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How does an RFID reader monostatic antenna both transmit and receive at the same time, are they CW or pulsed? I would like to extend the read range of a reader by using an amplifier between the antenna and the reader. Is that a feasible way to increase the range?

PeterJ
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Andrews
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  • What kind of RFID technology are you using: LF (135 kHz), HF (13.56 MHz) or UHF (around 900 MHz)? Depending on the technology there are several possibilities to increase the range using an additional amplifier. – pizzaani Apr 21 '14 at 10:43
  • UHF 865-928 MHz – Andrews Apr 21 '14 at 15:18

2 Answers2

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Passive RFID works because the tag varies the amount of signal power it absorbs/reflects. This is seen by the reader as a change in the "radiation resistance" (i.e., impedance) of the space around it, which causes small variations in the reader's antenna current and/or voltage. The reader looks for specific patterns in these varations in order to receive the tag data.

The range is limited primarily by the magnitude of these changes relative to other changes in antenna power that are due either to other objects in the environment, or to amplitude variations in the transmitter circuit itself.

You can't simply add an amplifier between the circuit and the antenna of an existing reader, because this would prevent the reader from sensing the radiation resistance. The reader itself needs to be designed to operate at a higher transmitted power level while maintaining (or even increasing) its signal-to-noise ratio for the tag's incoming signal.

Dave Tweed
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These are two questions.

(1) How does an RFID reader monostatic antenna both transmit and receive at the same time, are they CW or pulsed?

Through a magical device called a circulator. This is a (typically) three-port device. The energy flowing into port 1 is directed out port 2. The energy flowing into port 2 is directed out port 3, and likewise, the energy in port 3 is directed out port 1. Physically, these are often strong magnets that direct the energy flow.

Using a coupler, a CW (or any shaped signal) can be applied to port 1 and directed out the antenna. Typically, the amplifier is put in place before the coupler (i.e. the coupler is between the amplifier and the antenna). The return signal absorbed by the antenna is then directed out port 3 where it can then be demodulated and processed for data recovery.

(2) I would like to extend the read range of a reader by using an amplifier between the antenna and the reader. Is that a feasible way to increase the range?

Its a little tricky, but I think it will work. You, however, cannot just amplify the output signal. The incoming wave received by the antenna will not pass through the amplifier. You will need to use a circulator to isolate the outgoing signal, amplify this, pass it to a second circulator and to the antenna. The received signal from the antenna will need to be isolated and connected between the two circulators. There can still be some problems if you don't have a good match between the amplifier and first circulator as these reflections will appear on the antenna output.

In monostatic setups, your SNR (signal to noise ratio) performance is dominated by the circulator. A good circulator may provide 30 dB isolation between ports. So if you have a very high power signal applied to port 1, some of this will leak into port 3 (which is hopefully carrying your data). Most protocols get around this by modulating at a subcarrier which moves the data energy away from the CW transmit energy, BUT, the high power signal that leaks into your demodulation setup will destroy the range of your ADC and lower performance.

You would probably have much more luck moving to a bistatic setup, amplifying the transmit signal, and isolating the transmit and receive antennas as much as possible.

Stwerp
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