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.
-- 
Chris

Chris wrote:
> 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.
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

On Sun, 04 Nov 2007 22:14:06 GMT, Kevin 
wrote:

>Chris wrote:
>> 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.
>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....

Gavin  wrote:
> On Sun, 04 Nov 2007 22:14:06 GMT, Kevin 
> wrote:
>
>>Chris wrote:
>>> 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.
>>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)

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

Gavin wrote:
> On Sun, 04 Nov 2007 22:14:06 GMT, Kevin 
> wrote:
> 
>> Chris wrote:
>>> 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.
>> 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 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
> 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....
>

Chris wrote:
> 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.


Around 65-70 IIRC.

range is reduced as more models go up.

Kevin wrote:
> Gavin wrote:
>> On Sun, 04 Nov 2007 22:14:06 GMT, Kevin 
>> wrote:
>>
>>> Chris wrote:
>>>> 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.
>>> 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 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
>> 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....
>>

The Natural Philosopher <a@b.c> wrote:
> Chris wrote:
>> 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.
>
>
> Around 65-70 IIRC.

Unfortunately you don't RC.

> range is reduced as more models go up.

No it isn't.

Kevin  wrote:
> Gavin wrote:
>> On Sun, 04 Nov 2007 22:14:06 GMT, Kevin 
>> wrote:
>> 
>>> Chris wrote:
>>>> 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.
>>> 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 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

Nobody Here wrote:
> Kevin  wrote:
>> Gavin wrote:
>>> On Sun, 04 Nov 2007 22:14:06 GMT, Kevin 
>>> wrote:
>>>
>>>> Chris wrote:
>>>>> 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.
>>>> 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 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
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

On Mon, 05 Nov 2007 21:01:35 GMT, Kevin 
wrote:

>Nobody Here wrote:
>> Kevin  wrote:
>>> Gavin wrote:
>>>> On Sun, 04 Nov 2007 22:14:06 GMT, Kevin 
>>>> wrote:
>>>>
>>>>> Chris wrote:
>>>>>> 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.
>>>>> 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 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
>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


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.

Gavin wrote:
> On Mon, 05 Nov 2007 21:01:35 GMT, Kevin 
> wrote:
> 
>> Nobody Here wrote:
>>> Kevin  wrote:
>>>> Gavin wrote:
>>>>> On Sun, 04 Nov 2007 22:14:06 GMT, Kevin 
>>>>> wrote:
>>>>>
>>>>>> Chris wrote:
>>>>>>> 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.
>>>>>> 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 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
>> 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
> 
> 
> 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.
ah that makes sense

Kevin  wrote:
> Gavin wrote:
>> On Mon, 05 Nov 2007 21:01:35 GMT, Kevin 
>> wrote:
>>> 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
>> 
>> 
>> 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.
> 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.

Nobody Here wrote:
> The Natural Philosopher <a@b.c> wrote:
>> Chris wrote:
>>> 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.
>>
>> Around 65-70 IIRC.
> 
> 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.

The Natural Philosopher <a@b.c> wrote:
> Nobody Here wrote:
>> The Natural Philosopher <a@b.c> wrote:
>>> Chris wrote:
>>>> 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.
>>>
>>> Around 65-70 IIRC.
>> 
>> 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.

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.

In article , The Natural 
Philosopher <a@b.c> writes
>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?
-- 
Chris

Chris wrote:
> In article , The Natural 
> Philosopher <a@b.c> writes
>> 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%20BTCongress%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.

Chris wrote:
> In article , The Natural 
> Philosopher <a@b.c> writes
>> 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..

The Natural Philosopher <a@b.c> wrote:
> Chris wrote:
>> In article , The Natural 
>> Philosopher <a@b.c> writes
>>> 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..

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




"Nobody Here" <nobby@invalid.invalid> wrote in message 
news:13j21b0gv3fm1a7@corp.supernews.com...
> The Natural Philosopher <a@b.c> wrote:
>> Chris wrote:
>>> In article , The Natural
>>> Philosopher <a@b.c> writes
>>>> 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..
>
> 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
>
>

Peter Seddon  wrote:
> 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

On Mon, 05 Nov 2007 08:44:37 -0000, Nobody Here <nobby@invalid.invalid> wrote:

>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

Barry OGrady wrote:
> On Mon, 05 Nov 2007 08:44:37 -0000, Nobody Here <nobby@invalid.invalid> wrote:
> 
>> 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
> 
> Barry
> =====
> Home page
> http://members.iinet.net.au/~barry.og
> I do not represent atheists or atheism

In article , Barry OGrady 
 writes
>On Mon, 05 Nov 2007 08:44:37 -0000, Nobody Here <nobby@invalid.invalid> wrote:

>>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.
-- 
Chris

Chris  wrote:
> In article , Barry OGrady 
> writes
>>On Mon, 05 Nov 2007 08:44:37 -0000, Nobody Here <nobby@invalid.invalid> wrote:
>
>>>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.

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

Nobody Here wrote:
> Chris  wrote:
>> In article , Barry OGrady 
>>  writes
>>> On Mon, 05 Nov 2007 08:44:37 -0000, Nobody Here <nobby@invalid.invalid> wrote:
>>>> 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.
> 
> 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

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.