Is 900 mHz legal in the USA for RC??

I just got from a Canadian company an RC transmitter and receiver that uses 900mHz. It works OK, but the thought occurred to me that I dont even know if 900mHz is legal in the USA. Anyone know? Thanks. RD2

Reply to
RD2
Loading thread data ...

| I just got from a Canadian company an RC transmitter and receiver that | uses 900mHz. It works OK, but the thought occurred to me that I dont | even know if 900mHz is legal in the USA. Anyone know? Thanks. RD2

Yes, it's probably legal, as long as it's between 902 and 928 mHz, and the power emitted is low enough (and if it's legal in Canada, it probably is.)

The relevent part of the FCC regulations can be found here --

formatting link
Now, the AMA does not list the 900 mHz band as one of their approved bands for RC usage, but the AMA doesn't have any legal authority so that doesn't matter.

Reply to
Doug McLaren

Well, I just got word from my local hobby shop that either JR or Futaba is coming out with a system on that band...apparently it selects its own open channel and I was told that a hundred planes could fly at once...not practical..but still within the radio capability range. Synthesized receivers and transmitters are getting more prevalent........in five years I'll bet you that most JR, Futaba and Hitec radios, to name a few..all will offer synthesized transmiter and receivers in their standard range.... Face it, they can make a telephone you can talk all over the world on, it will take pictures and send them and you can play games onthem and who knows what else..all in a package smaller than a pack of cigarettes...so it was just a matter of time before it filtered down into the r/c industry... Frank Schwartz AMA123 remembering the tube, relay and dry battery single channel stuff I used to build and fly.....way back when......

Reply to
Frank Schwartz

What company!!?? this is the latest technology as I understand it. I would be very interested in contacting the company.

Reply to
Jim Slaughter

| What company!!?? this is the latest technology as I understand it. I | would be very interested in contacting the company.

Merely using the 900 mHz band is hardly the `latest technology'.

Now, hopefully they're using spread spectrum, and while that's probably not the _latest_ technology either, it would be almost perfect for our purposes, removing all problems with channel conflicts and many problems with interference from other sources in one fell swoop.

You can buy spread spectrum stuff now, using either 900 mHz or 2.4 gHz. The most `polished' stuff seems to be what you can get at

formatting link
and I've even seen their stuff at the local hobby shop -- and the prices are comparable to standard equipment.

Alas, the Spektrum stuff is geared for R/C car users right now, probably due to the limited range that the severe power limits on those bands give. The range would probably be good enough for planes

-- their FAQ claims 3000 feet -- but some people do seem to fly their big gliders that far away, and we've certainly gotten used to having a good amount of breathing room, so ...

You know, now that I read their stuff more carefully, I wonder if it's really spread spectrum at all. Their FAQ says this --

The DSM system scans the 2.4GHz band and finds a channel that is not being used. Once an open channel is found the transmitter begins transmitting on that unused channel. The DSM System has an available spectrum of 79 channels. In the unlikely event that the spectrum is full, the 80th system will not connect or cause any interference, going into "hold scan" until a channel is free.

... which does _not_ describe a spread spectrum system. But the next item in the FAQ says this --

Won't other 2.4Ghz devices like phones and wireless computers cause interference?

No, the FCC requires that all 2.4Ghz DSS devises be "smart"- incorporating collision avoidance such that when any DSS system is turned on, it scans the 2.4GHz band until a channel that is not being used is found then begins transmitting on that unused channel.

(DSS usually means `Digital Spread Spectrum'. Described at

formatting link
.)

So, it sounds like the Spektrum RC stuff is not spread sprectrum itself -- only that spread spectrum stuff should be smart enough to not interfere with it. They don't actually say that the Spektrum stuff is DSS, only that DSS equipment will not interfere with it. (They call the Spektrum stuff DSM, not DSS.)

Which makes it sound much less interesting. Since it automatically picks a channel, there should be very few problems with interference from other R/C users (unless somebody turns on their transmitter somewhere else, then drives over here where you're using the same channel without turning off first), but you could still get intereference from a strong narrow signal on the 2.4 gHz band if the channel your radio chose just happened to be the same. Unless the FAQ is just wrong.

formatting link
seems to suggest that people at the AMA think that the Spektrum stuff is spread spectrum -- but if their FAQ is accurate, it's _not_ spread spectrum at all.

Though the `AMA' article on spread spectrum did help me realize one thing here -- the AMA rules don't restrict you to using the frequencies listed on the AMA frequency charts. To be more specific, the AMA rule says --

  1. I will operate my model aircraft using only radio-control frequencies currently allowed by the Federal Communications Commission

and since the FCC does permit R/C usage on bands like 900 mHz, 2.4 gHz and 5.8 gHz (they pretty much allow anything there as long as the power is low enough), using R/C equipment on those bands wouldn't violate AMA guidelines. Which is good.

In any event, there are a few people who do sell spread spectrum R/C equipment that's `really' spread spectrum, but as far as I know so far it's all homebrewed and expensive, but maybe the Spektrum FAQ just oversimplifies things and it really is spread spectrum ...

Ok, enough rambling :)

| "RD2" wrote in message news: snipped-for-privacy@news.nls.net... | >I just got from a Canadian company an RC transmitter and receiver that | > uses 900mHz. It works OK, but the thought occurred to me that I dont | > even know if 900mHz is legal in the USA. Anyone know? Thanks. RD2

Reply to
Doug McLaren

The problem with spread spectrum at low price points is the lack of update resolution. This is critical when flying a model, but not critical when used for telephony.

Ed Cregger

Reply to
Ed Cregger

I think you are on to it, Ed. It's also true that you have to trade off update rate for number of control channels, and that is one of the reasons why 2 channel R/C car radios aren't quite the problem that 6-9 channel model airplane radios will be to design. I expect we'll see some form of delta modulation used to make efficient use of the signaling channel bandwidth, and probably different rates for primary flight controls than for aux functions like flaps and gear retract - that is already in the waveform design of some of the PCM radios.

Abel

Reply to
Abel Pranger

There have been reports of the Spektrum system being interfered with / interfering with wireless LANs and wireless video cameras.

Apparently there are some work around.

I believe it is not truly compatible in the computer network concept but rather it tries to find and avoid channels in use at powerup. If a powered up LAN card is not transmitting when the TX is powered up it may try to use that channel.

IMHO not ready for prime time yet.

Expensive even when compared with a all channel synth. TX & RX system.

Hugh

Reply to
Hugh Prescott

First things first Abel. It is good to see the weather does not have you down.

As for your comments I think they are closer than any other possible scenario I have heard of or talked with radio folks about.

Reply to
Six_O'Clock_High

Listen, the voice pager and even the digital pager market is almost dead. Why don't we petition the FCC for those channels and or others and dedicate that to R/C spread spectrum?

Reply to
Sport Pilot

| >The problem with spread spectrum at low price points is the lack of | >update resolution. This is critical when flying a model, but not | >critical when used for telephony. | | I think you are on to it, Ed. It's also true that you have to trade | off update rate for number of control channels, and that is one of the | reasons why 2 channel R/C car radios aren't quite the problem that 6-9 | channel model airplane radios will be to design.

Wouldn't that just be a matter of assigning enough bandwidth?

PPM systems update each channel 50 times per second (it varies, but let's use that as a goal.)

Assuming 16 channels with 16 bits per channel (which gives 65536 possible stick positions per channel, better than the Spektrum's

4096), a six byte `address' for the transmitter (like an ethernet MAC address) and a four byte checksum (if the checksum doesn't match, throw the entire packet out) and one more byte for things like setting failsafe values, that works out to 43 bytes/packet. Sending 50 packets per second, you'd get about 16 Kb/s.

We could also shave much of this off by only broadcasting as many channels as a given plane needs, or by going down to 10 bits/channel rather than 16 bits/channel. The Futaba PCM1024 system only uses 10 bits/channel, for example. Doing only 10 bits/channel and 8 channels it only works out to 21 bytes/packet, or 8 Kb/s.

802.11g can do 54 Mbps in in 30 mHz of bandwidth, so it seems reasonable that we could put our 16 Kb/s into 9 kHz of bandwidth, which is approximately the size of our channels now. Assume that we want to be able to support 50 simultaneous planes at once, and we'd need 450 kHz of bandwidth total, which is approximately what we have now in the 72 mHz band.

802.11g cards can decode a packet in just a few miliseconds, and do it cheaply, so it seems reasonable to believe that a R/C receiver could do the same. Certainly, doing everything in 20 ms (1/50th of a second) seems possible, so your latency would be around 1/25th of a second at worst. How good are your reflexes? :)

If more planes than 50 went up at once, the transmission rate would have to drop, so the latency would increase, but nobody would get `locked out' if everything was designed properly.

(Of course, these are all `back of the napkin' type calculations, so they should go with some grains of salt.)

Not all 900 mHz/2.4 gHz/5.8 gHz systems are spread spectrum, and so not all would play nice with a system like this. Ideally another band would be allocated strictly for spread spectrum R/C stuff, 500 - 1000 kHz, both air and ground (since they could play nicely together) but at this point, it seems to just be a political and economic problem, not a technical one. The technical parts needed are already out there, and relatively cheap. But somebody would have to go and make the equipment. Having the right chunk of bandwidth for it would certainly help provide incentive for that, but bandwidth is money, so ...

Reply to
Doug McLaren

That's sort of close to the datarate of a digital cellphone -- roughly

8Kbps or less per direction (GSM, CDMA, TDMA, etc.)

I don't think that's an appropriate comparison. A WiFi system (which uses OFDM modulation) has a much higher power and transistor (logic) budget than current R/C radio-systems (even PCM.) To maintain a radio link at the same distances as current R/C systems (running on 72MHz), you have to fight (1) higher transmit frequency requires more power (basic physics from the radio-link equation), and (2) different

True. If R/C systems could leverage the high-volume production of commodity laptop WiFi chipsets, the hobby companies wouldn't have to burn R&D designing the innards of the radio-module, they could just focus on the board-level integration and R/C application.

I'm not sure what you're proposing here. When multiple devices share a single WiFi-LAN, the data-rate drops because the network 'timeslices' between the devices. Each WiFi device's MAC unit detects the presence of others, and obeys a specified protocol to share the airwaves nicely (avoid collisions.)

Using the same approach in an R/C system is a bad idea, because it leads to (unpredictable) transmission delay between a given transmitter and receiver pair. One could make the argument that the datarate is already so slow (

Reply to
Monaks

I think you are referring to "Spread Spectrum" technology. It is in development for R/C.

Reply to
Tom

plantraco.com RD2

Reply to
RD2

Thanks. I was hoping it was something more than mini-micro-tiny-teeny, etc.

Reply to
Jim Slaughter

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.