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Radio - JR & Spektrum DSM > How Many Frequencies

Trace
Senior Heliman
Location: Wildwood, MO

Juan asked a question earlier and got an implied answer. I recall something like 22 or 24 flight stations spaced at close to 200 feet. Not exactly a mile after doing the math, but getting pretty darn close.

See what's really going on is DennisG got a taste of my DX7 connected to a sim projected on the wall in the library meeting room during the club mtg and now he's trying to convince himself not to buy one.

02-07-2007 Over year old.

MikeC
Key Veteran
Location: Wausau, WI

This is off of Spektrum's site

What happens if the band is full (80 users for surface or 40 users for aircraft) and I turn on my transmitter?

In the unlikely event that all channels are occupied, the next transmitter will scan the band indefinitely until open channels are available. The transmitter will then acquire the channel(s) and begin transmitting. Only then will the system connect.

So yes, only 40 possible at a time.

Impound may not be a requirement by the AMA but that doesn't mean that a specific event can't impose an impound. As more and more Spektrum radios are used, and especially at very large events like IRCHA, there will eventually be a problem if some type of impound isn't used. There will need to be some way of governing how long a person has their radio on since it will affect the guy's who are flying as well as the guy's doing setup.

Mike

Century Swift, Hirobo Sky Robo, Spektrum DX7

02-07-2007 Over year old.

Dennis G
Senior Heliman
Location: ST LOUIS, MO

Trace Flying the Sim

Does anyone know how many of the DX7s sold in the past 6 months.
Trace after flying your Sim at the club meeting I thought you should not be the only club member to have one of these Spectrum Radios. I should be the next in line to buy one. I ordered the radio and received it the other night.

Freya evo, V-Blades, Wildcat Fuel Distributor

02-07-2007 Over year old.

Trace
Senior Heliman
Location: Wildwood, MO

Hook, Line, and Sinker

02-07-2007 Over year old.

oldboldpilot
Key Veteran
Location: Southern California

Captain Chao,

No, sorry, if there are 80 radios on, then there are still (3160) - (80) = 3080 combos left, if your radio is designed to use a random pair of fqys.

Your odds of not being able to get yours going is, therefore, 80/3160 = 2-1/2 per cent - if ALL 80 are "on" when you turn on.

If none is "on" when you fire up, then you have a 2.5% chance of being shot down, IF all 80 turn on after you are in the air.

That's the math..

Question: are these transmitters designed to use a random pair of FQY's?

Helis are Man's Defiance of the Laws of Nature - OCHC

02-09-2007 Over year old.

Micro-Maniac
Elite Veteran
Location: Pasco,Washington Formerly: Captain Chaos

They don't freaquency hop if that's what you're saying - Each radio chooses 2 of 80 total channels and does not change channels until the radio's power is cycled - 80 channels / 2 channels per radio = 40 radios - I don't understand what this 3160 number is you're referring to because it doesn't apply to Spektrum® systems.

02-09-2007 Over year old.

oldboldpilot
Key Veteran
Location: Southern California

MikeC,

Eighty radios "on" doesn't begin to fill the "two-FQY-at-a-time" band.

Consider a radio "on" which uses the lowest FQY plus one other. There are 79 possible combos for this "lowest FQY plus one other FQY" transmitter.

That is, the chances another transmitter using the same lowest FQY will interfere with the first one is 1/78 or just under 3%.

Ah, but the odds of a second transmitter also using the same lowest FQY is 1/80 (if transmitters are using two FQYs at random). So the odds that you will be interfered with by any other transmitter is 2 one hundredths of a per cent.

So, if 50 transmitters suddenly come on after you take off (assuming all transmitters are using randomly assigned pairs of FQYs), there is a 1 per cent chance you will be shot down in flames - assuming that another rig using the very same two FQYs as yours means you are shot down (as he would be).

That is, given that you know there are 50 transmitters "off" until you get into the iar, then "on," they will "get you" just 1 flight out of 100 (assuming randomly assigned FQY pairs).

Anyway, the real question is whether or not the transmitters for sale use randomly assigned pairs of FQYs in this band.

If they do not, the odds of being shot down increase dramatically.

If they are, relax, you are in your mother's arms.

Helis are Man's Defiance of the Laws of Nature - OCHC

02-09-2007 Over year old.

Micro-Maniac
Elite Veteran
Location: Pasco,Washington Formerly: Captain Chaos

The radios scan to see what channels are already occupied and select 2 channels that are not occupied - If all 80 channels are occupied then the radio cannot acquire 2 channels for itself and will standby until 2 channels become available - If you start flying with no other radios on and then 50 radios turn on you will not be affected unless you cycle the TX power and loose your channels to another radio - Even then the RX will just go into fail-safe and ignore anything it hears from any source other than the radio it's bound to.

02-09-2007 Over year old.

oldboldpilot
Key Veteran
Location: Southern California

Dennis G,

You don't really care how many transmitters were sold in California or Hawaii. Yoou want to know how many will be at your flying site at the same time, huh?

The big question is whether or not transmitters are assigned pairs of FQYs at random across the 80 available FQYs. If they are, you are quite secure.

Helis are Man's Defiance of the Laws of Nature - OCHC

02-09-2007 Over year old.

pH7
Key Veteran
Location: Sterling Heights, MI - USA

oldboldpilot

It maybe COULD work the way you describe, but it does not. Read the Spektrum manual. When a spektrum Tx is turned on it picks two channels that are not in use by any other Spektrum Tx. When 40 transmitters have been turned on in a small area, then all 80 channels will be in use and the 41st Tx to be turned on will not find any available, as Captain Chaos said.

02-09-2007 Over year old.

MikeC
Key Veteran
Location: Wausau, WI

Quote
Eighty radios "on" doesn't begin to fill the "two-FQY-at-a-time" band.

80 radios on means 40 will be functioning and the other 40 will be waiting for someone to shut off. The last 40 TX's will not be transmitting because the band is full.

Quote
Ah, but the odds of a second transmitter also using the same lowest FQY is 1/80

No, the odds are zero. It won't transmit on a frequency that is already occupied.

Again:

This is off of Spektrum's site

What happens if the band is full (80 users for surface or 40 users for aircraft) and I turn on my transmitter?

In the unlikely event that all channels are occupied, the next transmitter will scan the band indefinitely until open channels are available. The transmitter will then acquire the channel(s) and begin transmitting. Only then will the system connect.

So yes, only 40 possible at a time.

Mike

Century Swift, Hirobo Sky Robo, Spektrum DX7

02-09-2007 Over year old.

Dennis G
Senior Heliman
Location: ST LOUIS, MO

I was just was curious to see how many Spectrum's Radios was sold already.
The other question I have is when you come to a big event like IRCHA that is not flat ground or you turn your radio on behind a metal shield area and then walk out to flight line there may be a chance that you lock onto someones freq. There flight line is long enough that this can happen.

Freya evo, V-Blades, Wildcat Fuel Distributor

02-09-2007 Over year old.

MikeC
Key Veteran
Location: Wausau, WI

That's a good point since these radios are line of sight. Or the guys across the road doing the scale stuff.

Mike

Century Swift, Hirobo Sky Robo, Spektrum DX7

02-09-2007 Over year old.

pH7
Key Veteran
Location: Sterling Heights, MI - USA

Someone who understands Spektrum better than I do should chime in - but I do know that the information transmitted has the transmitter ID in it and that the receiver ignores anything from another transmitter. I would expect that as long as the correct transmitter is significantly closer than the wrong transmitter, then there wouldn't be a problem.

02-10-2007 Over year old.

MikeC
Key Veteran
Location: Wausau, WI

Quote
but I do know that the information transmitted has the transmitter ID in it and that the receiver ignores anything from another transmitter.

I read early on with the introduction of the DX6 that the GUID in the TX is one of about a million codes. As it's name says it is a Globally Unique ID so there should never be another TX with the same code unless Spektrum at some time produces more than 1 million radios. But the fact remains that it is virtually impossible for any 2 TX's to be on the same frequency.

I can only think of 2 scenarios that would allow this. One is that 2 TX's are turned on at the same exact time. They're turned on, scan the band and both lock onto the same frequency. This would be highly unlikely as the timing of the 2 would have to be exactly the same. And even if it did happen it's highly unlikely that they would also lock the same second frequency. The other scenario is that a TX is turned on out of range of the other TX's, say left on when a guy was loading up his stuff at home and drives to the field with it on. Once he gets to the field, as luck has it, he's on the same freq. as someone else. Again, highly unlikely that both of the frequencies would be the same. At any rate we don't need to worry about it. It's certainly a lot better than any conventionl system.

Mike

Century Swift, Hirobo Sky Robo, Spektrum DX7

02-10-2007 Over year old.

Dennis G
Senior Heliman
Location: ST LOUIS, MO

Futaba Spektrum 2.4

Mike, Those are good points and I think the (binding) that the Spektrum radio is capable of marring up with the receiver would keep anything from happening.
I was at the E-Fest this past weekend and looks like Futabas is Coming out with there Spektrum Radio soon. I talk to one of the sponsored pilots and they are going to get there radios in March. I wonder if the futaba radios will work the same way. Does any one under Stan's how there system work.

Freya evo, V-Blades, Wildcat Fuel Distributor

02-12-2007 Over year old.

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