I just signed up tonight because I am a little stumped. I’m sure you’ve seen this general question a few times but I just cant seem to find the answer I want by searching.
I’m a hobby micro-controller/electronics/remote control enthusiast, I have been playing with PIC’s for about a year now. My current project requires a simple wireless link though. All I need to do is transmit a pin high signal from one micro-controller to another one at about 100’ away inside a moving object. So, after a wee bit of research I though XBEE’s are the way to go. So, I bought a pair of XBee 2mW Series 2.5 RPSMA units, a USB explorer and a couple of antenae. I made the mistake of assuming that the series 2.5 modules wire simply newer and greater versions of the series 1 modules.
After doing some more research and trying to figure out exactly how to configure the units for Digital I/0 passing I found it cannot be done. I really don’t want to learn how to do serial communication between two PIC as that seems overly complicated for what I want to do and I was hoping I could configure the units and then just start pulling pins up and down.
So, I have a couple of questions I would really like answered:
Is it true there is no way I can do digital I/O passing with series 2.5 modules without using the serial interface?
If the above is true, which modules would someone suggest I order so that I can do digital I/O passing. I would really like the RPSMA feature as the ability to remotely mount the antenae is quite important, otherwise I will have to have the XBee located externally of my steel enclosures. I can’t seem to find any modules that aren’t series 2.5 with the RPSMA feature.
RF module was tested and approved with 15 dBi antenna gain with 1 dB cable-loss (EIRP Maximum of 14 dBm). Any Omni-directional type antenna with 14 dBi gain or less can be used with no
cable-loss.
So it looks like this antenna will work, but wont be as powerful as the module can support. That is unless You need a 14dBi antenna. I don’t know much about antennae, other than you screw them onto the connector on the back of the device and don’t muddle them up between devices. Will I suffer a major loss of range?
Thanks for the reading waltr. I think I understand how it works now. The XBee Transmits with 1W of power this translates to about 1 dBm output. The FCC tested the XBee using a 15dBm antenna with a length of coax cable that created 1dBm of loss to the antenna, and found the module to operate safely, so they say anything up to 14dBm connected directly to the module is safe, even thought the module only transmits at 1dBm. So, the 5dBm antennas I have are overkill and I should have absolutely no problems, unless I use some sort of coax cable that has 4dBm of loss.
The XBee outputs 1mW of power which can also be expressed as 1dBm, 10 *log (Pout/ 1mW).
The FCC isn’t concerned with the modules safety only the radiated power that can affect other radio services. So the rule is that at the frequency and modulation type used by the XBee and being this is used in a Non-Licensed service the FCC Rules state what the maximum power that is radiated from the antenna may be and that is 14dBm EIRP.
Using a 5dBi antenna will keep you LEGAL. If you put a 22dBi antenna on the Xbee without any cable loss then there will be 22dBm of radiated power and you would be breaking the law and the FCC could fine you (or worse). But it would not hurt the XBee module.
Be careful with the units:
dB is actually unit less and is just an expression of the GAIN ratio on a Logarithmic scale. Example: If the power into a cable is 2W and the power out the other end is 1W than the Gain is 1/2. In dB this is 10log(Pout/Pin) or 10log(1/2) = -3dB or a loss of 3dB.
dBm is as I stated above.
dBi is the Logarithmic ratio of antenna Gain relative to that of a theoretical isotropic antenna (see the Wiki link in my last post).
W is Watt
mW is milliWatt or 1e-3 W
Please read those Wiki links and look up these other units in Wiki.
Ok, that makes sense, FCC says no more than 14dBi net output from the antenna for minimizing interference.
My physics background tells me you cant get more power than you put into a system, so when I was looking at the antenna gain of 5dBm and the XBee output of 1dBm I missed the different units and assumed I was getting free energy and was a wee bit confused.
So the 1dBm is the energy output of the RF module, the 5dBi is the multiplier of how the antenna transfers that energy to a radio wave.
So, simply using an antenna with a higher dBi (gain) will get you a higher energy output, but not necessarily a further range? Does it also decrease electrical efficiency?
Thanks for helping to teach me something new waltr.
There isn’t any magic with Gain from an antenna. Remember that the antenna gain figure is relative to an “Standard” isotropic antenna (why it is stated in dBi & EIRP). This antenna radiates power in all directions (a sphere) equally. For practical use there isn’t any need the radiated upward or downward, so a practical antenna radiates out the sides but not up or down. (see the Wiki on antenna gain and look at antenna patterns). Since all of the radiated power is now forced into a smaller area the intensity or energy density is increased and the measured power at some fixed distance is greater than with the isotropic antenna. This is the antenna GAIN.
Ok, that makes sense an antenna with a higher gain will make the energy pattern more like a vertically squished donut than a full fluffy donut as compared to an antenna with less gain. This keeps energy output by the RF module more dense therefore creating a stronger signal.
So assuming two antenna are in the same plane and both are oriented vertically the higher then gain the stronger the signal, but the less range the signal has in the vertical plane, so if you move on antenna high above the other you will loose strength faster than if you had an antenna with low gain.
Again, if I wanted to communicate with a plane from the ground I’d use a lower gain, and have a higher vertical strength but loose horizontal strength, but if I was communicating between ships Id want to use a higher gain with less vertical strength and more horizontal strength.
The FCC doesnt want us to use too high gain antenna because the energy density could become high enough in a narrow plane to cause interference with other radio transmissions.
Just so happens I will be communicating between a transmitter module on shore and a large model boat on a lake. Very little vertical strength is needed.
energy pattern more like a vertically squished donut than a full fluffy donut as compared to an antenna with less gain.
That is a correct picture of an antenna 'gain pattern.
if you move on antenna high above the other you will loose strength faster than if you had an antenna with low gain.
That is correct.
The received signal strength would decrease as the antenna pattern ‘points’ away for the source (works in both the receiver and transmitter side). This could be by having the antennas out of plane or by tilting the antenna.
This is why the application can dictate the antenna gain (pattern). If the antenna orientation con not be controlled (handheld phone for example) then a high gain antenna will cause problems due to drop-out in the antenna pattern NULLs. But if the antenna orientation can be fixed (phone cell tower antenna) then very high gain can be used.
So far we have been discussing Omnidirectional antennas (donut pattern) which does flatten as the gain is increased. Also consider a directional antenna like a multi-element TV antenna. These have high gain but the gain pattern is more like a flash light (torch if you’re in England) beam. This is why it is common to have a motor to rotate the antenna to achieve the beast reception.
Thanks a lot, waltr, your lessons have given me a much better perspective and understanding of how antennae work. I know feel confident that the antennae that I have will work as I want and that for future project I will have a better Idea of what I am looking for.
Back to the XBee’s after I had made my original post I was waiting for me series 2.5 XBee’s to come in the mail, they never did, so I phoned up the guys at sparkfun and it turns out I never made the order, everything was in my cart, but not ordered. So this morning I corrected my order and was able to get a pair of Series 1 modules from Digi. So, the I/O passing is laid out in the manual and should be a simple to implement.