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HelicopterMain Discussion › Ircha: How is impounding working with Spread​Spectrum radios?
08-09-2007 07:28 PM  10 years agoPost 21
Micro-Maniac

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Pasco,Washington​Formerly: Captain​Chaos

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72mhz is susceptible to a number of things including high power industrial stuff spilling over onto the band and things like metal-to-metal such as passing trains or the helis own bearings and other stuff (I'm pretty sure I've been taken down by irrigation systems flying crop circles) - 2.4ghz is well above metal-to-metal frequency and supposedly use of 2.4 is limited to 1 watt of transmission power - Like I said I've experienced a lot of 72mhz glitches over the years but so far no noticeable 2.4ghz glitches

I'm pretty sure you have to exceed the bandwidth of more than one 2.4ghz channel for things to start slowing down and dropping off - Spektrum systems do use 2 of 80 channels at once for this kind of redundancy - Other systems channel hop - So something not onboard able to take out an SS system would probably take out every 2.4ghz device in the vacinity with it - Unlikely

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08-10-2007 12:24 AM  10 years agoPost 22
tadawson

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Lewisville, TX

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My point was that Spektrum has 40 frequency "pairs" that can be active at any time. So, I still want to know, what happens if a 41st TX comes on air, and then enters the fray? Who dies? Since 2.4 is so directional, this could be as simple as walking out of a metal building . . . it does not necessarily have to be distance. Perhaps this is unlikely, but unlikely does NOT equal impossible. So I ask again, who dies - what (if any) algorithms exist in the Spektrum technology to address this kind of a frequency "collision" issue?

Oh, and I think you will find that WiFi tends to be more packed oriented and manages collisions, since like others have said, it's not realtime. As far as I know, all 2.4GHz R/C stuff is constant signal, with no collision detection/recovery capabilities.

- Tim

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08-10-2007 01:13 AM  10 years agoPost 23
RyanW

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Edmond, Oklahoma

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Back to the Topic

It is going great at IRCHA. I love the way they have dealt with this... everyone on 2.4 keeps there radio and respects the flight line use. Simple and it is working. If there becomes a problem, they will have a system to monitor and hopefully prevent abusing (hogging) flight stations by people, but so far so good. Good to have Spektrum!

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08-10-2007 01:14 AM  10 years agoPost 24
9387ASH

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UK

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My understanding is that the 41st Transmitter will not go to transmit because as far as it is concerned all 40 segments are in use. Once a segment pair becomes available, the 41st transmitter then moves up and can now transmit. Think of it as a large table with 40 seats, the 41st person cannot sit down until someone finishes and moces away from the table.

Also, (again as far as I understand) IF someone comes along that has been shielded from the others and is already ocupying a segment pair, because each system has an unique GUID (which the receiver will only recognise because it was "bound" to a transmitter) AND that the duty cycle is less than 100%, then the likelyhood of a transmitter causing interference to another receiver is still minimal. In reality, what are the chances that tranmitter No 41 will have exactly the same segment pair as someone else ?

This must be one of the advantages of having Frequency Diversity in the system.

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08-10-2007 01:14 AM  10 years agoPost 25
Micro-Maniac

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Pasco,Washington​Formerly: Captain​Chaos

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No one dies - The 40 and the 41st or 90th system that didn't see the 80 channels already in use when it initiated will all still work given the band-width of the channels and the GUID - They will continue to operate and just ignore each other - That is the reason for the large band-width and the GUID is to allow for the extra info carried on the channels by whatever else happens to show up on those channels while 1 or 40 Spektrum systems are already in use - As I said before it's not absolutely limited to 40 systems - There's plenty of room on the wide-band channels for more if such cirmumstances arise not just from other Spektrum systems but from whatever

Mac_Man seems to share my understanding of it all
The chairs at the 2.4ghz table are plently wide enough to share unlike the narrow ones at the 72mhz table where someone falls off and eats dirt if anything tries to share a seat

Thanks for the report RyanW - I'm glad they aren't impounded so we can really see how they handle the ultimate test of IRCHA

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08-10-2007 02:47 AM  10 years agoPost 26
drdot

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So. California,​Orange County.

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fwiw...

72 is not an RC only band...Some ham bands are very close, and the PRIMARY user of the band is Construction equip. such as cranes...We are secondary users, which is same as no protection at all...2.4 is here, why fight it!?

John.

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08-10-2007 03:59 AM  10 years agoPost 27
kangarooster

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Orlando Fl-USA

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2.4 is here, why fight it!?

Isn't that what they said about FM vs AM ?

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08-10-2007 05:56 AM  10 years agoPost 28
tadawson

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Lewisville, TX

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72 is not an RC only band...Some ham bands are very close, and the PRIMARY user of the band is Construction equip. such as cranes...We are secondary users, which is same as no protection at all...2.4 is here, why fight it!?
I won't argue that the entire 72 band is not an exclusive allocation, but there is no ham band anywhere near it - the ham allocations go from 50MHz to 144, skipping 72 by a mile . . . . And within the 72 MHz band, I think you will find that although there are things interspersed, that the R/C specific frequencies are a primary allocation. In the 6 meter (50MHz) ham bands, there is nothing but ham activity (at least in the US) which is why I fly there - never had a hit or interference, since all ham activity such as repeaters are subject to frequency coordination, and will NOT be allowed on an R/C designated segment . . . despite the "claims of doom" some folks make about repeaters in the R/C frequency space.

I still don't think 2.4 is as rugged as all the marketing blow claims it is, but it does appear to be an option for those who can't master a frequency board . . . .

- Tim

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08-10-2007 01:04 PM  10 years agoPost 29
w8qz

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Grand Rapids, MI -​USA

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As tadawson said, there are NO ham bands close to 72 MHz. 50 - 54 Mhz is well below; 144- 148 MHz is well above. There are other (non-ham)services close in around 72MHz, though: wireless microphone setups, for one.
My vote is for 50 MHz ham - there are (relatively) few hams that even have equipment for that band; of those that do (because it was included on a multiband radio) few will use it - because of fear of causing interference to TV channel 2, or due to lack of interest. Of those that do operate the band, most will be operating in the bottom 100 KHz or so, away from the designated 50 MHz RC frequencies.

"The helicopter is much easier to design than the aeroplane, but is worthless when done."

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08-10-2007 01:51 PM  10 years agoPost 30
Juggernaut

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Canada, Great White​North

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One item not mentioned, as you go higher in frequency, more power is required to achieve the same signal level for the same distance.
example:
At 72 MHz to travel 500 meters, the signal degrades by 65dBm
at 2.4GHz to travel the same distance the signal degrades by 95dBm
that's 30dB, basically requires an additional watt of power.

As to having the 41 radio come on line, that would mean that it would have to occupy another radios frequency, even though, each radio has been bonded to the RX, they are occupying the same airspace, they will interfere with each other, it's the same as two 72MHz radios on the same channel, it's like listening to two conversations at the same time, if you are lucky you'll get bits and pieces, but not the whole message. If the ID is sent, it may be corrupted, and ignored, if the message is received, and command structure is corrupt, it will ignore the message.
Simply put, anything that occupies the same frequency you are using is considered as interference



Finally learned to fly inverted, Helps if you stand on your head

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08-10-2007 04:21 PM  10 years agoPost 31
JKos

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Redondo Beach, CA

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> they are occupying the same airspace, they will interfere with each
> other, it's the same as two 72MHz radios on the same channel, it's
> like listening to two conversations at the same time

Not even close to the same as two 72 MHz radios on the same channel. Huge difference and that's the beauty of spread spectrum.

Someone said it above and was correct that Spektrum transmissions are not even close to 100% duty cycle; in fact, they are way down in the single digit percentage. That means the two frequencies being used by any one Spektrum transmitter are only occupied for very short periods of time every frame.

That 41st radio would a) be unlikely to choose the same two channels as any single one of the other 40 radios, b) be unlikely to be transmitting at the exact same time on those two frequencies, c) even if they do transmit at the same time, the "other" transmitter will just be noise to the unintended reciever(s).

> One item not mentioned, as you go higher in frequency, more power
> is required to achieve the same signal level for the same distance.

While true, the intent of that statement ignores the benefit of spread spectrum. With spread spectrum, you need much less signal level at the reciever to transfer the same data as a non-spread spectrum system.

- John

RR rules!

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08-10-2007 05:14 PM  10 years agoPost 32
trunkmunki

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Bangor

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AM is STILL the wave of the future!

One thing to consider, some of us don't live in areas chock-full of flying sites and clubs. Some of us fly more in our back yards or informal sites (i.e. a couple of guys in a large field). The problem here is when one of us is flying, and our neighbor 3/4 of a mile away turns on his transmitter, there is a serious possibility of a shoot down. Not to mention the guys that decide to use their aircraft-band TX for their boat.

So for everyone touting the frequency board as an end all, in some cases 2.4 is just more secure. In ALL of the clubs I have been in as I moved around, I never saw more than 10 guys AT THE FIELD, not to mention flying, at the same time. No, 2.4 is not absolutely bullet-proof, but 72MHz certainly is not either. The new stuff is just a little more convienient.

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08-10-2007 05:50 PM  10 years agoPost 33
tadawson

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Lewisville, TX

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There are other (non-ham)services close in around 72MHz, though: wireless microphone setups, for one.
FYI, most wireless these days has gone to UHF - 450 (fading) and 800 and up, which is the most common now . . . I have not seen a wireless mic setup offered on VHF in years . . . .

- Tim

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08-10-2007 05:57 PM  10 years agoPost 34
tadawson

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Lewisville, TX

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The problem here is when one of us is flying, and our neighbor 3/4 of a mile away turns on his transmitter, there is a serious possibility of a shoot down.
Assuming that both transmitters are running in spec with regard to power output, "capture effect" in FM should make this almost totally impossible - the signal from 3/4 mile away will not be strong enough to interfere with your TX. (I'm too lazy to go look up the difference in strength where capture takes effect, but you can hear it on an FM radio, when you are between two fairly strong stations - you don't hear both, you typically hear only one of the two, and they may pop back and forth). Shootdowns can only happen if the two R/C transmitters are close to each other, or the "offending" signal coming from farther away is much more powerful, causing it to hit the model at a similar signal strength as the primary transmitter.

On, and if anyone cares, I have been flying all my helis on 50MHz with PCM for 5+ years, and not a single glitch or other radio related problem . . . In my case, if it ain't broke, don't fix it!

- Tim, N8EAU

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08-10-2007 11:07 PM  10 years agoPost 35
MrMel

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Gotland

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On, and if anyone cares, I have been flying all my helis on 50MHz with PCM for 5+ years, and not a single glitch or other radio related problem . . . In my case, if it ain't broke, don't fix it!
I take it you never owned a High Voltage (10, 11, 12s) heli.
I rather go naked through our capital city then go back to to old FM stuff (for us 35Mhz)... Thats how much I love 2.4ghz stuff.

I can tell that 1/2 mile away is NOT enough to be safe from being shotdown, that I know...

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08-11-2007 03:49 AM  10 years agoPost 36
Trace

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Wildwood, MO

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Methinks a few folks need to read up on SS and how it functions, using FM analogy to argue how SS might fail is like unhooking a battery to tell someone they have water in their fuel.

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08-11-2007 06:02 AM  10 years agoPost 37
Micro-Maniac

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Pasco,Washington​Formerly: Captain​Chaos

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Methinks a few folks need to read up on SS and how it functions
There's a good idea
An SS receiver uses a locally generated replica pseudo noise code and a receiver correlator to separate only the desired coded information from all possible signals. A SS correlator can be thought of as a very special matched filter -- it responds only to signals that are encoded with a pseudo noise code that matches its own code. Thus, an SS correlator can be "tuned" to different codes simply by changing its local code. This correlator does not respond to man made, natural or artificial noise or interference. It responds only to SS signals with identical matched signal characteristics and encoded with the identical pseudo noise code
All SS systems have a threshold or tolerance level of interference beyond which useful communication ceases. This tolerance or threshold is related to the SS processing gain. Processing gain is essentially the ratio of the RF bandwidth to the information bandwidth.

A typical commercial direct sequence radio, might have a processing gain of from 11 to 16 dB, depending on data rate. It can tolerate total jammer power levels of from 0 to 5 dB stronger than the desired signal. Yes, the system can work at negative SNR in the RF bandwidth. Because of the processing gain of the receiver's correlator, the system functions at positive SNR on the baseband data.
"A case in point is a two-year demonstration project the Federal Communications Commission (FCC) authorized in May (1990) for Houston, Texas, and Orlando, Fla. In both places, a new spread spectrum personal communications network (PCN) will share the 1.85-1.9-gigahertz band with local electric and gas utilities. The FCC licensee, Millicom Inc., a New York City-based cellular telephone company, expects to enlist 45000 subscribers.

"The demonstration is intended to show that spread-spectrum users can share a frequency band with conventional microwave radio users--without one group interfering with the other -- thereby increasing the efficiency with which that band is used. . . . "

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08-11-2007 06:08 AM  10 years agoPost 38
Inspector Fuzz

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NLA

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All I know is......

I am pretty darn happy with my 2.4ghz system and I ain't ever goin back. If my micro electic walnut scale planes didn't need 72mghz, I would get rid of my last FM transmitter.
PREDICTION: 6 years from now RC Models will all be 2.4ghz and the frequency control boards will be gone from flying fields.
JEFF

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08-11-2007 06:16 AM  10 years agoPost 39
Eco8gator

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Palm Beach, FL

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Hello

I just came from IRCHA. Id still be there if it wasnt for my graduation...

But they definitally handled the freq's very well. Guys kept their 2.4 systems while the 72Mhz modules were impounded until checked out.

There were no issues reguarding freq conflicts that I know of. And my XPS system preformed flawlessly in my flybarless Logo 10 3D.

C

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08-11-2007 08:12 AM  10 years agoPost 40
tadawson

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Lewisville, TX

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I take it you never owned a High Voltage (10, 11, 12s) heli.
I rather go naked through our capital city then go back to to old FM stuff (for us 35Mhz)... Thats how much I love 2.4ghz stuff.
Perhaps I am a mutant, but myself, I'd rather run naked through anywhere of your choice than fly large electrics . . . all nitros here, save for a TRex 450, which I use only when desparate (and travelling) . . . .

- Tim

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