Circulators:

No this is not another name for water pumps or teaspoons but what is technically described under the term of  a non reciprocal junction. It is usually a three port ferro-magnetic device and it has distinct directional properties from port to port. They are usually seen as a block with three rf connectors on each corner, or sometimes two however one of the ports may be internally connected to a resistive load usually 50 ohms.

CIRCULATORS IN VARIOUS FORMS

Operation:   The three port device (think of it as a triangle)  relies on the fact that because of the ferro magnet coupling inside, rf energy is diverted in a direction from port 1 to port 2 but not into port 3, however if you swapped the connections over there would be great resistance to the signal flowing in the opposite direction from 2 to 1,  it would rather flow from 2 to 3. This feature occurs on all ports so you end up getting a "directivity" traveling in one direction around the triangle.: One immediate use that springs to mind is that of an automatic "gate" that can distribute the flow of signals from and to an antenna and that is a common use of the circulator. The ports have a measure of directivity or "isolation" measured in dB. eg a port to port isolation of 20 dB means that it conducts rf in one direction 100 times better than it does in reverse.The most misunderstood concept of circulators is that of isolation. 
Circulators do not provide isolation until they are terminated, and then the isolation between any two ports ( in the direction opposing the direction of circulation) is the return loss due to third port mismatch.

 Conect one port to an aerial one port to a receiver and the other to a transmitter (observing the directivity of course) and you immediately have 20 dB isolation of your transmitter and receiver. simple huh!  but there are a few constraints .They have loss and they are frequency concious. typical loss of a  matched circulator is usually measured in decimal fractions of a dB. Circulator operation is usually specified over a certian bandwidth of frequencies, i.e.  the amount of isolation in dB  over the frequency range it will provide the minimum value of port to port isolation. In order for the device to work as specified, it must be terminated in the designed impedance, typically 50 ohms.There are of course other designed impedances ie those circulators that are used in microwave waveguide.

INSIDE IT:

IN the VHF-UHF circulator, the coupling between ports occurs in flat insulated overlapping terminated copper sandwiched striplines that are positioned 120 degrees in rotation apart. see picture . These flat copper striplines are sandwhiched betweeen two disc shaped slabs of a ferrite material whose composition is optimised for the ferro-magnetic effect. There is a strong magnetic feild placed accross the discs such that any rf energy flowing in the striplines has to enhance or oppose this biasing magnetic feild, that is how the directivity is generated. The magnetic feild strength also has an effect on the frequecy of operation, and the degree of isolation.  This  change in magnetic feild will change the permiability of the ferrite that encloses the coupling striplines and alter their performance . The physically larger the coupling lines and ferrite discs,  the lower the frequency of operation , the small one I pulled to bits is out of  900 mhz equipment, cell phone I think,              Now you see all the variables you have to play with! It can be a real headache juggling these parameters to acheive what you want.   It is important that the circulator is tuned into the correct impedance if it is going to function as you would like it to perform. If you perform your tests and tuning using 50 ohm termination at each port , All  the repeater terminations   must be 50 ohms resistive or the circulator  will not perform well.

Circulators used at VHF and UHF (if you are lucky)  may have internal matching networks, usually in the form of L/C or just variable trimmer capacitors . This is a boon as it means you will be able to shift  the devices  operating frequency and change its degree of port to port isolation. Circulators picked up at junk sales tend to be produced for commercial frequencies (as that is where the money is ) In case you are wondering what they cost to buy, it is in the many hundreds of dollars region   for a device used on the standard commercial 20 W repeater. Purchase circulators as close to the frequency of intrest as possible, then you are more likely to have some degree of sucess in your venture. I found the best way to tune them, is to terminated all ports in 50 ohms and then tune for maximun rejection of signal at the wanted frequency (with the circulator connected in "reverse") then rotate through all three  ports tuning for the same .   You are wasting your time trying to tune in the forward direction you will just go round and round in circles getting more frustrated ( took me quite a few days to discover this!)

A properly tuned circulator on a repeater system can add up to an extra  20 dB transmitter/receiver isolation to that provided by the duplexer filters and only increase the system noise figure by a small fraction of a dB(0.2-0.4 loss through the circulator). They have another excellent use as well. If you place a suitable 50 ohm load on one port and place the circulator in the transmit feedline (with respect to the correct directivity), then if the antenna developes high SWR, the reflected power will be diverted back around into the 50 ohm load and the transmitter, protected from damage.