I have bought several Xbee 1mW wire antenna modules. My problem is quite an uncommon one: I need to reduce the transmission range to be at most 3-5 meters. Using the lowest transmission power, the modules still can achieve a transmission range of more than 15m (indoor, line of sight).
What I mean as transmission range is the separation distance which allows two modules to transmit data with good PRR (Packet reception rate at least 75%).
I tried wrapping the modules in aluminum foil, but I am looking for a more “technical” ways (e.g. replacing the antenna?)
Put them in a ferrous metal box with no opening larger than about 1/8 inch.
The signal strength in line of sight reduces by 3dB (half) each time you double the path length (distance). Inverse square law.
Example math:
1mW is max power of XBee non-Pro.
(dBm: decibels relative to one miliwatt = 0dBm)
at a few feet, the received signal strength (2.4GHz) will be about about -40dBm. Doube that distance and it becomes about -43dBm. Not much change.
The receiver sensitivity at a packet error rate (PER) of 1% is about -95dBm or so. That means the receiver will work reasonably well at that weak signal.
So you see, the path length and receiver sensitivity are an order of magnitude larger than a small change in path length.
A really good professional RF Tight Box is about 80dB of attenuation at this frequency. Old ammo box with good screen wire seals is like a pro RF tight box. Another way is using copper screen wire to make a cube with a frame.
What you want is an inline attenuator or “pad”. The problem is that your antenna isn’t a standard connector type. I wonder if you couldn’t make an adapter cable that plugs into your Xbee antenna “port” at one end and has a standard SMA connector at the other. Then all manner of SMA pads are available. You would need a new (SMA) antenna and perhaps an [adapter of some sort.
Why resort to intentionally ruining your antenna performance artificially. When you can use the functionality that the AT-commands allready provide? ATPL (Power-level) has 4 levels of power. Default it is set to 4, which means 0 dBm for the ‘normal’ 1mW Xbee Series 1 module. Parameter value 0 means it transmits -10 dBm.
EDIT: Ok, after re-reading Stevech’s post above, my suggestion is of limited use. But all bits help to take a bite. ( pun intended
Mee_n_mac has the right idea. However, that’s a big device to put inline with a little xbee. Google around for an RF attenuator circuit. [Here is one that contains 6 selectable stages, but you probably just need 1 stage that is always on. However, selecting and soldering those resistors close enough to the transmitter is going to be near impossible, and at this frequency and low power level, that tiny bit of circuit board trace leading to the antenna is probably enough “antenna” by itself. I do not know if the xbee (like most other RF devices) would react badly to simply cutting off the antenna, but at least it’s a relatively cheap experiment. A hand-held 2-way radio, for example, can fry itself fairly quickly if you try to transmit without an antenna. Just make sure you always transmit at the lowest possible power.](http://www.qsl.net/n9zia/wireless/pics/step_att.png)
re prior post… can’t use attenuators with OP’s XBees which he said are the ones with a wire antenna rather than a U.FL coax connector to which you could attach an attenuator. This is why I suggested the RF-box (Faraday cage).
Can you tell us why you want the transmission range to be so short? I’m wondering if there’s another approach that would achieve the same application goals. Also, the wire antenna is just soldered on, so if you’re fine with desoldering it, try that out if you want to test the results (voiding the warranty of course).
We are part of a research group on networked robotics and would like to perform experiments for multi-hop wireless networks. Therefore, it is of our interest the study of network topologies with relatively large diameter, which can be achieved doing experiments in large areas (if tx range is long) or in small areas (if tx range is short). For convenience, the latter is preferred. We made some experiments performing cuts to the Xbee wire antenna, the transmission range reduces to almost null if the antenna is removed. We are planning to try stevech approach by wrapping the antennas with small copper or aluminum pipes.
eduardofeo:
We are planning to try stevech approach by wrapping the antennas with small copper or aluminum pipes.
Keep in mind thost “pipes” need to be well grounded otherwise they just become part of the antenna. If you’re willing to remove the antenna, you can make a small pi or T attenuator out of resistors. Just trimming the antenna length will also work, though you take the chance that the resulting mismatch may reflect enough RF back to the transmitter to harm it. With only 1 mW of RF available I suspect it’s a small chance but …
