range issues with Xbee pro

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range issues with Xbee pro

Postby endrew » Sun May 24, 2009 4:45 am

hi,
in the specifications of the XBee Pro 900 Wire Antenna
https://www.sparkfun.com/commerce/produ ... ts_id=9097

it is noted that its range is:
Up to 6 miles (10 km) RF LOS with high gain antennas

its not clear to me if this range is achieved with the wire antenna the module comes with or should i use a different antenna to get this range...

Also if the range with the wire antenna is shorter - what will it be?

Thanks for any help!
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Postby stevech » Sun May 24, 2009 9:37 am

900MHz. To get miles of range, you'd be using yagi antennas.

You can accurately predict the range between two units if you do a "RF link budget". Google that. There are many on-line tools. Essentially...

Transmitter's power, in dBm. Example: 10mW = 10dBm, 1mW=0dBm.
add Transmitting antenna gain in dB
add Receiving antenna gain in dB
subtract path loss, calculated from physics, path length versus frequency.
subtract possible loss if the Fresnel zone isn't clear. This happens for long paths if the antennas are not high enough.
subtract a bit for coax cable/connector losses, if any.
this sums up to the received signal strength in dBm. Compare this to the required signal strength for a certain vendor product/mode.
The excess signal strength is called link margin. You'll want margin to accommodate occasional fading, say, a margin of 6dB. The cure for low margin is usually higher antenna gain. At 900MHz, this can be a 4 ft. long yagi with about 8dB gain as I recall.

Now the hard part: if the path is not line of sight (trees, buildings) you have to add to the path loss a figure depicting this. There are a few models of this, based on general measurements for cellular. But really, if there is low margin, you have to do this experimentally. Experience in wireless allows people to intuit what cannot work.

Also, don't forget possible interference - in No. America 902-928MHz is an unlicensed shared use band. For high reliability, some radios (including some that Digi makes) use frequency hopping to avoid interference.

so with some discipline, this can be more than guesswork and trial-and-error
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Postby endrew » Mon May 25, 2009 6:56 am

first of all - thank you for the detailed reply.

however - in the link above, the module comes with a wire antenna. I dont know how to get its gain...

I am only interested in LOS range, and I only need to get about 2Km range.
It will be very helpful if anyone can send a link, to the antenna that provides the longest range, or if anyone can estimate what range i can get with the wire antenna...

Last thing - one of the modules will be installed on a small aircraft, so I can not use directional antennas.

Thanks...
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Postby Philba » Mon May 25, 2009 10:19 am

do what steve suggested. It's still good advice. I think you are going to be disappointed in the performance, though.
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Postby stevech » Mon May 25, 2009 3:11 pm

endrew wrote:first of all - thank you for the detailed reply.

however - in the link above, the module comes with a wire antenna. I dont know how to get its gain...

I am only interested in LOS range, and I only need to get about 2Km range.
It will be very helpful if anyone can send a link, to the antenna that provides the longest range, or if anyone can estimate what range i can get with the wire antenna...

Last thing - one of the modules will be installed on a small aircraft, so I can not use directional antennas.

Thanks...

with the little "wire" antenna - give it a value of 2dBi gain. When you work through the numbers, you'll see that you can't get kM range with such.
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Postby endrew » Tue May 26, 2009 6:47 am

ok, I used this calculator to compute the path losses:
http://www.daycounter.com/Calculators/F ... ator.phtml

I entered
output power = 50mW ->17dbm
freq = 900Mhz
distance = 9600m
Gtx = 2dbi
Grx = 2dbi

the result I get:
Prx = -90.2dbm

now, according to the Xbee 900 manual, the Receiver Sensitivity is -100dbm so i should be able to get more than 10Km... which I know cant possibly be true...

any way I decided to use the Xbee XCS (100mW) and a 2dbi dipole antenna:
http://www.sparkfun.com/commerce/produc ... ts_id=9143

I hope it can provide a 2-3Km range. Ill be thankful to anyone who can tell me if I'll really get this range.
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Postby stevech » Tue May 26, 2009 10:51 am

endrew wrote:ok, I used this calculator to compute the path losses:
http://www.daycounter.com/Calculators/F ... ator.phtml

I entered
output power = 50mW ->17dbm
freq = 900Mhz
distance = 9600m
Gtx = 2dbi
Grx = 2dbi

the result I get:
Prx = -90.2dbm

now, according to the Xbee 900 manual, the Receiver Sensitivity is -100dbm so i should be able to get more than 10Km... which I know cant possibly be true...

any way I decided to use the Xbee XCS (100mW) and a 2dbi dipole antenna:
http://www.sparkfun.com/commerce/produc ... ts_id=9143

I hope it can provide a 2-3Km range. Ill be thankful to anyone who can tell me if I'll really get this range.


-100dBm is the claimed sensitivity to get a certain bit error rate, and in the absence of interference and fading. In practice, you'll need to allow 6dB or so for fading in line of sight. For interference, you either do freq. hopping (which some Digi 900MHz products do), or if the interference is brief, just delay and retransmit n times - presuming you checksum/CRC the message, or rely on such in the link layer.

Of course, the required signal strength depends on the channel bandwidth and modulation method. Narrower channel = lower modulation rate = better sensitivity, and so on.

The receiver noise figure has to be considered to, and down at -100dBm and weaker, and with channels hundreds of KHz wide, or more, it gets to be a consideration.

so a 100mW transmitter and essentially no antenna gain is marginal at 1Km line of sight. Beware the null in the antenna pattern of dipoles - parallel to the rod, off each end. "Pointing" th end of the dipole at the far end is not what you want to do!
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Postby endrew » Wed May 27, 2009 3:31 am

OK...
that was my best option for range using Xbee...

can you recommend a different low weight - wireless system that can provide better range?

thanks...
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Postby stevech » Wed May 27, 2009 8:07 pm

endrew wrote:OK...
that was my best option for range using Xbee...

can you recommend a different low weight - wireless system that can provide better range?

thanks...


nope. It's the best value. It might work at 1-2Km line of sight, reliably enough. Problem is getting enough Tx power on the lightweight platform side with a crude antenna and battery power.

I did get 3/4 mile range at 2.4GHz with the XBee Pro 2.4GHz version. Not much margin. One end had a 5dBi gain antenna, the other used the little PCB blue chip antenna. So if you're using 900MHz, it may be OK.

Digi makes a 1 watt 900MHz radio. XTend as I recall. MIght be too big.
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Postby drteeth26 » Tue Jun 02, 2009 10:45 pm

I don't have hard numbers, just my experience with the 900mhz xbee modules. With the wire antenna, line of sight I got somewhere around a quarter mile. It wasn't perfect line of sight at all...just walking around my neighborhood. I use them mostly inside in a theater, and have been able to get reception from the first to the third floors through a lot of concrete block. Our IT level wifi routers don't even come close to doing that.
The next bunch of xbees I get will be 900's with antenna connectors so I can start playing with higher db antennas.
One thing to remember with the 900's is they can use Mesh networking to effectively extend their range by bouncing messages from one node to another (an over simplified explanation I know).

--Dr. Teeth
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Postby stevech » Wed Jun 03, 2009 9:40 pm

with one end of the link being a "base station" and with a 5dBi gain antenna, and the other being a mobile with the PCB antenna, I got 1/2 mile reliable, and at 2.4GHz, with 60Mw XBee Pros.

With a similar gain antenna on one end and 900MHz, I'd expect 1+ mile line of sight. I did find in one project (not XBees but 900MHz) that trees attenuate 900MHz more than I expected.
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