Ferrite Circulator / Isolators
This article is in response to several queries in which individuals asked what is a Circulator or Isolator? What do they do? How do they do it? Do you need one?
A ferrite circulator is a three terminal device that will allow RF to flow between any two adjacent ports. This flow is restricted to one direction only. Below is a picture of a circulator with a 50-ohm load resistor attached to port 3. This unit is being loaned to Eddie Palmer, K4JP of Kingsport TN in an effort to determine if it will help in solving an intermod problem on the 146.61 repeater.
RF can flow from the input (port 1) to the output (port 2) as well as RF entering the output port can flow to the load (port 3) and finally RF can flow from port 3 (load) to port 1 (input). It helps if you think of it as a circle with the RF flowing in a clockwise direction with the three taps forming a Y.
A circulator is best thought of as a "Magic Box" containing three transmission lines spaced 120 degrees apart. These transmission lines are placed between two disks of ferrite material. On the other side of the ferrite is a non ferrous ground plane and then a magnet followed by a ferrous pole piece that shields the unit from external magnetic fields Usually on circulators designed for the VHF/UHF (50 - 512 MHz) frequencies will have a means of tuning each transmission line so that insertion loss and isolation can be optimized at the desired center frequency range. As a rule the cost is higher for units designed to operate on the lower frequencies. This is because the magnets and ferrite material is larger so that the larger transmission lines can be accommodated. The units are packaged in a metal enclosure that provides RF shielding as well heat dissipation. The circulators are most often mounted so as to promote efficient dissipation of the heat generated. The remainder is the "Magic" that makes it do what it does so well.
I do not profess to be an expert but, this is my understanding of the "Magic" that is inside the little box. When RF is introduced to any of the three ports two equal but opposite rotating magnetic fields are induced in the ferrite material. Provided the magnetic field produced by the magnets is of the proper strength the opposite rotating fields are cancelled over the adjacent transmission line and they are reinforced over the other two lines. This some how causes the RF to pass (this is the magic part) without being attenuated into the adjacent transmission line but not to the third line. Isolation or circulation is created thus the name of the device, Circulator as opposed to Framowitzor something.
How well does all of this work? You will typically see 25 to 30 dB of isolation in the reverse direction with an insertion loss of .25 to .5 dB. The load resistor used on port three is critical in the determination of how well the circulator performs. The transmission line and the load must be matched so as to provide as close as possible to a 1:1 SWR. When RF from the transmitter is applied to the circulator there is normally a 10 to 15 percent drop in the isolation. I am told that this is due to heating of the ferrite and magnetic materials as well as detuning of the transmission lines. When greater isolation is required additional circulators may be added in cascade. I have seen as many as three used in series. They may also be ordered with multiple units in one housing.
There is one caveat involving the use of ferrite circulators and that is ferrite is a non-linear device. It is because of this that circulators may generate harmonics of the transmitter applied to it. The harmonics can mix with other nearby transmitted signals to cause intermod etc. Yes, I know this is what we were trying to get rid of by using the circulator in the first place. Well, we have helped to prevent the generation of intermod in our transmitter and as a rule any harmonics generated by the ferrite material are of an extremely low level when compared to the level of the transmitter. This is why a harmonic filter or band pass cavity should always be used after the output of the ferrite circulator. The advantage of a bandpass cavity is that it will attenuate all out of band signals as well as any broadband noise generated by the transmitter.
I am a firm believer in the use of circulators at any location having a high RF density or if you wish to protect your transmitter from a mismatched load that can have disastrous results on your expensive amplifier.
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Last update 03/02/99