By Times 7, Jun 24, 2018
Both Multiplexers and Power Dividers are helpful devices to expand the number of antennas that can be connected to one reader’s port. One of the main benefits is to reduce the cost of a UHF RFID application by sharing expensive hardware. In this blog post, we explain the differences and what needs to be considered when selecting the right device for your application.
To understand what an RFID reader Multiplexer is we will quickly explain the general purpose of multiplexers (mux) and de-multiplexers (de-mux).
A multiplexer is a device that selects one of the several input signals and forwards it to an output.
A demultiplexer is a device that forwards an input signal to one of the several outputs.
Both multiplexer and de-multiplexer require switches to select the inputs and/or the outputs. These switches are powered, and thus mux and de-mux are active devices.
What is an RFID reader multiplexer?
An RFID reader multiplexer is a device that is a combination of a mux and a de-mux. It consists of one input/output port and many output/input ports. A single port of a mux/de-mux is usually connected to an RFID reader while the multiple ports are dedicated for the antenna connection.
It either forwards the signal from the RFID reader’s port to one of the several output ports or forwards the signals from one of the several input ports to the RFID reader’s port.
A built-in switch takes care of the signal switching between the ports and its switch timing.
The RFID multiplexer enables multiple antenna connectivity to a single port of the RFID reader. The magnitude of the signal switched is not affected significantly, regardless of the number of ports in a mux/de-mux.
That way, an 8-port RFID multiplexer, for example, can extend a 4-port reader into a 32-port RFID reader.
Some brands also call their mux a hub.
A power divider (splitter) is a device that divides the power. A 2-port power divider divides the input power into two outputs. The magnitude of the power is halved in the output ports.
The power divider is called a power combiner when used in reverse.
A mux will have constant power loss across the ports regardless of the number of ports. A 4-port, 8-port, and a 16-port mux will not have different losses per port.
A power divider would divide the power into ½ or ¼ depending on the number of available ports. A greater power reduction is experienced in each port as the number of ports is increased.
A mux is an active device. It requires DC power and control signals to operate.
A power divider is a passive device. It does not need any extra input than the RF input.
Not all ports in a multi-port mux are turned on at the same time. The RF power is switched between the ports. Only one connected antenna will be energized at a time, and the switching speed is so fast that the antennas will not miss a tag read.
All the ports in a multi-port power divider get the power equally and at the same time.
Very high isolation between the ports is achieved. This is essential to avoid cross-tag reads between the antennas. Isolation is usually in the range of 35 dB or more.
The port isolation is little less compared to a Mux. Typical port isolation is around 20 dB or more. Cross tag reads may become an issue.
Has minimal or no impact in antenna’s beam or cancellation.
When the power divider is not used in the right way, RF fields can get canceled, and the antenna’s RF beam can be altered significantly.
No RF expertise required to install a Mux. The Mux will have to be controlled by the RFID reader`s software.
RF expertise is essential to install the power dividers and to achieve a working solution. An incorrectly installed power divider would dramatically spoil the performance of the RF.
No custom antenna alteration is possible
Custom antenna alteration is viable. Antenna`s beam-width, beam angle, etc. can be changed.
On an average, an 8-port mux cost around $220 to $250 (USD)
A high performing 8-way Power dividers costs around $260 (USD)
An RFID mux will have losses in the range of 1 to 1.5 dB (depending on the manufacturer) regardless of the number of ports (4, 8 or 16). The RF switches and other logic within a mux would induce this loss. The loss is termed as the insertion loss, and it is same across all the ports in the RFID multiplexer.
A power divider will have two losses associated, a) the inherited insertion loss and b) the splitting loss. The insertion loss increases as the number of ports are increased. Insertion loss also depends on power divider’s manufacturer type. A typical 3-port, 4-port, 6-port, 8-port and a 12-port power divider will have an insertion loss of 0.8 dB, 1.1 dB, 1.5 dB, 2 dB and 3 dB, respectively at each port. In addition to this, power lost during the power-split for a typical 3-port, 4-port, 6-port, 8-port and a 12-port power divider is 4.7 dB, 6 dB, 7.7 dB, 9 dB and 10.7 dB respectively.
Let’s assume you use a 1-port RFID reader that can deliver 31.5 dBm maximum output power and you wanted to use a coaxial cable between the RFID reader and the multiplexer / the power divider. The cable losses are known, which is 1.5 dB. The actual power that will be delivered by the cable will be 30 dBm (after the loss).
When a 4-port mux is used:
The insertion loss data is provided in most of the mux manufacturers’ datasheets. Let’s assume the insertion loss for the 4-port mux is 0.9 dB and when the reader mentioned above configuration is applied, the 4-port mux will yield an output power of 29.1 dBm in each port of the 4-port mux.
Input = 30 dBm (1 Watt)
Output = 29.1 dBm (0.812 Watts) per port
When a 4-port power divider is used:
The insertion loss data is provided in most of the power divider manufacturers’ datasheets. Let’s assume the insertion loss for the 4-port power divider is 1.1 dB and when the reader mentioned above configuration is applied, the 4-port power divider will yield an output power of 22.9 dB in each port of the 4-port power divider.
Input = 30 dBm (1 Watt)
Output = 22.9 dBm (0.194 Watts) per port
A power divider, as the name suggests would divide the power equally to all the available output ports. Consequently, a 4-port power divider divides the power equally among the four output ports, which corresponds to a 1/4th split.
A mux would not divide the power. It would rather switch the power between the ports, and the power is made available to all the ports of the same magnitude at different times.
A mux will not induce any RF characteristic changes on the antennas that are connected to its multiple ports. This is mainly because the mux has a built-in switch that would switch the RFID reader’s output to each of the connected antenna ports at different times. Each antenna will turn on at different times.
A power divider can alter the antenna’s performance dramatically when it is not used correctly. Things like a) location of the antennas, b) cable lengths of the connected antennas and c) antenna orientation are critical during installation.
Location of the antennas:
When two antennas are intended to be located next to each other, a separation distance of at least 2 feet (600 mm) is recommended. When the antennas are closer, say 150 mm, then the antennas’ beams are altered.
When the cable lengths are different for two antennas, the RF phase is altered between the antennas. This, in combination with the antenna’s location, can alter the antenna’s beam angle.
When antennas are installed in a portal or a door-way, two antennas facing each other can cancel the fields, and a null-zone can be experienced. Two antennas with a different polarization can be used to overcome this effect, but the corresponding antenna’s cross-polarization levels need to be considered to avoid any detrimental effects. It is not recommended to use a power divider in this scenario.
A Mux will not be a problem, and it is safer to use, even for those, who are new to RF.
The use of power divider may lead to the breach of rules and regulations. The following is an example;
Let`s assume you are in the USA and you connect a 6 dBi antenna to an RFID reader. The reader power is set to 31 dBm with a cable loss of 1 dB. This configuration yields 4 W EIRP, which is the maximum allowed power specification for UHF RFID systems in the USA region. This is prescribed by the Federal Communications Commission (FCC). You are within the limit now, and you do not breach the law.
Say you wanted to connect two antennas using a 2-way power divider and the two antennas are located right next to each other. You may think that you have connected 2 x 6 dBi antennas to a power divider but what happens is that the beams of those two antennas would combine to yield an ~8 dBi gain. This will cause you to breach the law because you are now generating 6.3 W EIRP.
It is not recommended to use the power dividers by a non-RF expert.
A near-field antenna and a far-field antenna’s beam may not combine if the near-field antenna is a pure near-field only antenna. Some near-field antennas in the market are loop antennas which have far-field gain associated. These antennas will impact the over-all RF beam when a power combiner is used. The RF beam realized can be unpredictable and can cause nulls zones too.
The only way to ensure that this does not happen is to separate the antenna by a distance of 2 feet or so.
It is recommended to use a multiplexer in most of the scenarios.