A unidirectional device, also known as a unidirectional filter or unidirectional isolator, evolved from the principle of microwave components—the circulator. Specifically designed for wireless paging transmitters, it absorbs the energy of interference signals entering the transmitter through the antenna feed system from external sources. It also protects the transmitter and reduces failure rates.
The unidirectional filter primarily consists of precision-sintered and ground rotating magnets and permanent magnets, coupled with microstrip inductors, capacitors, resistors, and cavities to form a coupled, resonant, and filtering circuit. Under the influence of a rotating magnetic field, electromagnetic wave signals exhibit unidirectional transmission characteristics. Signals rotating 120 degrees exit the output port with minimal loss; External signals entering through the output port rotate 120 degrees in the same direction before being absorbed at the input port, dissipating as thermal energy (see Figure 4).
Electrical characteristics of unidirectional filters: Forward insertion loss must be extremely low, typically below 0.3 to 0.5 dB. Reverse characteristics require maximum energy absorption across the widest possible frequency bandwidth, ideally achieving over 25 dB attenuation within a 20 MHz bandwidth. Thus, a unidirectional filter functions by absorbing interference signals backfed from external sources around the transmitter's main frequency, thereby suppressing intermodulation products.
The technical specifications for a unidirectional filter primarily consist of three key metrics:
(1) Forward Insertion Loss
When a unidirectional filter is added to a transmitter, power loss ranges from 0.3 to 0.5 dB (equivalent to approximately 7 to 10 W for a 100 W transmitter output).
(2) Reverse Isolation and Reverse Isolation Bandwidth
This metric indicates the ability to absorb external interference signals. The National Radio Regulatory Commission's Class II standard requires performance exceeding 25 dB within a bandwidth of ±10 MHz around the center frequency fo, and exceeding 30 dB within a bandwidth of ±5 MHz around fo.
Class III unidirectional devices employ a dual-junction configuration (i.e., two unidirectional devices combined), achieving a reverse isolation of better than 40 dB within a bandwidth of fo ± 10 MHz and better than 50 dB within fo ± 5 MHz. However, due to the relatively high insertion loss (resulting from the combined loss of two unidirectional devices), users are often unwilling to accept this design.
(3) VSWR
VSWR is a critical metric for evaluating whether a unidirectional coupler interface is properly matched. It should be less than 1:2:1, with high-quality designs achieving less than 1:1:1. Specifically, at 100W transmit power, the reflected power should not exceed 0.8W.
