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multiple antenna

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Andrew_C View Drop Down

Joined: 05 October 2010
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    Posted: 15 October 2011 at 6:17am
Hiya, I have a reader with a nice antenna attached to it. However I want this reader to be able to process tags from multiple locations. Can I simply attach another antenna to the same solder contacts (and hope and pray) or would the antenna conflict - only one antenna will be in use at a time, no chance of both being interfered.

Ta, Andy
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amal View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amal Quote  Post ReplyReply Direct Link To This Post Posted: 15 October 2011 at 11:18am
if the reader is for passive tags, then no you can't attach another antenna. you could try to attach a multiplexer though, which would allow one antenna to be active at one time, then switch to the other antenna. As long as both antennas were tuned correctly when they are active on the multiplexor, it should work. 

The tuning is the issue though... loop antennas are resonators and if you have your capacitive elements on the reader side of the multiplexor, then the loop antennas will need to be identical in every way, including the length of wire connecting them to the multiplexor. You can't connect one loop antenna to the reader with a 3 foot piece of wire/coax, and then connect the other loop antenna to the multiplexer using a 50 foot piece of wire/coax... the impedence will be different and your tuning capacitors cannot be adjusted properly for both loops... so one antenna will work and the other one won't.

Commercial multiplexers for passive loop antennas actually actively tune and detune as they switch antennas. This is good for two reasons; first, if you have your antennas next to each other where their fields would overlap (like in a smart shelf application), while one antenna is active, the other one is actively detuned by the multiplexer so it will not resonate and interfere with the active antenna. second, a good multiplexer will attempt to properly tune the currently active antenna to ensure it resonates well.

if you are using an active system, then you could probably solder it on and pray. active systems leverage the electric field to transmit and receive, just like cell phones, FRS radios, or just about any other "traditional" radio you can think of. if attaching the a second antenna directly does not work, then a multiplexer definitely will. The book has a chapter on building a flip-flop based multiplexer for an active RFID system.
Amal ;)

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