By Times 7, Sep 27, 2018
Times-7 design and manufacture world-leading RFID antennas. These are the linkage between RFID-tagged items and RFID-readers. As specialists in the antenna technology which receives tag data and passes this onto the reader unit, we have deep knowledge in the efficient use of radio frequency electromagnetic waves as a tool for better business.
An RFID reader powers an antenna to read an RFID tag.
Because there are different levels of electromagnetic signaling between the reader port and the antenna, it is very important these signals are balanced - or you risk badly damaging the reader.
We explain how the antenna sense mechanism protects the reader and why this would never happen with a Times-7 antenna.
How RFID readers work
UHF RFID readers are transceivers (having both a transmitter and receiver) in a single box, equipped with both monostatic OR bistatic antenna ports.
A monostatic antenna port can transmit and receive the radio frequency (RF) signals at different times.
A bistatic port can only do one of these jobs, and only ever one. A bistatic port requires a dedicated antenna for transmission and a dedicated antenna for reception.
Readers with monostatic ports transmit the RF signal for a short period through its antenna to the tag, then immediately switches to reception mode through the same antenna to receive data back from the tag.
Most UHF RFID readers have monostatic ports as it requires fewer antennas and reduces the cost and complexity of an RFID system.
These reader transmitters typically have a modulator, to modulate (vary the strength of) the signal, and a power amplifier to amplify (increase) the modulated signal.
The signal is then conducted as electromagnetic waves through the antenna.
Reader receivers have different demodulation stages. The antenna receives signals from tags which have a very low magnitude (or strength). This signal is fed into the reader’s low noise amplifier (LNA), which increases the signal in order to allow efficient demodulation.
Monostatic ports use a device called a duplexer - which isolates the reader’s transmitter and receiver portion. A duplexer component called a circulator allows a transmitter to send a signal to the antenna for broadcasting, and direct the tag’s data from the antenna back to the reader’s receiver module.
Why an RFID reader requires an antenna sense mechanism
An interesting phenomenon of high-frequency signals is what is known as signal reflection due to impedance matching.
When the load impedance (load which passes the signal voltage from the source to the destination) is not matched with the generator’s impedance, the signal gets reflected.
The RFID transmitter is the generator, producing RF modulated signals, with the antenna acting as the load.
The antenna’s impedance must match the transmitter port’s impedance for efficient signal transmission.
RF signals can only flow through impedance-matched ports. An open-circuited and short-circuited port will offer a maximum signal reflection (which is not what you want).
Take for example if an antenna is not connected to an RFID reader port, and it is being operated at full power. As this port is open-circuited, maximum reflection is offered. More than 99% of the power is reflected and is fed to the transmitter’s power amplifier.
Over time, the reader is damaged, and cannot read tags efficiently. Prolonged signal reflection can blow up the reader, leaving it unusable.
How an RFID reader senses an antenna is connected
Readers with an antenna sense mechanism ensure the transmitter is only turned on when an antenna is connected to its port.
There are two main sensing mechanisms in commercial RFID readers:
A reader with a Voltage Standing Wave Ratio (VSWR) sensing module checks the port’s VSWR before transmitting a signal.
VSWR measures the percentage of power reflection whenever the transmitter is turned on. An open-circuited or short-circuited reader port will have a large VSWR value, and the module will close down power transmission through the port.
Some readers look for DC resistance within the antenna as a means of ensuring its connectivity with the reader port.
The target DC resistance is proprietary to the reader’s manufacturer but commonly uses sense resistors between 50 Ω to 10 KΩ.
Which is the best RFID reader sensing mechanism?
Antennas with very good VSWR can only be detected when they have a sense resistor.
However, sense resistor enabled antennas with poor VSWR can still damage the transmitter’s power amplifier.
This is because the VSWR sensing mechanism not only makes sure an antenna is connected to the reader’s port, and also watches for damage within the antenna or antenna cable which potentially create an impedance mismatch - and trigger the VSWR module to close down power transmission.
The DC resistance sense mechanism operates with the assumption the connected antenna has good VSWR.
DC resistance readings can still be obtained from a crushed cable or a physically damaged antenna.
This means a DC sensing mechanism may not be effective enough to ensure the power amplifier is not under threat in some circumstances.
Some readers can’t handle a high-power reflection (VSWR) and will shut down or get damaged. Times-7 antennas provide a low VSWR and a DC sense resistance for antenna detection. Thus the readers are protected, which enables reliable application performance.
Times-7 are RFID antenna experts, with deep knowledge across the tags-antenna-reader continuum. We can help you with your particular RFID challenge - or put you in contact with one of our partners. We are located in New Zealand and globally connected. Get in touch today at firstname.lastname@example.org