Some guidelines on picking wireless frequencies

Wireless can be something of a black art, and there are a dizzying array of standards and choices. Here are some simple guidelines to help you pick wisely:

  • --We operate in four frequency ranges: 72Mhz (RC gear), 900 Mhz (video and telemetry), 1.3 Ghz (video) and 2.4 GHz (RC, video and telemetry), not including GPS, which is a passive receiver only.
  • --It is a BAD IDEA to have transmitters and receivers in the same frequency range right next to each other, so you'll want to choose your equipment to avoid this.
  • --In general, the lower the frequency the longer the range because it can go around obstructions better, so 900 Mhz video tends to have longer range than 2.4Ghz video. But digital transmission technology can more than compensate for that, so it's not a hard-and-fast rule. For instance, a high-end spread-spectrum 2.4GHz wireless video setup can outperform a lower-end 900 Mhz one.
  • --Some frequency ranges are more crowded than others. For instance, indoors and in urban environments 2.4GHz has to compete with everything from WiFi to Bluetooth to cordless phones. 900Mhz just competes with some cordless phones.


So, for example, you could try these configurations:

 

Configuration One: RC: 2.4Ghz. Xbee 900Mhz. Video: 1.3 Ghz

 

Configuration Two: RC 2.4 Ghz, Xbee 900Mhz, Video 2.4Ghz, with antenna as far from RC as possible

 

Configuration Three: RC: 72 Mhz, Xbee 900Mhz, Video 2.4 Ghz.

Views: 3737

Comment by Xander on January 19, 2009 at 8:38pm
Perhaps it's more trouble than it's worth, but have you considered dropping the RC gear entirely and using the Xbee for remote control? Seems like using the Xbee for all communications besides video could simplify things in the long run. I'm considering doing so with the project I'm planning.

3D Robotics
Comment by Chris Anderson on January 19, 2009 at 8:44pm
Yes, reverse engineering a half-century of RC development definitely counts as more trouble than it's worth ;-)

Seriously, it's a huge amount of work for no benefit that I can think of off the top of my head. (The only reason to do it would be for a ground-based autopilot, which might make sense for quadcopters or stunt helis, but doesn't apply to the fixed wing stuff we do.)
Comment by James Ross on January 19, 2009 at 9:08pm
The benefit would be that you'd free up that part of the spectrum. Autopilots like the Piccolo autopilot by Cloud Cap Technology integrate data and RC on the 900 MHz band. The remote control plugs right into the base station.
Comment by Jack Crossfire on January 20, 2009 at 12:00am
If you want to sell your autopilot, skip the XBee & use stock equipment. Most of the buyers come from an RC background & already have radio sets. If you want to do it for yourself, by all means build RC control & telemetry on XBee PROs. After all, Chinese build everything from scratch.

The XBee PRO is probably as good as any $300 radio set. You can build a triple redundant full duplex XBee PRO link for less than a $pektrum.
Comment by Howard Gordon on January 20, 2009 at 9:07am
It really depends on what you are trying to accomplish. From my perspective, attaching 50-year-old technology to a modern control system is a big step backwards, and the system (controls, telemetry and video) should be designed from the ground-up as a digital system with single data link. However, this requires a higher bandwidth communication channel than the half-duplex 80kbps you get via a single XBee Pro. We use 2.4GHz 802.11g for this purpose, but range is limited. There are some promising technologies in the 900MHz range that ultimately should provide the best solution, at least for my objectives.

100KM
Comment by crystal garris on January 20, 2009 at 6:34pm
A 2.4 MHz transmitter can even swamp a 900 MHz receiver . what people refer to as "swamping" is actually when the AGC of the receiver drops the gain to low for decent reception the best plan is to use 2 different bands like you suggest but also add a low , high or band pass filter to the receiver .
Comment by Pat on January 21, 2009 at 8:38pm
If you end up experiencing decreased performance of the GPS receiver due to the 900Mhz video transmitter, the following post from rcgroups may help you. I'm not an RF guy, so I don't claim to know if this is a good fix or not, but the fact that it is a notch filter makes sense to me and for the cost I would give it a shot.
Comment by Jack Crossfire on January 21, 2009 at 9:13pm
For a good time, go to http://www.xtremepowersystems.net/universal.php where they accidentally show you the $130 XTremeLink universal module & it's just an XBee with an adaptor board. The adaptor board probably has a super cheap PIC microprocessor on it. Forget 40% markup. How about 200% markup.


This is the sad truth about most RC electronics. They're just the simplest cheapest parts repackaged & marked way up because most people in RC don't know a thing about designing circuits.
Comment by spagoziak on June 9, 2011 at 1:02pm

I'm just beginning my reading on ArduPilot and everything that it entails... I'm completely impressed and plan to buy the necessary bits once I have money available.

 

In the meantime, my reading has lead me to the value of telemetry, in this case for the purpose of using ArduTracker (FPV antenna tracker).  Near as I can tell, there isn't an xbee (or equivalent) transmission system available at frequencies other than 2.4ghz and 900mhz... both of which are in use on my rig (RC and FPV respectively).

 

1: Is there a 5.8ghz telemetry-compatible system available?  

2: Is there any chance of a telemetry system being cheaper than $150?  That price is killing me :(


Moderator
Comment by Ruwan on January 9, 2012 at 9:37pm

Hmm, why do I see no difference in Configuration one and Configuration three. So to use Video 1.3GHz do we have to have HAM licence?

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