In RF systems, there is often the need to provide or enhance the isolation between different signal paths. A key component used for this purpose are RF Isolators. Isolation is the degree in which two or more RF paths are coupled. Given the nature of electromagnetics and RF coupling, any and all conductive pathways are coupled to some degree. In some cases, this isn’t an issue, as the frequency ranges or filtering components can handle undesirable signal content or interference. There are many cases, especially in precision communications and sensing applications, it is critical to have high levels of isolation between specific signal paths.
Key Takeaways:
An RF isolator is a two-port (or derived from a three-port circulator) device that allows signal transmission in the “forward” direction while providing isolation (or absorbing) in the reverse direction.
They are typically built with ferrite materials and a static magnetic bias (in microwave designs) to achieve non-reciprocal behaviour (i.e., different behaviour forwards vs reverse).
In practical systems such as radar, wireless communications, distributed antenna systems (DAS), test labs and amplifier systems, RF isolators are used to protect sensitive equipment from reflected signals.
Key performance parameters include insertion loss (how much signal is lost going forward), isolation (how well reverse signals are blocked or absorbed), power rating, and frequency band. Example: some devices offer isolation levels of ~20 dB minimum, insertion loss as low as 0.35 dB and power ratings up to 1000 W.
Choosing the right isolator means matching the frequency band, connector type, power handling, and insertion loss/VSWR requirements of your system.
Proper use of isolators contributes to system stability, reduces unwanted reflections, prevents equipment damage, and improves overall performance in RF/microwave signal chains.
Isolation is generally measured as the difference in power level between an injected signal and the leaked signal that is coupled into another port on a different signal path. The figure-of-merit, Isolation, is most often expressed in decibels (dB), so a higher negative value means that there is less coupling, or more isolation if the value is expressed in positive polarity. In many cases, the isolation between a switch when closed is on the order of -20 dB to -65 dB, with -65 dB being exceptional Isolation.
The role of an RF Isolator, such as a RF Coaxial Isolator based on a RF Circulator, is to only allow for signal energy traveling in one direction to pass through the Isolator with minimal insertion loss. For signals traveling in the other direction, the signal energy is terminated in a matched load, or terminator, and absorbed. This function prevents the injection of signal energy traveling in any other direction than the desired signal path direction from entering through a node in the signal path.
A prime example of the use of an RF Isolator is to place the Isolator at the output of a device or node that may experience damage or desensitization to substantial reflections of the signal energy. This can be the case in a radar or communications system where the transit and receive paths are switched to the antenna. When switching off from the transmitter there may still be some high power transmit signal energy at the input of the switch may be reflected back to the transmitter power amplifier (PA) and signal chain. Having a RF Isolator at the output of the PA prevents a significant portion of the reflected signal energy from entering the PA output node.
FAQs: Frequently Asked Questions
Q1: What is the difference between an RF isolator and an RF circulator?
A: A circulator is a three-port non-reciprocal device where energy entering one port travels to the next port in rotation. An isolator can be thought of as a circulator with one port terminated (absorbed) so only two ports are active, effectively allowing signal one way only.
Q2: Why do I need an RF isolator in my system?
A: If your system has a source, amplifier or other sensitive component, reflected power (from mismatched loads or antenna reflections) can cause instability, degraded performance or even damage. An isolator helps absorb or redirect reflected signals, so your forward path remains clean and predictable.
Q3: What key specs should I check when selecting an RF isolator?
A: Important specs include frequency range (bandwidth), insertion loss (forward path loss), isolation (how much reverse signal is blocked/absorbed), power rating (average and/or peak), VSWR (voltage standing wave ratio), and connector type. For example, one model from Fairview has 4-8 GHz, Type-N female connectors, 50 W forward power, 18 dB isolation minimum, and 0.6 dB max insertion loss.
Q4: Can an isolator be used in both directions?
A: No—the purpose of an isolator is to define a forward direction and suppress or absorb reverse signals. If you swapped the ports without regard to direction, the isolation would not be guaranteed, and the device may not function as intended.
Q5: What happens if I use an isolator with the wrong frequency band or connector type?
A: Using a device outside its rated frequency band may lead to degraded isolation, higher insertion loss, poor VSWR and increased reflections. A mismatch in connector type may add unwanted losses, reflections or mechanical incompatibility. It’s essential to match all relevant parameters.
