I'm working on a project that requires wireless communication between two devices but only within a small distance (2-3 feet at most). I have settled on an using RF (2.4 GHz) to communicate between them, but need to be able to limit my range.
Would tuning my antenna to a harmonic of 2.4 GHz work? Or would this result in an unreliable communication link? I know antennas are VERY complicated, but could I just play with reducing the length of the antenna?
reducing the length of the antenna causes power to reflect back into the amplifier, causing problems. The creature you probably want is called a PAD, its a small network of resistors that attenuates the signal while maintaining correct impedances at the antenna, and output amplifier. If yours is a single antenna system this may knock down your receiver sensitivity as a side effect, and thats VERY BAD in non spread spectrum systems and just plain BAD in spread spectrum. many 2.4 ghz devices use a switch between the transmitter and receiver to select which are connected to the antenna.
I'm sure there are websites about SWR nowdays, so you should google "standing wave ratio" which is kinda misnamed, it is the relation between power leaving the antenna (good) and power reflected back from the antenna (bad)
It depends on your device, if this is bluetooth and short ranged to begin with , or something a lot more professional.
for two to ten devices, get a pad from say Minicircuits, for more, have the device redesigned by a pro. 2.4 ghz is well past the point where things get difficult for a non rf person.
Actually, a lot of low-power antennas you see in the ISM band there are dipoles, patch antennas, etc. -- nothing particularly fancy.
For something like a dipole, sure, that'll work fine, and progressively decrease your gain as you shorten it (assuming you started with a "quarter waver" or shorter antenna to start with -- that'd be roughly 1 1/4").
Can't you get your mates to ask the bloke from the other country who doesn't need to know what they are chatting about to swap seats?
Otherwise they will be waving waveguides over his head and that will look a bit suspicious. Unless they claim that reception of BBC2 is a bit bad in the confluence chamber.
Reducing your effective radiated power would reduce the range but in a very mushy way, and with the risk that normal reception is compromised should the slightest irregularity happen (dirt, humidity, object in the path etc.)
If you goal is to absolutely make sure that communication will fail when moving, say, 10ft away you need a more elaborate scheme. For example the receiver can transmit right back and the total delay (plus possibly phase if xmit back happens at a lower frequency) of that return echo is measured. IOW you tolerate x amount of latency and no more. However, this is not at all a trivial task at such a short distance.
In ultrasound we do this stuff all the time but there we usually have a few microseconds.
why the range limit? are you concerned about others jamming your application? or your jamming somebody elses? would IR work instead of RF? or coded RF transmission to keep your data seperate from others?
3 feet is not very far. For example, I have a Laipac 434 MHz receiver whose specs say it has a -100 dBm sensitivity. This receiver, with a 1% efficient antenna, should almost be useful at 3 feet from a transmitter when putting
10 microwatts into its 1% efficient antenna.
Assuming you are using a low-power transmitter which will not be hurt by its load:
Use a half-inch (or less) antenna to transmit (1/10 wavelength).
Use a use a receiver with terrible sensitivity.
Use a half-inch (or less) antenna to receive.
Adjust a combination of the above to get the range you desire.
Due to reflections and other variations, I think it will be a bit difficult to adjust the range to between 2 and 3 feet and expect reliable operation. But, please let us know if you achieve this and how you do it.
Disconnect the antenna... Otherwise, reduing the transmit power at both ends seems OK.
Best Regards
Jens
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Why not just use straight coupling between coils ? IIRC the field falls off as the cube of distance, so nobody is going to be eavesdropping unless they are right on top of you.
To cope with 'null' spots, use multiple receive coils in different orientations and switch between them for the strongest signal. Google 'diversity'
Yeah, I saw that. I'm just not sure that the lowest transmission setting (-18 dBm) will be low enough to restrict the range to what I need. I don't have a circuit ready to test it, so I'm trying to hedge my bets a bit...
If I were a smarter man and could use a faster microprocessor, this would the ideal method. However, I'm naught but a simple hobbyist trying to tackle a project way out of my league. And with a maximum desired range of about 3 feet (if my calculations are anywhere near correct), the signal will be received about 3 nanoseconds after it's transmitted. Not the kind of thing my little 20MHz microprocessor is going to be able to measure.
As you say, not at all a trivial task.
By the way, by strange coincidence, my wife just had an ultrasound yesterday and we found out that everything's fine with our baby. I don't know what you do with ultrasound, but I can't escape the feeling that I should say thank you. So, thank you. Keep up the good work.
Basically, I'm designing a system involving one or more stationary "base stations" and corresponding wireless "tags". A "bases station" needs to be able to detect and react when its associated tag comes within range (again, 2 or 3 feet).
The wireless "tags" need to be as small as possible (2 inches square at the very most), and should use as little battery power as possible.
Using IR was my initial idea. However, the fact that LEDs wear out over time has me worried that the tags would lose range over time. Plus, I can't always guarantee that the "tags" are going to be facing the "base stations" in any particular way, which makes IR troublesome.
On the other hand, now you've got me rethinking it. Implementing IR would be quite a bit simpler than using the 2.5 GHz transmitter I'm looking at. And, LED wearout and directionality may not be as troublesome as I first thought. I should be able to get by with an IR LED and an IR phototransistor circuit as a receiver, and should be able to easily adjust the LED current to restrict the range... Hmmmmm... I'll have to look into that again, thanks.
Would it be admitting too much ignorance to say that that's all waaaay above my head? RF is *way* beyond me, which is why I chose an integrated transceiver solution. However, I'll look up 'diversity' and will hopefully either gain some knowledge of antennas or inter-racial relations. Sounds good either way!
I hope you are interleaving the playing of Mozart to the sprog in utero with readings from the sacred 'Art of Electronics'. (;-)
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OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
This stuff is usually done analog or at least the latency assessment part is analog. In that domain 3nsec is almost an eternity these days. Plus the total time available would be round trip, in addition to whatever the transceiver needs to create the echo.
True. Fishing out the true signature of the echo isn't easy and without that a latency measurement would become inaccurate. Although, if your requirement isn't 3ft plus minus a few inches it could be done. You would need a blazingly fast comparator but the good news is that nowadays we have chips that can almost do it. When I started out it was all discretes, nothing integrated was fast enough (although a butchered uA733 came close).
Glad to hear that. My happiest moments were usually when watching our newly designed machines being used in the clinic. Now I am working more in cardiac ultrasound and the most sophisticated systems are typically used in difficult cases. Sometimes when I see the images there in the cath lab all I can do is quietly pray for the patient.
You can use audio-frequency magnetic fields, with a tuned receiver. Systems like this are already in use for security purposes. Extremely simple. My e-mail address works, if you are interested to go further.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
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