Most off-the-shelf reader antennas I've found are in the neighborhood of 4 inches or more in size. What's the practical lower limit, given the frequencies at which we're operating?
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To answer your question, I reached out to two companies that make RFID antennas, Times-7 and MTI Wireless Edge.
Dr. Prabakar Parthiban, Times-7's head of engineering, responded: "In practical terms, for far-field reader antennas, a 3-inch to 4-inch patch antenna is the smallest size for an efficient radiating antenna. These antennas can typically read at distances of more than 12 feet. It's possible to create circular patch antennas as small as 1-inch by 1-inch, but the efficiency will be very low. Chip antennas made in monolithic microwave integrated circuits (MMICs) can be as small as 0.1 inch, but they are generally monopoles and hence linearly polarized."
Reuven Drori, MTI Wireless Edge's VP of RFID sales and marketing, noted that antenna gain and size are proportional. As he explained, "The higher the gain, the larger the antenna. A smaller antenna will produce lower gain and a shorter reading range." He also referred to a paper he wrote, titled "Choosing RFID Antennas."
I hope these responses answered your question.
Founder and Editor, RFID Journal
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Thanks so much, this has been very helpful!