 27Mhz to 40 then to 72 and 35 AM and FM.
Same system used in the single stick with next to it one of the RF decks. (with a VN46 Vmosfet )
Being born in a wee country that gets occasionally trampled upon by almost all its bigger neibours has not only disadvantages, it allowed me to get my hands on most RC and electronic magazines of the period : French-German-British-American- Dutch. And very rich they were in such projects all of them.
<![if !vml]><![endif]>I have kept most of them so if you’r looking for something 1977-2003 period I have most . After, either they ceased to exist (RCM, le hautparleur,radio plan, ETI, practical electronics,…..) or I couldn’t be bothered by RTF reviews or 2.4Ghz WiFi based Chinese plastic junk.
Current effort is in an Ace commander single stick case with Kraft stick and SLM button, with futaba 35Mhz synthetized rf deck and M*2K encoder. It is visible in 3d pic.
<![if !vml]><![endif]> Sorry for being immodest but hey, that’s also part of the hobby !
Thanks for the input Georges !...as you can see it has got us all thinking now about
‘Single Stick’ control and where and why did it get buried ?
Georges Bery’s range of home brewed transmitters brings back many happy memories in Micron days trying prototypes of triple axis stick transmitters. Having grown up with early pulse proportional systems that used just one control stick (Galloping Ghost), for rudder and elevator control, the search for increasing the number of controls to throttle etc inevitably grew around that single established joystick. In those early days, any additional controls to the model were simply ‘add-ons’ to the already established ‘single stick’. Throttle control was the first to come and usually involved a small lever on the Left side of the transmitter or a couple of buttons to give Hi or Low throttle. Development then progressed around that established ‘single joystick’ to allow Left/Right movement to give aileron control. Push Forward, for down elevator, Pull Back for up elevator. And TWIST the joystick clock or anticlockwise to effect Rudder movement. This ‘new’ development was called a Triple Axis Joystick and remained for a short period with many transmitter manufacturers.
<![if !vml]><![endif]>Triple Axis Transmitters however had a few ergonomic problems. (1) They were normally only available for right handed people. (2) Holding the transmitter with the only hand left, presented a problem. Often a neck-strap helped with this. (3) Where to put the throttle lever ??. Often it was put on the front LHS of the case with a neck-strap used to support the transmitter. Several users however preferred to ‘cuddled’ the Tx box with the left arm, with the right hand fingers/thumb on the top of the joystick. So the throttle lever had to be where the left hand fingers were. i.e on the right side of the ‘Box’ somewhere.
Now considering that, these transmitters should also be available for ‘Lefties’ with their different preferences of throttle…….You can see that multiple production presented problems.
Note, the picture to the left shows a typical ‘Single Stick’ transmitter. This one is a ‘cuddle’ type for Right Hand users only. Much more information is available at the original website. written by Bob Aberle. http://www.modelaircraft.org/mag/FAQ/answers/faq-q31.htm Well worth a visit. (What’s that black thing on the side of the Tx ?)...well you will have to pay a visit to the website above !
Some bright spark however rapidly produced a solution. Simply put two Dual-Axis joysticks at the top front of the transmitter case ! Get the fliers to use ‘thumbs on top’ of the sticks and (lefties included) could sort out which control movement was best for them. The transmitter in this way was securely supported with both hands without the need for a neck-strap.
It is interesting that well after the marketing and acceptance of the ‘standard’ two joystick R/C transmitter, some USA model aerobatic championship fliers were still using ‘Bespoke’ Three Axis Joystick transmitters.
<![if !vml]><![endif]>Probably the best website for ‘Single Stick’ radio control history is Duie Matenkosky’s website http://www.singlestickstuff.com Duie shows his collection of six working transmitters along with lots of background history. Duie still flies ‘Single Stick’ and has done for 30 or forty years ! His website is professional and very thought provoking about why this form of control layout appears to have got left behind. It is the nearest thing to the controls of full size flying.
Duie Matenkosky’s collection of ‘Single Stick transmitters >
(Much better to visit his website above)
<![if !vml]><![endif]>I have two of the readily available Quadcopters (Drones). Both supplied with the ‘normal’ R/C transmitter layout, (two dual axis stick units). I am totally convinced that if a ‘Single Stick transmitter’ version was available for these machines, then control of them would be so much easier for the hobby flier. Movement Left and Right….Forward and Backwards seems to be mastered without too much time being involved but mixing this with correct direction of flight takes some practice with two sticks. The rotary bit of the ‘Single Stick’ would make life much easier with drones.
<![if !vml]><![endif]>David Caudrey rummaged around the stuff that one collects with a lifetime in this hobby and came up with, ‘Would You Believe It’, A (‘Single Stick Joystick !). Well, to be ‘mechanically’ correct , should we really call it a Triple Axis Joystick’? …Well, if there is a possibility of ‘Single Stick’ control layout for model Radio Control transmitters re-<![if !vml]><![endif]>inventing itself, then perhaps we should start by using the terminology of the industry which are presently using thousands of these things annually. The Games and Robotics manufacturers have found the obvious advantages with these joysticks as well as the Space and general industrial applications.
Unfortunately, the ‘Industrial’ specification comes at a cost and although readily available, the present breed of analogue Triple axis Sticks are expensive for use by model radio control manufacturers. I will include just one of the superb triple axis industrial versions here that sell in their thousands. (see Right picture)>>>>>>>>
The upper image of David’s findings we have little idea of origin. To us it looks a Skyleader possibility or perhaps an early fleet Transmitter. Without doubt, it was not used by ‘Flight Link’ as they manufactured their own design triple axis stick system which was to be honest, mechanically very professional. I have an urge to get David’s findings into an existing transmitter that I have lying around and see if I can re-live the unique feeling of being part of the model !
<![if !vml]><![endif]><![if !vml]><![endif]>The picture to the left shows a fairly rare example of just one of many Heathkit’s radio control transmitters available around 1970ish. YES ! it’s a triple axis stick version, or should we say Single stick ? Either way, this unconventional shape and narrow dimensions allowed either hand to securely hold the transmitter case. Now, with a slider control conveniently positioned for thumb use on both sides of the case, throttle and one extra channel could be available for both RH and LH users. This basic layout does get around ‘the ergonomic problems’ mentioned above allowing quantity production to suit all model and drone fliers.
With a quality plastic case, click type in-flight trims, LCD display and 2.4 GHz, Mmmm, could this be a possible way forward for manufacturing designers of a future Single Stick R/C Transmitter ? (see very rough sketch right) >>
May the ‘Single Stick’ be with You !
<![if !vml]><![endif]>LAPTOP TOUCHPADS ARE BEING TRIED IN PLACE OF R/C JOYSTICKS. Although difficult to get any technical information or pictures about this development, at least one R/C manufacturer has been involved in the development of an electronic joystick. Making use of a modified laptop ‘Touchpad’, it has no moving parts. Moving the thumb on the pad produces a similar effect to a normal dual axis joystick. Lifting the thumb from the pad returns the servos to centre within 10 milliseconds without the several rebounds of the mechanical type. Swiping the outside edges of the pad produces in-flight trims. (each swipe, giving just one degree movement at the servo). Automatic in-flight trim is said to be a possibility and could probably be installed in the production versions.
Availability of two stick transmitter versions is aimed at early April 2015.
May the Happiness be with you !
<![if !vml]><![endif]> ORIGINALLY BOUGHT FOR CARAVAN USE, CHARGING BATTERY FOR LIGHTING AND 12V TELEVISION, THIS PETROL GENERATOR ALSO SUPPLIES A 240V OUTPUT SUITABLE FOR LAPTOPS AND SENSITIVE ELECTRIC FLIGHT CHARGERS.
The IMPAX 800 is quite light and easy to carry. Two litres of petrol will last all day and with a multi-mains extension socket will power up to six fast chargers at the flying site. Running with the economy switch is just 1800 RPM, almost tick-over speed for this 60cc Rapier engine. So noise level is low. Up against the fence of your flying site, you will never hear it whilst flying. The internal technology uses many magnets on the rotating crankshaft to produce healthy DC volts (rectified) which drives a sophisticated inverter circuit producing a very controlled, pure sine-wave 240 volt AC output. In many respects, it compares favourably with the Honda portable generator presently available in the UK retailing at £820. This little Gem is readily available, UK wide for just £189. Ideal for R/C club use! The picture shows the control panel of the charger and the exhaust end. Also with a side panel removed, shows the compact layout of air filter, carburettor, oil filler plug, and large effective exhaust silencer. Obviously, of Chinese manufacture but the Chinese technologists and manufacturers probably lead the world of technology at the moment. If you want quantity ‘cheap and cheerful’ products, then they can supply. On the other hand, if you want a high quality product, in quantity (which this generator certainly is), then these Guys are certainly capable. The same generator is available using different names and colour in Australia, America and Canada.
<![if !vml]><![endif]><![if !vml]><![endif]>ARE DIESELS AND GLOW MOTORS NOW A DYING BREED FOR MODEL AIRPLANE ENGINES?
David Coudrey is at the moment renovating a gift from a flying friend. Namely a Flair ‘Puppeteer’. One major intension is to change the existing Super Tigre 40 with an equivalent power brushless motor. The picture to the left shows the significant difference between the IC motor and the Electric replacement. The electric motor body is less diameter than the ST40 crankcase, much less length to the prop driver and no carburettor or piston, cylinder, glow plug. There is no need for a silencer too.
It is easy to see how ‘Electric Model Flight’ has progressed over the last three decades. David went into some of his usual simple scientific stuff coming up with the fact that this conversion would save 327gms (11.5 ounces!)
Against the equivalent conventional Glow Motor power propulsion system.
Weight of IC combo – ST40 + silencer + throttle servo + 10 x 5 prop + half full 6oz fuel tank + Rx battery 2300mah = 697 g.
Weight of electric motor + 5O amp ESC + 2200 3 cell Lipo. (30C) + 10 x 5 prop (electric) = 370g
To give an idea of this in simple terms, it means that 40 US Dollar coins or 34 UK pound coins would have to be used as ballast in the front cowl, around the electric motor to bring the model back into correct Centre of Gravity trim for stable flight ! However, David is working on shifting the electric bits forward to drastically reduce this necessity for ballast. We, Will keep you informed of progress!….
OK..Regarding the ‘intro’ of this article, ‘’are Diesels and Glow motors dying’’, Certainly not ! ! The fascination with miniature precision gas engines, Diesel, Glow, and Petrol will last forever with the model enthusiast! LIKE MOTORBIKES….IT’S THE SOUND THAT MATTERS ! !
<![if !vml]><![endif]>Update April 2015… David has constructed an all welded aluminium extension motor mount. Attached (right) is a picture of the Puppeteer motor mount with the nose cover (from a Febreeze aluminum aerosol can!). The cover hides and secures lead sheet which is wrapped around the <![if !vml]><![endif]>mount to get the balance point right. It is now painted drab olive to match the fuselage and nose ring. ‘I realised that I didn’t know where the balance point should be but found it on the ’net. . Looks as if I shall need to add a bit more lead than the 100 grams already fitted. I will send a photo of the aircraft soon’. With the slight side and down thrust built in, and also those allen screws, it all looks a very well engineered unit.
Interesting when you look at the picture to the left…How did that motor get into that All Welded extension mount ? Mmmm….
The finished electrified version of the Puppeteer is shown in this picture and has just had its maiden flight. ‘A BIT TWITCHY’ was the finding of David C and for the next session more lead is going to be added inside the front cowling to move the CG forward. This should help with stability and handling.
More flights to come !
OK ! another change of subject now…..
A newcomers guide of the 2.4 Gig ‘Spread Spectrum’ radio control
These notes about ‘Spread Spectrum Technology’, ‘Frequency Hopping’ and the use of the ‘2.4 Gig band’ for model radio control are intended for newcomers. They introduce the most exciting innovation for radio control models in the last decade.
Frequency bands allotted for model control are usually wide enough to fit in around thirty crystal controlled channels. Each channel with its plug-in crystals can control a model without radio interference to any of the other 29 frequency channels. This method has been used from the mid sixties and has, and still is, a superb well developed and proven system for model control. HOWEVER accidental switching on of a transmitter on the same frequency at your flying site can be catastrophic for the model in the air at that time.
<![if !vml]><![endif]>Radio Control equipment using the 2.4 Gig (gigahertz) band however, ‘share’ this radio band in a very different way. They use all the frequency channels in a quick random type scatter, This method ‘spreads’ the transmission of each system over the whole of the 2.4 Gigahertz radio band. ANYBODY CAN SWITCH THEIR TRANSMITTER ON AT ANY TIME WITHOUT INTERFERING WITH ANY OTHER USERS MODEL.
Each transmitter spends so little of its time on any single frequency channel, that virtually zero interference is produced to any of the other model control systems.
As a result of this continual ‘frequency hopping’ of the whole (or part) of 2.4 Gig radio band, many R/C systems can be used alongside each other without interference. All the systems simply ‘SHARE’ the same 2.4 Gig radio band.
The 2.4 Gig radio band is the largest radio band that R/C modellers can use (it has the equivalent of up to 80 channels!) but there are other commercial users of the band. In practice (based on recent years) 2.4 Gig model radio control has proven to be interference free from other users, Partly, perhaps because other commercial users are outside the range of your club flying site.
2.4 Gig transmitters use a rapid random like ‘frequency hopping’ transmission but each transmitter is produced to have its own random like sequence of its ‘frequency hopping’ called a ‘code’. With 80 channels to hop around in, the chances of two transmitters with an identical frequency hopping sequence, is one in several billion! To find those two identical transmitters down at your local club site is about the same as being run over by a bus several times and living to tell the tale!
Manufactured frequency hopping receivers have no idea what transmitter they are going to end up with! And they<![if !vml]><![endif]> have no frequency spot sequence built into them! As a result, they just sit and freeze their servos with any transmitter that tries to control them. Fortunately, they have a tiny memory chip built into the front end which is empty. The receiver will remember the random sequence ‘code’ of any transmitter within a second of pressing the tiny button on the side of the receiver. This is called ‘binding’ the receiver to a particular transmitter. Because all other transmitters have a different frequency sequence (‘code’), the receiver will refuse to work with them unless the ‘binding’ process is done again with a different transmitter.
The number of Frequency Hopping (Spread Spectrum) systems that can work together is quoted by different manufacturers from 20 systems to ‘over 200 systems’ (Graupner HoTT system). Both of these figures are in excess of a ‘safe’ number of model aircraft in the sky simultaneously down at your local club. Perhaps the lower figure of 20 could equal a model boat club activity on a very exciting Summer, day!
2.4 gigahertz receivers have a very short aerial, just 3 centimetres at the end of a short 5/7cm coax flex. The effective 3cm bit is best outside the model if possible. In a boat, the 3cm end bit must be above the water level. 2.4 Gig is virtually ‘line of site’ contact between your Transmitter and the Reciever aerial. Thought has to be given to metal type objects that get in the way of this, i.e. batteries, motors, helicopter metal parts and carbon fibre. Longer coax replacement aerial flexes are available but intended for replacement of transmitter aerials. If applied to receivers the longer coax covering unfortunately produces less range owing to coax signal loss. Model boat application probably OK but aircraft, some serious thought needed.
The signal intensity from the short (around 12 cm) transmitter aerial forms a doughnut shape around the aerial axis. The least signal strength (range) is pointing the aerial at the model. The 45 degree (click) angle would seem to be the most practical optimum for aircraft water and land models.
Spread spectrum (frequency hopping) systems ‘share’ the 2.4Gig radio band. This results in a small loss of range as the number of users at the model site increases. A particularly active hobby exhibition venue where 2.4 Gig systems are used extensively for aircraft, boats and land models simultaneously, may well find some range reduction. Particularly with aircraft which use the greatest range.
So what are just the practical advantages of using ‘Spread Spectrum’ ‘Frequency Hopping’ ‘2.4 Gig Radio’ for model control?
No more expensive box of plug-in crystals to change frequency! (around £10 a pair, 15$ US)
No more long cumbersome transmitter aerial to bend/break or poke someone’s eye out!
No need to check who is on what frequency!
No need to display a frequency flag!
No need to get a frequency peg from transmitter control!
No need to worry about testing the model controls and motor, just switch on and test!
No need to worry about ‘metal to metal’ or electric motor interference, 2.4 Gig is immune!
20 or more models can be controlled at one venue!
A 2.4 Gig transmitter and receiver ‘Combo’ can cost as little as £26.00!, Just use all your existing batteries and servos. See Page7.
The 2.4 GHz SS band has been now accepted for the future of Radio Control Models (2015)
Some interesting facts about our existing Radio Control Frequencies………
The 27 MHz band still available for R/C use including…
Marine and Citizens Band Radio. Speech.
Model aircraft control.
Model control boats and surface vehicles.
Toys model control.
Industrial.. very little, phased out.
The 35/36/40*/72 MHz band…These are protected frequencies for Radio Control Models.
Model aircraft only. No other users legally allowed.
*New Zealand only. EU 40MHz is for surface models.
The 2.4 Gigahertz Model Radio Control band…is however shared with many other people…
Wireless mouse keyboards.
Baby monitors (audio).
Baby monitors (Video).
Bluetooth (short range personal area networks).
Car alarm hand sets.
Microwave ovens (domestic).
Television video senders.
Garage door wireless type.
Radio control toys.
RF activated sulphur street lighting (phasing out).
ISM (industrial scientific medical) research projects.
WISP wireless internet service providers.
Wireless Video for security cameras.
ZigBee wireless data networks.
High speed point to point links that use propriety protocols.
Plastics pre heaters.
Building to Building video and speech up to 50 miles.
Radio Ham usage up to 1500Watts for carrier wave and single sideband Greater than 50 miles range.
Over Counter 2.4 Gigahertz transmitters 50 miles capable.
Police Robot surveillance vehicles. Video feedback. (bomb squad).
Military video flying surveillance drones. (30 miles+).
Flying spy cameras, domestic, video transmission. Police use.
Model aircraft (all types).
Model boats (all types) (except under water)
Model land vehicles, cars, tanks etc.
Commercial applications are being added to this list as you read this text. We simply share this band with whoever decides to use it. There is little or no control.
Model aircraft radio control systems use ‘Spread Spectrum’ techniques on the 2.4 Gigahertz band. The transmitter ‘scatters’ its transmission
over many of the frequency channels in a random like fashion. This allows a maximum of signal to get to the receiver.
This method allows other users (fixed frequency and Spread Spectrum types) to use the band too.
Most commercial transmissions are confined to Towns and Cities and are short range devices but when considered all together,
can produce asignificant amount of background interference.
Industrial, Ham Radio and video systems with up to 50 miles range are also in the cook-pot of possible future problems for the use of the 2.4 Gigahertz band for radio control models.
A ‘good’ flying site would be several miles away from Towns and Cities. Also several miles from Industrial complexes.
If you can see the stars at night from your flying site…..then you got a good site for 2.4 Gig R/C.
If you have got this far reading about the 2.4 Gig phenomena then (pat yourself on the back!) I must now pass you on to a Radio control Guru that I met on the web. This guy has lived the journey of 2.4 Gig Model Radio Control and has the background to predict the possible future of this system…….
Spread-Spectrum RC Systems, How they started and how we got there.
By Barry Lennox NZ.
A few weeks ago, Barrie Russell (of the MFHB club NZ) leaned on me looking for an article This is a bit of a struggle, as I feel like I have written on just about everything that modelers could possibly be interested in, but there’s still one or two yet, (But not too many more).
<![if !vml]><![endif]>Several years ago, I did suggest that one day we would all be flying with Spread Spectrum (SS) radios, but it’s hard to see the future, otherwise I’d be very, very, very rich indeed ! However, one thing that has fascinated me for years is SS communications, and have been working with them since the mid 90’s.
Firstly, there’s two main varieties that you will come across. The first is a frequency hopper (FH) It’s pretty self-explanatory, instead of just sitting on one channel, say 35.02, it just hops about over maybe 20-50 channels at many times per second. So if one or two or even several channels are busy, or jammed, the data will still get through, albeit at a reduced rate sometimes. (Futaba uses this version)
Here’s a screen shot from a spectrum analyser covering the 2.4 GHz band from 2.4 to 2.485 GHz with a FrSKy FH transmitter in action. You can see about 50+ channels that are being hopped over. Note that this was averaged over 15 seconds, so there are many “hits” on each channel.
The second type is Direct Sequence Spread Spectrum (DSSS) Here the <![if !vml]><![endif]>narrow band signal is mixed with a pseudo-random one that is much wider, or broadband. The resulting signal takes on the characteristics of the wider signal, so narrow-band noise or interference gets suppressed by the width of the new signal. If this seems a little hard to understand, don’t worry about it, just regard it as trickery by maths, however, it does work very well. (Spectrum and the first JR sets use this)
Here’s a sketch of how the DSSS works. (See Right) >>>>>>>>>
Notice how the signal is dramatically spread, in fact it generally looks like noise. But it can be reconstructed into the original narrow-band signal, IF the code is known. (and that’s a BIG IF, as we will see later). Remember the spreading (and de-spreading code) is pseudo-random. If it was truly random, there would be no way of ever recovering the original signal.
Here’s a spectrum analyzer display of a DSSS transmitter, note how it’s not much above the noise level. And that’s followed by another display, nothing has changed, except that the DSSS mode is now turned off. So it’s now a narrow-band signal, just like say, on 35.05 MHz.
Next, shown below, is a spectrum analyzer plot of a Spektrum DX-6. It can be seen, there are two signals, one at 2433 MHz, and the other at 2473 MHz, that are separated by 40 MHz. The signals are sent alternately. Zooming in on the 2433 MHz signal reveal that it has about a 830 kHz bandwidth. Remember that our normal narrow-band radios have a bandwidth on only about 12 Khz, so the Spektrum signal has been spread by a factor of about 70 times, plus, the signal is alternately transmitted on two quite separate channels for redundancy.
<![if !vml]><![endif]>“Which is best” is the question all poised on your lips. More on that later, and the real answer is “It all depends” However, the above plot of the Spektrum does give us a clue: That the best system will combine elements of both FH and DSSS.
So, where did all this trickery start? The answer is much further back than you might think, certainly long before any of us existed. The very first suggestion was from Nikola Tesla, who filed a patent in July 1900 (the 1903 U.S. Patent 723,188 and U.S. Patent 725,605).
Tesla came up with the idea after demonstrating the world's first radio-controlled submersible boat in 1898, when it became apparent the wireless signals controlling the boat needed to be secure from "being disturbed, intercepted, or interfered with in any way." His patents covered two fundamentally different techniques for achieving immunity to interference, both of which functioned by altering the carrier frequency. The first had a transmitter that worked simultaneously at two or more separate frequencies and a receiver in which each of the individual transmitted frequencies had to be tuned in, in order for the control circuitry to respond. The second technique used a variable-frequency transmitter controlled by an encoding wheel that altered the transmitted frequency in a predetermined manner.
So far, so good, Piece’o’ cake really. However, the huge (insurmountable actually then) problem was to ensure that the transmitter and receiver stayed in synchronization with each other. He never solved that bogey.
The next mention of frequency hopping is in a very old text by the radio pioneer Jonathan Zenneck's book Wireless Telegraphy (German, 1908, English translation McGraw Hill, 1915), and Zenneck claims that Telefunken had already tried it several years earlier. Zenneck's book was a leading text of the time, and was highly regarded as the “Wireless Bible” It is now believed that the German military made limited use of frequency hopping for communication between a few fixed command points in World War I to prevent eavesdropping by British forces, who did not have the technology to follow the sequence, or were even aware of it. However, no equipment, documentation or reliable memories can be found these days.
Then in 1929, a Leonard Danilewicz who was working for the Polish General Staff's Cipher Bureau, proposed a system for secret wireless telegraphy that : “was a truly barbaric idea consisting in constant changes of transmitter frequency” Incidentally, he was also involved in developing parts of the infamous Enigma cipher machine, widely used by all German forces in WW2.
The most celebrated version of frequency hopping was that of the very attractive actress Hedy Lamarr and composer/friend George Antheil, who were experimenting with “alternative” music and the electrical control of musical instruments. In 1942 this pair received U.S. Patent 2,292,387 for their "Secret Communications System". Lamarr had learned at defense meetings she had attended with her former husband Friedrich Mandl that radio-guided missiles' signals could easily be jammed. So, Hedy and George concocted a version of frequency hopping that used a piano-roll to hop among 88 frequencies, and was intended to make radio-guided torpedoes harder for enemies to detect or to jam.
As far as is known, they never solved the huge problem of synchronization, to produce a fielded system. Not surprising really, can you imagine trying to get one piano-roll working correctly, let alone two of them in synchronization!
It has become a popular myth that they invented SS comms, that is not true, what they patented was the use of the piano roll. But the media and the Internet never lets facts get in the way of a “human interest” spin !
<![if !vml]><![endif]>Incidentally, many years later in the 1970’s Hedy Lamar got a not very complimentary lampooning in that zany spoof movie “Blazing Saddles” She really took exception to this, and she and the producer settled out of court for some undisclosed sum.
For several years during the 1990s, the boxes of CorelDRAW's software suites were graced by a large Corel-drawn image of Hedy Lamarr, in tribute to her pre-computer scientific discoveries. These pictures were winners in CorelDRAW's yearly software suite cover design contests. Far from being flattered, however, Lamarr, once again, sued Corel for using the image without her permission. Corel countered that she did not own rights to the image. They reached an another undisclosed , but rumored to be $250,000, settlement in 1999. Here’s that picture;
The first really successful SS system was used during WW2 and was developed over a couple of years by about 600 engineers, technicians and mathematicians at Bell Labs. This was known as SIGSALY or “Project X”
SIGSALY was a secure voice system that was used to communicate between major command and HQ centers from 1943 onwards. There were only 12 terminals ever made, and no wonder, A SIGSALY terminal was massive. Consisting of 40 racks of equipment, it weighed over 50 tons, and used about 30 kW of power, necessitating an air-conditioned room to hold it. The system was cumbersome, but it worked very effectively. When the Allies invaded Germany, a SIGINT investigative team discovered that the Germans had never even come close to cracking it.
Bell Labs had solved the synchronization problem by recording random noise on a phonograph record (younger readers, ask your dad what these are!) The records were played on turntables, but since the timing – the clock synchronization – between the two terminals had to be precise, the turntables were by no means just ordinary record-players. They were precision devices, and the rotation rate of the turntables was carefully controlled, but the system owed as much to extremely complex mathematics as superb engineering. The records only held 20 minutes of random noise, but this was mathematically expanded to cover a whole Crypto-period, being one day.
This was the first working SS system. Incidentally, the principles involved were still classified until the 1970’s.
Then, one of the lessons learnt by the Western forces during the Vietnam conflict was that just about any kid with a cheap scanner and a few add-ons could intercept, direction-find, jam and generally mess-up your communications quite badly.
The response to this was to turn to once again, SS systems. The ground forces use the 30-88 MHz band with FM, and the system developed was known as SINCGARS, and frequency hops 111 times a second. Nearly every western military service uses this nowadays with around 600,000 sets being manufactured. Air forces use a different band, modulation and system. It’s AM in the 225 to 400 MHz band and the system they developed is HAVE QUICK. Again, just about every western air force now uses HAVE QUICK routinely. These systems offer a reasonable degree of protection against unsophisticated interception and jamming techniques. Initially, they were regarded as being a secure system, or close to it, but of course, within about 2 years a number of manufacturers (including Rohde & Schwarz, Marconi, Racal, Harris, Thomson-CSF and Tadiran) all offered systems that could once again could intercept, direction-find, jam and generally mess-up your fancy new SS radios.
You may have noticed that all these system use frequency hopping. The other commonly used technique (DSSS) is also widely used these days in many communications systems, but probably the one you will all be most familiar with is GPS. The GPS signal from the satellites is actually very narrow band, about 70 Hz wide, but it spread out over 1 MHz for transmission. I.e. about 14,300 times ! And that’s just for the commercial CA code, the “P-Code” which is only available to restricted users is spread out to about 10 MHz, ie, 10 times the CA code.
There’s many more, ie; Bluetooth, which is a frequency hopper, hopping between channels at 1600 times per second. And then there’s 3G cellphones, Wireless LANs, and all manner of medical, security and industrial application, even cheap garage door openers, remote meter readers, remote tyre pressure monitoring on some high-end cars, and baby monitors.
So how does all this fit into model control? Well, several years ago, a clever company in the USA, Cypress Semiconductors, introduced a cute little chip, a complete 2.4 GHz SS radio on a chip, the Cypress CYWUSB6935 Transceivers The datasheet described it as a programmable Radio System-on-Chip (PRoC™) device and is the world’s first low cost flash programmable microcontroller with an integrated 2.4-GHz radio transceiver. Their list of suggested uses included about 20 market sectors, including “toys” It’s interesting to note that in the early days, Cypress had a lot of datasheets and application notes on the Internet on this family of chips. This covered amongst other things, the somewhat mysterious “binding” process. Then, a few years ago, these were removed without explanation, however, the prudent ones of us, stored local copies at the time! There’s a message there, If you see something you want on the Internet, grab it now, it may be too late in the morning!
Anyway, an equally clever engineer, Paul Beard, noticed that this device might make a very good RC system. It’s power output was a little low at just 1 milliWatt, but an external amplifier-on-a-chip, the SE2526A, soon fixed that. Hence the original DX-6 was born, (although originally for RC car control) it also used some older JR mechanics, and cases.
Needless to say, technology does not stand still, and now both FH and DSSS systems are readily available form many vendors. This has come about by the relentless march of technology, smarter engineering, reverse engineering, and dare I say it; Chinese copies! Incidentally the original CYWUSB6935 is now obsolete
So, where are we at now? The various systems are spreading rapidly, with every major manufacturer, and many smaller ones all offering SS systems.
The major advantage is that they allow interoperation without worrying about pegboards and the idiot that might shoot you down.
They also do not suffer from electrical interference we tend to get with poorly installed electric motors. Please do note that they are NOT immune to interference as is often claimed. And in fact, they are dead easy to interfere with. However, the good news is that interference off motors, ESCs, etc, runs “out of puff” at a little over 1 GHz, so by operating at 2.4 GHz, the problem is neatly bypassed.
To make this clearer, here is a screen-shot of a spectrum analyzer looking at an old, unsuppressed and noisy electric motor. The white marker dot in the centre is sitting at 1.5GHz, and the “spike” on the right is a FrSky transmitter. Notice there’s no significant energy past about 1.2 GHz. Also, note that each graticle line is 10dB (or 10x) so the motor noise down at around 20-50 MHz is about 1000 x greater than the background noise at 2.45GHz.
<![if !vml]><![endif]>So, now you can see why 2.4GHz systems apparently “handle interference very well” They don’t really, but quite simply, the interference is not there !
And of course, they are very cheap indeed.
On the other side, there’s several systems, and none of them are compatible, so you have to make a bit of choice and stick with it. But, they are very cheap if you want to change horses. You should also be aware that they are not quite as immune to interference as the marketers and advertisers would have you believe. A strong signal will knock them out very easily. But it’s also true that MOST users of the 2.4GHz band are restricted to low power. Some are not however, and you must understand and accept that this band is a bit of a “Wild West” where pretty much anything goes, and what’s more, you must accept interference whenever and wherever it happens, it’s clearly part of the terms of the Licence granted to these devices.
There was reportedly some significant problems with some of the early systems, such as being shipped with the same binding code, and the sensitivity of some sets to low battery voltage, but most of these matters have now been discovered and remedied.
Now, back to the question, which one (FH or DSSS) is better. Well, there’s no simple answer to that, it all depends on the RF environment you are operating in, and what the threats are. A simplistic view is that DSSS ensures data robustness, and FHSS allows the wireless signal to "hop" to new channels once interference becomes too great.
It is interesting to note the following comment from Spektrum:-- “Originally, Spektrum engineers started their development with FHSS-based systems because they were relatively easy and inexpensive to develop. However, it was soon discovered that FHSS had several limitations that would prevent it from being the optimal solution for RC. While more difficult and costly to develop, our engineers began experimenting with Direct Sequencing Spread Spectrum and optimized the modulation scheme to overcome critical response and re-link issues. In addition, DSSS offered 18dB increase of processing gain for significant improvements in range. With years of development and testing the DSSS modulation scheme was optimized for RC car use and Spektrum’s DSM system was born.” (So, now the wheel has turned full circle and Spektrum has also moved to FH systems.)
OK, so which is better? Part of the answer is given by various texts, including the daddy of them all. “Spread Spectrum” by RC Dixon, but be warned, the maths is heavy going!
There was an excellent article in “RF Design” a few years ago, comparing FH and DSSS systems in the face of severe interference. The slight edge was held by DSSS systems, all other things being equal, but in our environment with moving transmitters and receivers and all the other variables, things are far from equal. And most systems now combine some elements of both FH and DSSS systems. Ie, they might be a DSSS system, but retain the ability to hop around a bit if the RF environment gets a bit busy.
<![if !vml]><![endif]>So, the answer today, will be: what works best for you !!
Nowadays, most SS RC systems operate in the 2.400 to 2.483.5 GHz band. The precise terms vary a bit around the world, some countries permit 100mW, some less, usually 10 mW and some more. Here in NZ, we operate under a no-charge GURL (General User Radio License) and it’s a bit of a free-for-all with many thousands of users with WiFi, cordless phones, alarms, data-links, microwave ovens, Ham radio transmitters and many other devices. However, there are conditions, and these include:
· All equipment must be type-approved.
· Power must not exceed 4 Watts EIRP (That includes the antenna gain)
· If you interfere with a radio or spectrum license holder, you must stop.
· RSM does not investigate interference to General User Licences because frequency use is on a shared basis and the chief executive does not accept liability under any circumstances for any loss or damage of any kind occasioned by the unavailability of frequencies or interference to reception.
There, I didn’t answer the question, but hope that you have found this article interesting.
Some of our readers may be aware that Barry is the current President of ‘Model Flying New Zealand’. For those that don't, I asked him for a short "Bio" to put the author into perspective, and here is what Barry wrote....
A baby boomer now, semi-retired, semi-consulting, and living on a few acres in Nth Canterbury with wife of 41 years, plus a dog, 3 chooks, 3 alpacas, 2 cats and a pond-full of goldfish.
I have spent nearly all my working life in the electronics industry, much of it in the RNZAF as an Avionics Engineering Officer, but also several years in commercial aerospace, secure/special communications, the IT industry and a UAV start-up. Qualifications are many and varied civil and military ones. I still am a practicing engineer in things like RF engineering, EMC, electromagnetics, antennas, etc.
I have been modeling for many years, on and off. And have dabbled in most areas, free-flight, control line, rubber-powered, gliders,sport and scale. This all started when a 3rd form Ham friend suggested a joint project, I build the model and he built the RC gear. The transmitter was a very wobbly, non-xtal controlled, approx on 27MHz with a couple of 3D6 valves liberated from Army stores by a friendly Cadets Army Seargent. While the receiver used a hearing-aid valve, and a couple of these new-fangled transistors, imported from Henry's Radio in London. We had a bit of success, but the constant supply of batteries to feed these things cost a fair chunk of pocket money.
Barry Lennox 2013
Many Thanks to the Hawkes Bay Flying Club for co-operation with this article. …….
FURTHER CONTENT MAY BE ADDED….AND HEY! THANKS FOR READING.