Futaba Fasst vs Spektrum radio link

[GooseF22]

In general terms, video signals are very dirty...meaning alot of side lobes, and the orientation of the ground plane under the antenna is critical. We use old circuit board for the ground plane.

The "hole" on a pole antenna, not only is directly above the antenna, but also directly below it, so we tried to place the ground plane so it shields the receiver. Still there are alot of trons floating around. And the Video 2.4 systems do not incorporate collision avoidance for the most part, as well as being broad instead of narrow bandwidth.

Thats a long way of saying, that the guy next to you running a 2.4 video could affect you, but his plane would have to be close to your plane, not his receiver. But.....the video systems use alot of power, so there are alot of variables.

In general I would suggest if someone is using a 2.4 downlink, keep your distance from his plane. If the video system was turned on first, so the DX could avoid it, it worked...

in the same scenario, the FASST system didn't fare as well, but that failure was on the same plane, and I don't know which system was turned on first, or what channel the video was on, but the plane was a loss.

With that in mind, a 900 mhz video is best OR a 900 mhz radio link.....

If its any consolation for 72, orienting the ground plane and separating the transmitter from the receiver has to be done for all the setups no matter on 72, 900, or 2.4 or the noise threshold can overwhelm the gain control of the receiver.

[GooseF22]

	Quote: Originally Posted by Zeeb
	D*mn good question Sam... I'm feeling better about it after seeing Goose's answer, but I'm gona have to chew on it for a bit... I say that because one of the guys I fly with has an electric powered Cub with the DX-7 system and when he bought the video system for it, the manufacturer told him he'd have to go with the 900 stuff for the video when using the 2.4GHz for his flight control. So his system shouldn't affect my 2.4 at all, but the stuff is getting more popular. He's only flown it once as it's still in the "get everything working stage", but the video system makes his servos hum when it's turned on. They seem to work okay, but that tells me there's some "extraneous" signals getting into stuff they shouldn't.

Tell him to make a ground plane under the vid tx antenna. a circular shaped circuit board.

[GooseF22]

Hey,
I read this off the Linksys website, who make 2.4 routers and network gear. I thought it was a good simple explanation.

What is Spread Spectrum?

Spread Spectrum technology is a wideband radio frequency technique developed by the military for use in
reliable, secure, mission-critical communications systems. It is designed to trade off bandwidth efficiency for
reliability, integrity, and security. In other words, more bandwidth is consumed than in the case of narrowband
transmission, but the trade-off produces a signal that is, in effect, louder and thus easier to detect, provided that
the receiver knows the parameters of the spread-spectrum signal being broadcast. If a receiver is not tuned to
the right frequency, a spread-spectrum signal looks like background noise. There are two main alternatives,
Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS).

What is DSSS? What is FHSS? And what are their differences?

Frequency-Hopping Spread-Spectrum (FHSS) uses a narrowband carrier that changes frequency in a pattern that
is known to both transmitter and receiver. Properly synchronized, the net effect is to maintain a single logical
channel. To an unintended receiver, FHSS appears to be short-duration impulse noise. Direct-Sequence Spread-
Spectrum (DSSS) generates a redundant bit pattern for each bit to be transmitted. This bit pattern is called a chip
(or chipping code). The longer the chip, the greater the probability that the original data can be recovered. Even if
one or more bits in the chip are damaged during transmission, statistical techniques embedded in the radio can
recover the original data without the need for retransmission. To an unintended receiver, DSSS appears as low
power wideband noise and is rejected (ignored) by most narrowband receivers.

[RadiMani]

Has anybody tested when some noise applied on the line of the satellite receiver? The line convey the digital data to main rx. When the satellite receiver to be placed remote place from main rx, the 3-wire cable must be extended. In this case, some electric nosie will be easy to intrude. For example, for gasoline powered engine, or jet, the ignition or controller's noise is very huge....

[wildhare]

	Quote: Originally Posted by RadiMani
	Has anybody tested when some noise applied on the line of the satellite receiver? The line convey the digital data to main rx. When the satellite receiver to be placed remote place from main rx, the 3-wire cable must be extended. In this case, some electric nosie will be easy to intrude. For example, for gasoline powered engine, or jet, the ignition or controller's noise is very huge....

I think that the hundreds of people flying the Spektrum/JR technology with gas engines (myself included) have pretty well shown that this is not a problem. I am quite sure that they used a signaling system that is noise averse.

TF

[Aly]

	Quote: Originally Posted by NormS
	Because of the different pseudorandom sequences in different transmitters, two or more DSSS systems can actually use the same channel at the same time with no interference (this is how CDMA cell phones work) but I don’t think Spektrum makes an attempt to do this because they seem to have a limit of 40 systems at a time.

This is off Spektrum's FAQ: (http://spektrumrc.com/DSM/FAQ.aspx)

What happens if two transmitters are on the same frequency?
To be FCC legal, all 2.4Ghz devices must incorporate a collision avoidance system that prevents the system from transmitting on an already occupied frequency. The chance of two transmitters occupying the same frequency is highly unlikely. If two transmitters should somehow end up on the same frequency, other safeguards, such as the GUID and proprietary time base coding, will prevent interference. Of course, with the aircraft system, the transmitter is transmitting on two frequencies simultaneously. The odds of two transmitters transmitting on the same two frequencies is even, more unlikely but again, should this happen, other safe guards will prevent interference.

It seems Spektrum systems will work when operating on the same frequencies (be it Spektrum or other) as other devices. It appears they have the 40 systems limit due to FCC requirements.