[NSRCA-discussion] SPECKTRUM Modules for Futaba and JR

[J.Oddino]

I'm having a real problem understanding what an "unused frequency" is.  First of all nothing that transmits information transmits on one frequency as it must use a band of frequencies.  SS intentionally uses a wide band of frequencies and the wider this band is relative to the information bandwidth the more processing gain you get and that is good.  The Direct Sequence system "chips" the baseband information at some high rate with a pseudo random code that should be unique to your transmitter in order to spread the bandwidth.  With different codes it should therefore be possible to operate many RC systems simultaneously using the same band of frequencies. 

Now getting back to "unused frequencies" , which we will interpret as unused channels about one megahertz wide, what happens if a system (not necessarily an RC system) that uses Frequency Hopping turns on and spreads its info over more than one channel?  Or just decides to use the same  channel that you are using?  Maybe the FCC rules prohibit this but I'd hate to depend on the government to make sure my airplane doesn't get shot down.  I've got to believe that "unused" really means that the ambient noise level is below some acceptable value and I'd like to think we can really operate a lot more than 40 users in an 80 MHz band.

It sure would be nice to get an understandable, but more technical description of this system and its limitations.

Jim O
  ----- Original Message ----- 
  From: JonLowe at aol.com 
  To: nsrca-discussion at lists.nsrca.org 
  Sent: Thursday, February 22, 2007 7:26 PM
  Subject: Re: [NSRCA-discussion] SPECKTRUM Modules for Futaba and JR


  In a message dated 2/22/2007 6:52:04 PM Central Standard Time, patterndude at tx.rr.com writes:
    What does Spectral capacity - 40 systems mean?  Is this the number of tranmitters that can operate in range of each other, or is this 40 channels or what?
    --Lance

  This is grossly simplified, but the 2.4 ghz band has about 80 frequencies.  Each Spektrum unit picks two unused frequencies and transmits on both.  80/2=40 transmitters that can operate at once.  It is a requirement of the FCC that things on this band don't interfere with each other, and transmitter power is strictly regulated, therefore anything using the band has to check before transmitting.  You have no idea which two frequencies you are on (nor do you care), as they change each time the transmitter turns on.

  I've flown my Dad's Arresti III with the DX7 extensively.  Absolutely solid link, with no fear of getting shot down.  I have been waiting for something like the modules so I could use a decent transmitter for things like throttle curves, conditional mixes, etc.

  I don't want to be at a field that has 40 airplanes in the air at once!

  Jon Lowe





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