The truth about the XBee

[Jack Crossfire]

For those of you planning to use a 2.4Ghz XBee system for RC, some bits have leaked out on the paparazzi mailing list.

Am 10.09.2007 um Uhr haben Sie geschrieben:
> It worked well up to 500m, then we lost the uplink and later the
> downlink.

> > Right now I have like 2 km down link and about 10m up link.

> the video signal is broad, loud and constantly sending. You would need
> some very well tuned narrow filter to separate the XBee receive path
> from that.

The answer is, expect the same problems with the 2.4Ghz XBee that U had at 72Mhz. It's not optimized for controlling aircraft and falls over with TV transmitters. A purpose built 2.4Ghz diversity RC system is still required for the uplink.

[Unterhausen]

What were they using for video transmission? There is always going to be a problem if you use a high power Tx near a Rx. The Zigbee by itself seems to have fairly good range, much higher than 500m.


> the video signal is broad, loud and constantly sending. You would need
> some very well tuned narrow filter to separate the XBee receive path
> from that.

The answer is, expect the same problems with the 2.4Ghz XBee that U had at 72Mhz. It's not optimized for controlling aircraft and falls over with TV transmitters. A purpose built 2.4Ghz diversity RC system is still required for the uplink.

[AntonK]

The video signal he is refering to is not TV, it is RF as in wireless video equipment on the same 2.4Ghz frequency. Ive seen teh Xbee Pro work to 3km in a rural environment, in an urban environment this number goes way down due to wifi though.

Anton K

[_helitron_]

The only problem here is the SAME frequency band of 2,4 GHz !

//Erwin

[Unterhausen]

The video signal he is refering to is not TV, it is RF as in wireless video equipment on the same 2.4Ghz frequency. Ive seen teh Xbee Pro work to 3km in a rural environment, in an urban environment this number goes way down due to wifi though.

Anton KI assumed it was a cctv downlink. Is it on the same frequency, or on 900MHz? I'd be interested in details. Most people that have 2.4GHz video systems are reporting nearly zero range with Spektrum systems, so 500 m. is not a bad range under those conditions. My impression is that the 2.4GHz systems are skirting the rules for transmission power. Generally, 2.4GHz systems are reported to be able to coexist with 900MHz systems and Wifi for that matter.

[Jack Crossfire]

> Most people that have 2.4GHz video systems are reporting nearly zero
> range with Spektrum systems

So word of mouth is slowly revealing a parallel reality which is entirely different than what the product promoters claimed. The Paparazzi notes were regarding 2.4Ghz video and 2.4Ghz XBee Pro on the same airframe. 2.4Ghz video also interferes with 72Mhz, but it isn't zero range. Guess the only reliable data connection is the big, heavy, expensive 900Mhz modem.

[workshop]

My Xtend module from Maxstream is pretty small, light and simple to use...

[Jack Crossfire]

$180, 2.5" x 1.5" is too big and expensive. Think $32, 1" x1", compatible with 2.4Ghz TV transmitters. Was hoping someone could point to a movie showing an XBee controlling an aircraft with a 2.4Ghz TV transmitter on it. Looks like the 72Mhz transmitter and its unreliable, measly 3kbit is staying around for uplink. Was hoping the XBee would provide 115kbit uplink and immunity from TV transmitters but the price curve simply isn't there yet.

[XJet]

I ran a check of the XBee Pro modules alongside a cheap 600mW 2.4GHz video sender and found that although it raised the noise floor on a couple of the XBee's channels, the majority remained completely unaffected.

Perhaps my video sender was better than average quality (with less splatter) -- I don't know.

i do have plans however to use the XBeePro (for telemetry) alongside this transmitter in a UAV we're testing at the moment. It will be interesting to see whether the real problem is desensitization of the XBeePro's receiver rather than spectral pollution from the video sender.

[vector_vortex]

I was under the impression that zigbee is a Direct Sequence Spread Spectrum modulation technique which spreads the output power over a greater bandwidth, As opposed to Frequency Hopping Spread Spectrum like the XTend and Aerocomm RF modules, As I understand it a DSSS while allowing for greater bandwidth and hence transmission data rate is inferior in terms of transmission distance compared to FHSS which dumps the power into the smaller bandwidth and allows a narrower and hence more sensitive receive amps.

[Jack Crossfire]

Since these TV transmitters also interfere with GPS, you can't expect a 2.4Ghz part to do any better.

[XJet]

I've used the 600mW 2.4GHz video senders we've got very extensively with GPS-guided UAVs and never encountered any problems at all.

[Jack Crossfire]

We're not getting anywhere. What is the make & model of the 600mW 2.4Ghz video sender, the distance between it and the GPS part, the distance between the two XBee modules that showed no interference from the TV transmitter, the distance between the XBee module and the TV transmitter, the maximum range of the XBee parts with the TV transmitter, the distance between the TV transmitter and nearby objects? And still there is no video of flying using the XBees to control an aircraft with a TV transmitter.

Without specifics, we can't assume this part is going to work.

[XJet]

We're using the transmitters sold by BlackWidow AV.

This transmitter sits about a foot from the GPS receiver in our blended-body UAVs. The Telemetry transmitter (900MHz or 2.4GHz depending on the market) is also about a foot away.

We get no interaction with the GPS or the 900MHz gear and are about to trial the 2.4GHz XBeePro in this configuration.

[AntonK]

You can expect that the 2.4 video TXs will interfere with just about everything onboard. They are very noisy, and the higher the power the greater the noise level of course. We have seen them interfere with both IR sensor signals and gyro/accel analog signals. They interefere with the 900Mhz transmission to some degree, and of course on 2.4 Ghz xbee transmission. Radio equipment is loud, and I like Martin's analogy, you have equipment with sensitive ears sitting along side whats equivalent to a large subwoofer. Using good antennas, directional antennas pointed away from components, and even isolating components in metal foil, and ferrite all helps to some degree. Good luck! It is a very difficult problem.

[XJet]

Gosh, I guess things must work differently in the southern hemisphere, we haven't had any of those problems with our 2.4GHz video senders and other gear interacting adversely.

[Unterhausen]

Gosh, I guess things must work differently in the southern hemisphere, we haven't had any of those problems with our 2.4GHz video senders and other gear interacting adversely.I don't think that all 2.4GHz systems are the same. The cheap ones you can buy here for around $100 are apparently rather noisy.

[_helitron_]

The 2,4 GHz stuff is not functioning only in the southern hemisphere without problems, XJet, but also here in Europe :D ! I've in use in my Cularis FPV-motorglider: a 500 mW BWAV 2.4 GHz video Tx, only 10 cm/4" (!) away a standard 35 MHz Futaba PCM Rx and 25 cm/10" away the GPS Rx (1575.42 MHz) for the OSD and rudder-home function. I fly away up to 2 km (!) and had til today not one failsafe (I see a failsafe immediately on the OSD screen respectively later on the video) and no problems with the GPS !

//Erwin

[typicalaimster]

Hmm interesting. I've seen 2.4ghz Video walk all over my GPS. However once I add some distance (12 inches) between my GPS and video TX I'm fine.

Now 2.4ghz (Non Spread) and and 2.4ghz (Spread) doesn't play nicely here.

I've had great success with a mixture of 2.4gig, 900mhz, and 72mhz all in the same ship.