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Model Forum / Radio Controlled / Air Models / December 20072.4 GHz - Is there a limit?
When using Futaba 2.4 GHz is there a limit to the number of models that can be in the air at the same time? Apparently the system changes frequency every 2 milliseconds. How many frequencies are there to change between? What happens if many people are using 2.4 GHz? Anything which will help me to properly understand the technology will be welcomed and appreciated.  SignatureChris > When using Futaba 2.4 GHz is there a limit to the number of models that > can be in the air at the same time? [quoted text clipped - 6 lines] > Anything which will help me to properly understand the technology will > be welcomed and appreciated. there are 80 channels and as each set uses 2 then its a minimum of 40, it locks one one main channel and has a back up channel >> When using Futaba 2.4 GHz is there a limit to the number of models that >> can be in the air at the same time? [quoted text clipped - 8 lines] >there are 80 channels and as each set uses 2 then its a minimum of 40, >it locks one one main channel and has a back up channel Nope thats the Spektum system, the Futaba switches the channels it uses every 200milliseconds. (Called spread spectrum) I suppose both are good and bad, if the spectrum starts up and picks two channels if something then sits on them causing interference, the TX won't change to avoid the interference, that said the futaba hops channels and can hop onto something in use albeit for a very short time. Depending on the circumstances it could be costly either way. Back to the original post in theory it's 40 models, as each system will have a main frequency and a backup, in practice I guess more than 10 and you'll be wondering which ones yours. We had 4 Weston Cougars in the air yesterday at the same time and I was rather glad I was flying something different.... >>> When using Futaba 2.4 GHz is there a limit to the number of models that >>> can be in the air at the same time? [quoted text clipped - 11 lines] > Nope thats the Spektum system, the Futaba switches the channels it > uses every 200milliseconds. (Called spread spectrum) The Spectrum system is also spread spectrum as I understand it, because it uses the full width of the (?)1MHz channel to transmit effectively very low bandwith data, and that means that the reciever can resolve the signal even below the noise floor of the surrounding RF environment, and it is much less affected by narrowband interference in the band in use. I'm not sure if the Futaba suystem does that too, but I'd guess it does. > I suppose both are good and bad, if the spectrum starts up and picks > two channels if something then sits on them causing interference, the > TX won't change to avoid the interference, that said the futaba hops > channels and can hop onto something in use albeit for a very short > time. Depending on the circumstances it could be costly either way. There are some good discussions around the web, particularly on rcgroups, by people who are very knoledgable about the two systems, if you want to compare. Personally, I'd say that neither is significantly better or worst than the other, and all far better in most applications than 35 or 72MHz narrowband stuff. There are some difficulties apparently with carbon planes - because the aerial is so short it's not easy to get it outside the fuselage and the carbon fibre apparently blocks the signal quite effectively. I've heard that some jet pilots won't use it because if the signal is lost the rebinding time when it's reestablished can be significant at the speeds they travel. > Back to the original post in theory it's 40 models, as each system > will have a main frequency and a backup, in practice I guess more than > 10 and you'll be wondering which ones yours. We had 4 Weston Cougars > in the air yesterday at the same time and I was rather glad I was > flying something different.... Both systems will support more models than can be safely in the air at once. I think there's very little to choose from between the two - it just depends on your liking of either JR or Futaba. The two systems are incompatible, so whatever choice you make now will stick with you forever. The new JR sets that are coming out are compatible with Spektrum receivers and I guess JR receivers with Spektrum transmitters. My only real worry is that there is no competition - both systems are proprietary to the manufacturers and that locks you in unlike 35MHz where you could swap between JR, Futaba and anyone else to your heart's content and everything was compatible. How long will it be until one of them comes out with a "new improved digital servo interface" that's not based on pulse width but on a digital signal path between the Rx and the servo? There'd be many potential benifits of that approach, but it would also make the lock-in complete. Ah, but then I'm a cynic - and a pleased Spektrum owner. Norbert >There are some difficulties apparently >with carbon planes - because the aerial is so short it's not easy to >get it outside the fuselage and the carbon fibre apparently blocks the >signal quite effectively. Can't you feed the antenna via coax so the entire antenna is outside? >Norbert Barry ===== Home page http://members.iinet.net.au/~barry.og I do not represent atheists or atheism >> There are some difficulties apparently >> with carbon planes - because the aerial is so short it's not easy to >> get it outside the fuselage and the carbon fibre apparently blocks the >> signal quite effectively. > > Can't you feed the antenna via coax so the entire antenna is outside? Probably.But 2.4Ghz coax and connectors are not that easy to come by. I guess some of the bits sold for WiFi might work. >> Norbert > [quoted text clipped - 3 lines] > http://members.iinet.net.au/~barry.og > I do not represent atheists or atheism >>There are some difficulties apparently >>with carbon planes - because the aerial is so short it's not easy to >>get it outside the fuselage and the carbon fibre apparently blocks the >>signal quite effectively. >Can't you feed the antenna via coax so the entire antenna is outside? You don't need to do that. On the Futaba RX there are two aerials - each about one inch long - connected to the RX by thin wires about four inches long. It's only the end inch that's the aerial. So - no problem. SignatureChris >>>There are some difficulties apparently >>>with carbon planes - because the aerial is so short it's not easy to [quoted text clipped - 8 lines] > It's only the end inch that's the aerial. > So - no problem. Oh is that right? I've not seen a Futaba Rx. The Spektrum ones aren't too good because the antennas are connected directly to the PCBs. I'm also not sure if they're two separate antennas or supposed to be a dipole. Whatever, there's no way of getting them out of the model. There is a third brand of 2.4GHz stuff out, the name escapes me at the moment, and it seems to have antennas attached via proper RF coaxial connectors, so I guess in principle you oculd mount them wherever. However there'd be a weight penalty there although it'd be minimal in a largish model, of course. Nobby >>>> There are some difficulties apparently >>>> with carbon planes - because the aerial is so short it's not easy to [quoted text clipped - 17 lines] > > Nobby IIRC there are two separate systems to deal with multipath. One uses discrete channels and needs two receivers, and the other hops around fast enough to find a frequency that works in a reasonable time frame on a single receiver.. So you may find that not all 2.4GHz systems are created anything like equal. >>> When using Futaba 2.4 GHz is there a limit to the number of models that >>> can be in the air at the same time? [quoted text clipped - 11 lines] > Nope thats the Spektum system, the Futaba switches the channels it > uses every 200milliseconds. (Called spread spectrum) I stand corrected but wont that cock up a Spektrum set if the Futaba set decides to transmit on its frequency for 200ms ?? I suppose both are good and bad, if the spectrum starts up and picks > two channels if something then sits on them causing interference, the > TX won't change to avoid the interference, that said the futaba hops [quoted text clipped - 6 lines] > in the air yesterday at the same time and I was rather glad I was > flying something different.... >>>> When using Futaba 2.4 GHz is there a limit to the number of models >>>> that can be in the air at the same time? [quoted text clipped - 14 lines] > I stand corrected but wont that cock up a Spektrum set if the Futaba set > decides to transmit on its frequency for 200ms ?? I think there is a lot of digital signing going on. So the wost that can happen is your signal gets lost. Its not possible really for another signal to 'take over' your receiver. i.e. crowded airspace does not result in interference, so much as servo slowdown and signal loss. If you have digital terestrial TV, you will have seen the result. Not other channels ghosting on yours, just complete loss of sound or picture. oR both. > I suppose both are good and bad, if the spectrum starts up and picks >> two channels if something then sits on them causing interference, the [quoted text clipped - 7 lines] >> in the air yesterday at the same time and I was rather glad I was >> flying something different.... >>>> When using Futaba 2.4 GHz is there a limit to the number of models that >>>> can be in the air at the same time? [quoted text clipped - 14 lines] > I stand corrected but wont that cock up a Spektrum set if the Futaba set > decides to transmit on its frequency for 200ms ?? Not necessarily, because the Spektrum system uses two channels. Also I think (but I'm not positively sure) that the Futaba system listens for free channels as it hops around and won't use a noisy one. Also, the nature of the in-channel spread spectrum means it's likely that even if two transmitters were on the same channel both receivers would still function. Spread spectrum is bizzare and doesn't obey the rules of narrowband comms as used for 35MHz systems. Norbert >>>>> When using Futaba 2.4 GHz is there a limit to the number of models that >>>>> can be in the air at the same time? [quoted text clipped - 25 lines] > > Norbert It seems to be taking off in our club but bizarrely you still need to put a peg on the peg board? not sure of the reasoning as a separate club rule limits the number of models in the air at any one time >>>>>> When using Futaba 2.4 GHz is there a limit to the number of models that >>>>>> can be in the air at the same time? [quoted text clipped - 28 lines] >put a peg on the peg board? not sure of the reasoning as a separate club >rule limits the number of models in the air at any one time We have that rule too, Not for any frequency clash issues but because we don't have anyone exclusivly on 2.4, most people stillhave some models on 35Mhz and a 2nd Tx. It's to stop people getting out of the habit of putting a flag out when they revert back to 35Mhz now and again. Once everyone has gone 2.4 we can do away with it, but thats not likely to be anytime soon. >>>>>>> When using Futaba 2.4 GHz is there a limit to the number of models that >>>>>>> can be in the air at the same time? [quoted text clipped - 35 lines] > Once everyone has gone 2.4 we can do away with it, but thats not > likely to be anytime soon. ah that makes sense >>> It seems to be taking off in our club but bizarrely you still need to >>> put a peg on the peg board? not sure of the reasoning as a separate club [quoted text clipped - 9 lines] >> likely to be anytime soon. > ah that makes sense It's more a matter of reducing errors. If you see 4 people flying and only three pegs, is that because the forth is using a 2.4GHz system, or is it because they's forgotten to put a 35MHz peg on? It's better to have at least as many pegs as there are pilots (you will often have more pegs, of course, because of people using their Tx on the ground). That as well as the problem of people still using both systems, of course. > When using Futaba 2.4 GHz is there a limit to the number of models that > can be in the air at the same time? [quoted text clipped - 6 lines] > Anything which will help me to properly understand the technology will > be welcomed and appreciated. Around 65-70 IIRC. range is reduced as more models go up. >> When using Futaba 2.4 GHz is there a limit to the number of models that >> can be in the air at the same time? [quoted text clipped - 8 lines] > > Around 65-70 IIRC. Unfortunately you don't RC. > range is reduced as more models go up. No it isn't. >>> When using Futaba 2.4 GHz is there a limit to the number of models that >>> can be in the air at the same time? [quoted text clipped - 10 lines] > > Unfortunately you don't RC. No., 40 it speems. >> range is reduced as more models go up. > > No it isn't. I think you will find with spread spectrum that is exactly what happens actually. The other models will be almost out of band noise: at some point the S/N ratio will drop and range will reduce. Still don't take my word for it - look up the theory yourself. >>>> When using Futaba 2.4 GHz is there a limit to the number of models that >>>> can be in the air at the same time? [quoted text clipped - 24 lines] > > Still don't take my word for it - look up the theory yourself. Don't need to. Yes, at some point it will, long after all the bands are full, and given that the Txes choose free bands in the first place and (presumably) won't bind if all the bands are full, you'll never be in a position to notice. So, the answer is still "No it isn't". I say "presumably" because I don't know what the system behaviour is when the bands fill up, but I'd guess that the Spektrum system will simply refuse to connect and I don't know what the Futaba system will do. >Still don't take my word for it - look up the theory yourself. I would like to do that! Have you any good, authoritative links? SignatureChris >> Still don't take my word for it - look up the theory yourself. > > I would like to do that! > Have you any good, authoritative links? <http://grouper.ieee.org/groups/1451/5/Coexistence%20Papers/Paper%20Coexistence%2 0BTCongress%20-%202001-%20v1%20Monaco%20Coexistence%20Tools.pdf> is not bad. Basically as the number of transmitters goes up the probability of a collision between two goes up. If they are using frequency hopping. That mean some or all of the data is lost. In situations of extreme range that will mean that MORE packets are lost - some due to being weak, others due to being stamped on. I suspect that the range issue is more one of getting a lowering of the data rates till it drops to nothing. Probably controls will seem sluggish or delayed at extreme range. More transmitters in the air will make that worse. Other transmitters appear as general background noise to the receiver. What will NOT happen is glitching. The receiver will either respond to packets intended for it, or it won't respond at all. I am trying to find te exact frequency hopping specs for Futaba and Spektrum in order to make better estimate of how things degrade, but of course no one is telling. >> Still don't take my word for it - look up the theory yourself. > > I would like to do that! > Have you any good, authoritative links? Hmm. Some confusion. It appears that Spektrum maybe doesn't use spread spectrum at all - just pulls a pair of channels out of the 80 odd available and uses those. http://www.rcgroups.com/forums/showpost.php?p=7886722&postcount=15 Futaba allegedly *does* use spread spectrum via frequency hopping, which gives some interesting issues with respect to how the two will interoperate. if that *is* how the Spektrum works, then it will be 'up to 40 models up and no interference'., I THOUGHT it was some form of spread spectrum tho..at least on the latst.. However with futaba it will be 'unlimited models., with performance degrading* according to the square root of how many are up (more or less). Interesting. *Slower response, less accurate response, less range for a given speed of response. I'll dig deeper... Anyway, 40 models is enough,and I'd go for a 2.4gig set anyday once they stop being so proprietary and I can get Royal Evo type programming on one.. >>> Still don't take my word for it - look up the theory yourself. >> [quoted text clipped - 16 lines] > and no interference'., I THOUGHT it was some form of spread spectrum > tho..at least on the latst.. The spektrum system is a CDMA-like spread, over each full (?)megahertz band, so it's a direct sequence spread spectrum coding rather than a frequency hopping one. I don't know if the Futaba system combines DSSS with FHSS, though. There is a discussion somewhere on rcgroups that describes it all pretty well, and also mentions the cypress chipset that's used. I think it's this one here: http://www.rcgroups.com/forums/showthread.php?t=721024 I am interested in this discussion as I, alongside a growing number of modellers, still operate on 35MHz (I am in the UK) but transmit video in the 2.4GHz band from a model. This is a good reason to continue the peg system as I don't want to turn up and switch on my video if someone is already flying on the video channel I will use. The transmitter uses FM for the video. regards Peter >>>> Still don't take my word for it - look up the theory yourself. >>> [quoted text clipped - 28 lines] > > http://www.rcgroups.com/forums/showthread.php?t=721024 > I am interested in this discussion as I, alongside a growing number of > modellers, still operate on 35MHz (I am in the UK) but transmit video in the > 2.4GHz band from a model. This is a good reason to continue the peg system > as I don't want to turn up and switch on my video if someone is already > flying on the video channel I will use. The transmitter uses FM for the > video. The 2.4GHz radios "should" be able to handle it. The 2.4GHz band is a shareed band anyway, there can be any number of emitters in and around the band, but the whole point of the spread spectrum approach is that you can mix signals in the band - signals that don't belong to a transmitter/receiver pair just look like noise to the receiver and can more often than not be handled by the decoder. As I said before, it's like magic - wideband radio links are non-obvious in how they work compared to narrowband links like the 35MHz RC system. On a narrowband link if there is *anything* transmitting on your narrowband, it is either powerful enough to block your signal completely, or weak enough not to - think of 2 transmitters on the same channel but separated by 10 miles. Wideband is different in that if there are two transmitters operating on the same channel they might and probably will degrade each link, but there is not the on/off behaviour of the narrowband system. Also, narrowband noise, which is most of what we'd call "interference", has a very small effect on the wideband link, because it only affects a small part of the bandwidth, and the rest of the signal gets through. Think of it a bit like listening to music. If there is no noise, you can hear music perfectly. If someone starts humming a single note, you can still hear the music, it's slightly degraded, but nevertheless almost as good as the noise-free stuff. The only information that's lost from the original music is the note that's now obscured by the humming person, all the other notes are still as clear as they were. Even better, because you know how music works, you can most likely guess when a note is being obscured, and if you want you could reconstruct a perfect bit of music by correcting the error intruduced by the hummer. Nevertheless, you should check your video transmitter against a 2.4GHz link, perhaps by doing a range check on a 2.4GHz model with your own model sitting right next to it transmitting video. or even better stick your video transmitter in a 2.4GHz model and range check it (on the ground I mean!). Nobby |
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