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Why Must Transmit and Receive Antennas Be Positioned Closely Together?
I recently found a fixed reader product description advising monostatic over bistatic configurations in item-level tracking (ILT) applications. The document stated: "ILT is difficult for bistatic systems, because the transmit and receive antennas must be positioned so closely together that the signal from the transmit antenna couples strongly back into the receive antenna. The primary reason that some readers use bistatic antennas is to provide isolation between the transmit and receive signals, a condition not typically available in ILT environments. Readers equipped with monostatic antennas work better for ILT." The statement "the transmit and receive antennas must be positioned so closely together" is confusing to me, as I don't understand why the send and receive antennas must be very close. Moreover, if the send and receive antenna are in a cross-circular polar configuration, the problem seems to be solved. What do you think of this statement?
The document you quoted, "Five Factors for Success: UHF Gen 2 Readers," was published by Impinj, a leading RFID reader and tag IC provider. Therefore, I reached out to Impinj and asked them to clarify the statement. A spokesperson explained:
"Products tagged at the item level often have rather small, short-range tags which may be applied to materials that significantly attenuate radio signals. An example would be tags applied to cosmetics or liquid-dose pharmaceuticals. In order to read such items, the reader antenna must be positioned close to the tag. A bistatic reader requires another antenna—the receive antenna—which also must be close to the tag. In the scenario described, the read zone is within the radiating near field of the antennas, where the propagating far field has not yet developed and the concept of circular polarization is undefined. Therefore, one cannot rely on opposite circularly polarized antennas for transmitter-to-receiver isolation. Bistatic readers typically have receivers which cannot tolerate high levels of transmit signal and the cross-coupling between the two antennas in a short range ILT read zone is sufficient to saturate the receiver. A much more robust system uses a receiver that can tolerate significant transmit signal such that it is capable of either bistatic or monostatic operation. For a system that is monostatic-capable, it is cheaper and more reliable to use a single antenna or a plurality of antennas operating mono statically."
—Mark Roberti, Founder and Editor, RFID Journal
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