2.4Ghz = no more glitches... sure?!!? [Archive]

dogg5306

05-11-2007, 07:23 PM

Hi there,

I'm a 35Mhz 14MZ regular user and I'm planning to get the 14 channels 2.4Ghz FASST module and receivers as soon as they will be available... but I've got one question about it :roll:

With Spektrum you've basically got 2 receivers and 4 antennas (AR9000)... on Futaba FASST system you get 1 receiver and 2 antennas and the system can be "extended" by linking the receiver to another one and so on...

Question: Why do we have such a possibility if 2.4Ghz is supposed to be glitch free :?:

Danal Estes

05-11-2007, 10:41 PM

Because the wavelength of 2.4 is very short. There are tiny "holes" or "nulls", about the size of your clenched fist (or a little smaller). If a single RX/antenna gets in one of these nulls... problem. The good news is that if a null exists, you can pretty much be assured that a few inches away is a peak. So, two antennas (or two RX/antenna assemblies) a few inches apart assure good reception.

Note that the nulls can exist in free air space for complex multipathing reasons, and/or they can be created by "shadowing" from masses aboard your aircraft like the engine, the fuel tank, etc. In a moving aircraft, they can rapidly move around. Antenna/spatial diversity completly solves this issue.

The "Glitch Free" mostly refers to the generation of RF noise aboard your aircraft. A bearing on a helicopter, or a metal link vibrating against other metal on a plane, or the ignition system on a gasser, or the ESC on an electric, all generate RF noise. This RF noise turns out to be overlap the 35 and 72 Mhz bands, but does not extend anywhere near the 2.4Ghz band. The second "Glitch Free" is because these are SPREAD. Spread systems tend to see any RF energy that's not using their exact "Spreading Code" (sometimes called a chipping code) as noise. The explanation of exactly how this works is long and arduous. The best one I've seen is in relation to GPS systems, which use a very similar concept to operate "below the noise floor", that is, they use "digital gain" from the spreading/despreading to operate at S/N ratios that classic radio theory would say are impossible...

And, of course, "Glitch Free" sort of extends into "Shootdown free" because there are no longer channels as we understand them. The systems interoperate (in various complex ways) so that they ignore each other. I've seen several Spektrum users standing side-side, just a few feet apart. Try that with 35 or 72!! Even on widely separated channels, 3 rd order IM will kill you. That's why pilot stations are separated...

Anyway, the multi antenna is for spatial diversity to avoid nulls and shadowing. The glitch free part is very real.