BACKGROUND ART The NRD Guide is a structure wherein parallel conducting plates are arranged apart from each other by the distance of a half wavelength of the frequency to be used or less and wherein a dielectric strip having the same height as the gap between the two conducting plates and a certain constant width is inserted in between the two parallel conducting plates.

A circulator, which is the representative nonreciprocal circuit device in the millimeter wave band, is a circuit device indispensable for the application of an isolator, an injection synchronous amplifier, a modulator and so forth.

However, the circulator may have some insertion loss of from 4dB to 5dB due to the unnecessary mode (TE, LSE modes) generated between an NRD Guide and a Ferrite resonator. Thus, it may be difficult to get desirable output characteristics.

Also, the frequency bandwidth of frequency range having the isolation level over 20dB is at best about 1 GHz. Thus, it is difficult for a modulator to detect desired signal because the range that may include higher order components of signals is narrowed.

Accordingly, a circulator for the NRD Guide using a permanent magnet as illustrated in Figure 2 has a problem in that it is impossible to maintain the low loss rate,

which is the advantage of the NRD Guide. Therefore, such circulator in the related art eeds for communicating a large quantity of information in a broad band of signals.

DISCLOSURE OF THE INVENTION The purpose of the present invention is to provide a circulator using a permanent magnet that may maintain the low loss characteristics of the NRD Guide and may have the broadband characteristics at the same time.

In other words, the present invention maintains the low loss characteristics of the NRD Guide by reducing loss through inserting mode suppressors (10,11,12) into the circulator that uses a permanent magnet and the present invention implements that stair form by narrowing the widths of mode suppressors (13,14,15) in order to broaden the width of the frequency range that has isolation characteristics over 20dB.

The first preferred embodiment of the present invention is implemented by mounting a permanent magnet (6) between the upper/lower conducting plates (1,2), composing a Ferrite resonator (illustrated in Figure 5) by assembling a Ferrite (7), a Teflon tube (9) and a Ferrite (8) sequentially along the central axis of the permanent magnet, arranging NRD Guides (3,4,5) around the Ferrite resonator with the gap of 120° from one another as illustrated in Figure 1, and inserting mode suppressors (10,11,12) in between the respective NRD Guides (3,4,5) and the Ferrite resonator.

The second preferred embodiment of the present invention is implemented by inserting mode suppressors (13,14,15) to compose circulator in the shape of stairs, which are the mode suppressors as inserted in the first preferred embodiment but which have the narrower width than the NRD Guides.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a perspective view of a circulator according to the first preferred embodiment, wherein mode suppressors are inserted in order to minimize the transmission loss of the circulator.

Fig. 2 illustrates a perspective view of a circulator that has the ordinary structure in the related art.

Fig. 3 is a schematic diagram that illustrates the basic principle of a circulator that uses NRD Guides.

Fig. 4 is a schematic diagram that illustrates the modes in a circulator using NRD Guides.

Fig. 5 illustrates a perspective view of a Ferrite resonator that is inserted into the center of a circulator.

Fig. 6 illustrates a perspective view of an NRD Guide and a disassembled mode suppressor.

Fig. 7 is a graph that illustrates transmission losses in the cases where mode suppressors are inserted into three terminals of a circulator and where no mode suppressors is inserted.

Fig. 8 illustrates a perspective view of a circulator for the NRD Guide according to the second preferred embodiment, that may have the broadband characteristics by composing circulator in the shape of stairs through narrowing the width of inserted mode suppressors.

Fig. 9 is a graph that illustrates the measured impedance that varies corresponding to frequency when the width of mode suppressors is not made narrow

for the stair shape.

Fig. 10 is a graph that illustrates the measured impedance that varies corresponding to frequency when the circulator has the shape of stairs by narrowing the width of mode suppressors.

Fig. 11 is a graph that illustrates the transmission losses in the cases where the width of mode suppressors is narrower than the width of the NRD Guides according to the second preferred embodiment and where the width of mode suppressors is not narrowed.

**Description of the codes of important parts of the diagrams** 1 : Upper Conducting Plate 2 : Lower Conducting Plate 3 : NRD Guide of Input Port 4: NRD Guide of Isolation Port 5: NRD Guide of Output Port 6: Permanent Magnet 7,8: Ferrite 9: Teflon Tube 10: Mode Suppressor of Input Port 11: Mode Suppressor of Isolation Port 12 : Mode Suppressor of Output Port 13 : Mode Suppressor of Input Port Narrower than the NRD Guide 14: Mode Suppressor of Isolation Port Narrower than the NRD Guide 15: Mode Suppressor of Output Port Narrower than the NRD Guide 16: NRD Guide

17: Thin Teflon Transmission Line sl++er) Film INPUT : Input Port OUTPUT (2) : Output Port Isolated3) : Isolation Port BEST MODE FOR CARRYING OUT THE INVENTION The preferred embodiments of the present invention will now be explained in detail with references to the attached drawings.

(First preferred embodiment) Figure 1 illustrates a perspective view of the circulator according to the first preferred embodiment of the present invention, wherein mode suppressors (10,11,12) are inserted in between NRD Guides (3,4,5) and a Ferrite resonator that comprises Ferrites (7,8) and a Teflon tube (9).

Because PTFE (Poly Tetra Fluor Ethylene) that has the permittivity of 2.04 is used as the NRD Guide of the present invention and because the present invention is implemented in the 50GHz range, the height of the transmission line, which is the same as the gap between the upper and lower conducting plates (1,2), is 2.7mm and the width of transmission line is 2.4mm in the present invention. Also, Ferrites (7,8) of the present invention are made by ultrasonic wave processing of the Ferrites of TDK Co., Ltd., that has the permittivity of 15 and is 1800G and whose tangent loss is 0.0008.

The Ferrites have the circular shape, the diameter of which is 3.37mm and the Ferrites are ground for the proper thickness to be used.

The general structure of a circulator for the NRD Guide and its operation

principle will be explained in detail hereinafter with references to Figures 2,3,4 and 5.

Fbre-. 2 ilust-rates the=structure of a. circulatorfor the NRD Guide in the related art. A permanent magnet (6) is inserted in the center of the upper/lower conducting plates (1,2). A Ferrite (7), a Teflon tube (9) and a Ferrite (8) are assembled sequentially along the central axis of the permanent magnet, constructing a Ferrite resonator. Figure 5 is a perspective view of the Ferrite resonator.

NRD Guides (3,4,5) are arranged around the Ferrite resonator with the gap of 120°, making three ports. The role of each port will be explained as illustrated in Figure 3 and Figure 4.

Because the unnecessary mode like the LSE mode is generated from a bent part or asymmetrical/discontinuous region of an NRD Guide and affects the circuit, such unnecessary mode may be generated in a nonreciprocal device such as an asymmetrical circulator.

The cause of the unnecessary mode's generation in the related art will be examined hereinafter. In a circulator for the NRD Guide illustrated in Figure 3, wherein port0 is an input port, port (2) is an output port and port3) is an isolation port, if only the LSM mode enters the input port, the magnetic field of the primary transmission wave, the LSM mode, exists only in the cross section of the axis direction as illustrated in Figure 4. The electromagnetic field distribution in a resonator while a circulator is operated shows that the LSM mode is outputted through port ( (5) because the magnetic field exists in the axis directional cross section. However, in port 30 (4), the magnetic field forms a ring shaped loop in the cross section in the horizontal direction instead of in the axis direction. Thus, it may be determined that the LSM mode does not exist in port (3) (4). On the other hand, because the magnetic field distribution of the

LSE mode forms a loop in the horizontal direction cross section, it is easy for the LSE mode to ; e--xigt i.

Consequently, the inputted LSM mode is not outputted through port (3) (4) but outputted through port (2) (5) as illustrated in Figure 3. However, at the same time, the LSE mode is generated in port (4), causing the insertion loss. Therefore, the insertion loss may be reduced if such unnecessary mode is suppressed.

In order to suppress the unnecessary mode that is generated when a circulator using the permanent magnet is operated, mode suppressors (10,11,12) are inserted in between the Ferrite resonator and the NRD Guide of the respective ports as illustrated in Figure 6 according to the present invention.

The LSM mode is not affected at all by the insertion of a metal strip (18) because the center section of the NRD Guide is made of the electric wall. In contrast, if a metal strip is inserted, the LSE mode may be removed from the usable frequency range by increasing the cut-off frequency because the electric field of the LSE mode is parallel with the center section. Nonetheless, the TEM mode might be generated against the LSE mode. Thus, the present invention intends to suppress all of the unnecessary modes by composing the metal strip (18) in the form of a X/4 choke.

The mode suppressor is 0.4mm wide at the narrow part and is 2.4mm wide at the wide part. The wavelength of the TEM wave, which is plane wave, in the dielectric material is equal because the electromagnetic field is distributed densely within the NRD Guide. Accordingly, the length of each section of the choke is 0.95mm, that is 1/4 of 3.8mm, the wavelength of the plane wave in the transmission line.

Figure 7 illustrates the transmission characteristics of Figure 1 and Figure 2.

The circulator shown in Figure 2 that uses a permanent magnet has the transmission

loss of 4dB on the average in the 50GHz range and the circulator according to the first preferred embodiment shown in Figure 1 that uses a permanent magnet has the transmission loss of below 1dB on the average in the 50GHz range.

(Second preferred embodiment) Figure 8 is a perspective view of the second preferred embodiment of the present invention. In the second preferred embodiment, the width of the mode suppressors (10,11,12) of the first preferred embodiment in between the NRD Guides (3,4,5) and the Ferrite resonator is narrowed, making the circulator of second preferred embodiment in the form of stairs.

Here, in the second preferred embodiment, the Ferrites are 0.342mm thick and the bias magnetic field (the strength of the permanent magnet) is 13200e.

It is found that the impedance is completely matched in the center frequency of 50GHz because the impedance is located mostly at the center of smith chart as illustrated in Figure 9. The insertion of mode suppressors (13,14,15) that has the narrowed width and the length equal to a half wavelength of the center frequency in the circulator using a permanent magnet as illustrated in Figure 10 does not significantly affect the characteristics of the center frequency.

However, the reactance component is changed corresponding to the change of center frequency. Although the reactance component of narrowed mode suppressors (13,14,15) changes corresponding to the change of center frequency, it is possible to implement the broadband because the impedance matching condition is satisfied over the wide frequency range.

Preferably, mode suppressors (13,14,15) are 1. 9mm wide, which is narrower than 2.4mm, the width of the NRD Guide. The length of mode suppressors is 5.8mm,

which is equal to a half wavelength of 50GHz multiplied by an odd number.

Figure 9 and Figure 10 commonly concern the condition where the strength of the permanent magnet (bias magnetic field) is 13200e, the Ferrites (7,8) have the diameter of 3.37mm and the thickness of 0.342mm and the Teflon tube (9) has the height of 2.016mm, the diameter of 3.37mm and the inner diameter of 3mm. Figure 9 is a graph that illustrates the impedance trajectory corresponding to the center frequency when the width of mode suppressors (10,11,12) is the same as the width of the NRD Guides, whereas Figure 10 is a graph that illustrates the impedance trajectory corresponding to the center frequency when the width of mode suppressors (13,14,15) is about 1.9mm, which is narrower than the width of NRD Guides.

Figure 11 illustrates the result of the respective insertion loss/isolation characteristics of the first preferred embodiment and the second preferred embodiment of the present invention. If mode suppressors have the same width as the NRD Guides, the bandwidth that has the isolation characteristics over 20dB is about 1 GHz. If mode suppressors are narrower than the NRD Guides as in the circulator using a permanent magnet according to the second preferred embodiment, the bandwidth that has the isolation characteristics over 20dB is over 2GHz and the low loss characteristics of the first preferred embodiment is maintained.

In summary, a circulator that uses an ordinary permanent magnet in the related art, illustrated in Figure 2, has the loss of 4dB due to the asymmetric characteristics.

However, the circulator of the first preferred embodiment in which mode Suppressors are inserted may maintain the low loss characteristics of the NRD Guide by suppressing the unnecessary mode caused by the asymmetric characteristics of the circulator using a permanent magnet. The circulator of the second preferred embodiment may maintain

the low loss characteristics of the NRD Guide by inserting mode suppressors as in the second preferred embodiment and may have the broadband characteristics at the same time by narrowing the width of the mode suppressors.

The foregoing embodiments and advantages of the present invention are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. The claims are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.

INDUSTRIAL APPLICABILITY As explained above, the circulator of the present invention that uses a permanent magnet may remove the heat-emitting symptom generated at the electromagnet used in the related art, eliminate the need for consumption of the electricity and make it unnecessary to use the electric source circuit added when the electromagnet is used.

Also, the circulator of the present invention that uses a permanent magnet may accomplish the desired output characteristics because it reduces the insertion loss of electric waves by inserting mode suppressors in between the respective NRD Guides and the Ferrite resonator.

Furthermore, the present invention may broaden the bandwidth of the usable frequency by narrowing the width of the mode suppressors so that the mode suppressors become narrower than the NRD Guides.

According to the present invention, the circulator using the permanent magnet may attain the advantages as a nonreciprocal device of a modulator, amplifier or etc. because the circulator of the present invention using the permanent magnet has the superior characteristics of low loss and broadband.