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Topic: XBee Antenna (Read 1 time) previous topic - next topic

arduinoPi

I have one of these https://www.sparkfun.com/products/10421
I would like to know if I can just use a wire to extend the antenna since the XBee is in a large robot and I would like to keep signal strength up.

Jack Christensen

How far does it need to go? The spec on that particular module is 300 feet indoor/urban, up to 2 miles outdoor line-of-sight.
MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

arduinoPi

I just didn't want the interior of the robot to interfere with the signal. The xbee it's self will be low to the ground so I was hoping to put a 5 foot wire to the top of the robot. Would this work?

kg4wsv

It's an antenna, not a random piece of wire.  The length of the antenna element is a function of the frequency.

Depending on the power of your transmitter and how bad the mismatch is, it can cause behavior anywhere from "it doesn't work" to "you just let the magic smoke out of the power amplifier".

There are xbees with coax connectors so you can use an antenna with a short feedline.

Another solution is to move the whole xbee, but there are limits to how far you can run the TTL level serial connection between the xbee and whatever it's plugged in to.  Those limits depend on noise, and a robot can be a noisy environment.

-j

arduinoPi

I am looking at the Xbee right now and it appears to just be a single wire no complete circuit. It is similar to a antenna on a radio

arduinoPi

my knowledge on Antennae is lacking I think I am going to do some research.

kg4wsv

Quote
it appears to just be a single wire no complete circuit


You're half right.  It probably is a single piece of wire, but it has some specific characteristics - most importantly, its length is equal to 1/4 of the wavelength of the signal being radiated (as measured in the conductor, which is slightly different than the free space wavelength).  This geometry allows the signal to radiate.

If it was an inefficient design (incorrect length, etc), at least some of that power would be reflected back into the power amplifier (special transistor) output, which could damage the amplifier.  At best, the signal isn't getting out of the radio as intended.

-j

arduinoPi

What can I do to increase range?

Jack Christensen

#8
Mar 17, 2013, 12:27 am Last Edit: Mar 17, 2013, 12:31 am by Jack Christensen Reason: 1

What can I do to increase range?


Again: What kind of range does it get now, and what is the requirement? What exactly is the problem we are trying to solve here?

If the XBee is truly "inside" the robot, that would be an issue if the robot's skin consists of significant metal parts. Even lacking a metallic skin, keep the XBee away from large metallic components. Consider using an XBee model with an RP-SMA connector (e.g. XBP24BZ7SIT-004, and note also the extension cables and antennas on that page) and get an antenna made for the job, e.g. https://www.sparkfun.com/products/558
MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

kg4wsv

A note/caveat to Jack's advice:  you can add coax, but not too much.

If you're using the 2.4GHz xbee (which is most of them), coax attenuates the signal quite badly.  I wouldn't use more than a few inches of coax, 10 or 12 max at 24.GHz.

The U.FL connector xbees can also use an antenna with coax (I think all the U.FL antennas have a feedline, but I could be wrong).

Don't forget that the xbee is a serial device, so it may be feasible to move the whole xbee to a different location.

More information would be helpful.  Like Jack says, what problem are we trying to solve?

-j

arduinoPi

The robot I have is 4 feet tall. The body is made of wood and it has a outer shell of aluminum flashing. The electronics including the XBee are in the center of the robot.
I was hoping to put a 1 foot antenna above the robot for better range and to reduce interference with an on board wireless netgear router.

I already have a pair of these https://www.sparkfun.com/products/10421 so I would prefer not to have to buy a different kind.

Jack Christensen

#11
Mar 18, 2013, 06:25 pm Last Edit: Mar 18, 2013, 06:29 pm by Jack Christensen Reason: 1

The robot I have is 4 feet tall. The body is made of wood and it has a outer shell of aluminum flashing.


So essentially one of these. They have some interesting properties. I wouldn't think much will be heard from an XBee or any other radio transmitter inside that robot.

Quote

The electronics including the XBee are in the center of the robot.


I'd look at relocating the XBee to some position where it can see the light of day without a layer of aluminum (or aluminium for GM et al.) in the way. The serial data lines to the XBee should be ok running a few feet.

Quote

I was hoping to put a 1 foot antenna above the robot for better range and to reduce interference with an on board wireless netgear router.


A Wi-Fi router will not interface to an XBee, so that would be futile. It'd be overkill even if it did; the Pro XBee should have more than ample range, but it can't be in a can.
MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

TrailerTrash

i have this on my wish list to replace my 2W.
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