Typical junction circulators are composed of ferrite material—usually circular, hexagonal, or triangular in shape—surrounded by a uniform dielectric. Microstrip traces are then patterned on top with ports emanating at equal intervals in the azimuthal direction. This approach to circulator design poses problems for circuit layout. To solve these problems, Ryan S. Adams, Jeffrey L. Young, and Benton O'Neil from the University of Idaho (Moscow, ID) developed a microstrip circulator topology with a rectangular ferrite region.
The rectangular ferrite region allows the ports of the circulator to diverge from the ferrite region at right angles. This ferrite has the following parameters: 4πMS = 2000 G, ΔH = 300 Oe, εf = 12.4, tan δ = 0.00025, and HC = 1.6 Oe. The applied field is 1710 Oe, which creates an internal field of approximately 0. The ferrite puck is 3.1 mm wide and 5.37 mm long.
Because the topology does not present rotational symmetry, unique load impedances were required for each port. A full-wave, electromagnetic solver was used to find the S-parameters of the ferrite puck and therefore the circulator impedances. By plotting the simulated circulator impedance data, the researchers could see the best locations for the matching networks. The matching networks and ferrite region were then conjoined to form the circulator.
Because the geometry is not symmetric, the device's properties are not symmetrical. This aspect makes the design more complex. Yet it also can lead to very positive results in other areas, such as a 15-dB isolation specification from 5 to 15 GHz between two of the ports. See "Novel Microstrip, Rectangular Ferrite, Circulator," Microwave And Optical Technology Letters, May 2007, p. 1036.
