Two types of near-field microwave imaging probes

We consider two types of near-field microwave imaging probes. One employs the small circular ridged aperture and the other employs unsymmetrical transmission cavity whose closed ends comprise the small circular aperture with no ridge and capacitive iris of double ridge type which is placed half waveguide wavelength apart from the circular aperture. The relationship between working principles of the two types of probes is discussed in both transmission efficiency through a small aperture and spatial confinement of the electromagnetic energy.


I. Introduction
when a uniform plane electromagnetic wave is incident upon the small circular aperture in an infinite conducting plane, the transmitted power P t through the aperture is very small. Specifically, the transmission cross section (TCS) T of the aperture whose diameter D is much smaller than the wavelength is proportional to (D/ ) 4 ·D 2 according to the Bethe's theory [1]. Here the TCS T is defined so that the transmitted power P t through the aperture may be given by the multiplication of the incident power density P inc [W/m 2 ] and T [m 2 ], i.e., P t = P inc T.
It has been found that the transmission efficiency can be significantly enhanced by modifying the aperture shape as in the circular ridged aperture in the infinite thin conducting plane or by employing a small coupling apertureto-cavity-to-aperture system in a thick conducting plane. It is interesting to note also that the TCS T for both the two structures can be increased to the maximum value of T = 3 2 /4 [m 2 ] as a upper bound [2]. This means that incident power on the much larger area than actual aperture area is funneled into the physical aperture and transmitted through it and radiated into the opposite half space to incident side regardless of the actual aperture area under the transmission resonance where the transmitted power P t becomes maximum. The near-field microscopy employs a probe of a subwavelength size and an object that is mounted in the near field of the probe, so that the spatial resolution is determined by the coupling hole size of the probe rather than by the wavelength. Several designs of the microwave near-field probe have been used, such as a circular aperture [3] and open waveguide [4]. A smaller coupling circular aperture provides better resolution, although this is not very practical because the transmitted power is very low as mentioned above. So several kinds of apertures such as long resonant slit and anular aperture have been studied to overcome such a low transmission efficiency problem [5]. In general, the transmission efficiency through a small coupling aperture and the spatial confinement of the electromagnetic energy are taken as two conflicting characteristics of the compromise problem which we should solve in the design procedure of near-field imaging probe. The aim of this work is to consider coupling aperture structures for a near-field microwave imaging probe whose transmission efficiency can be significantly enhanced while the coupling hole size is maintained as small as possible for better resolution.

II. Design of two types of near-field microwave imaging probes and numerical results.
We are going to deal with the two types of probes which are fed by a rectangular waveguide. First, we consider the probe which comprises a small ridged aperture cut in the end plate of the rectangular waveguide.

2-1. Circular ridged aperture type
We first obtain geometrical parameters (ridge width S, gap G, and aperture radius RA in Fig. 1(b)) of the resonant circular ridged apertur be located roughly midway in th Glisson (RWG) method [2]. From the Fig. 2, this type of pr impedance matching and smal advantage of this type is, as se appearing later, enabling to tran

Bis
It is well known that the reson sized small hole can be incorpo and the small holes in the cente the iris couples through the h waveguide wavelength g , the r as shown in Fig. 3(a). re in an infinite conducting plane so that the t he X-band range (8.2 ~ 12.4 GHz) by using th g. 1 shows the structure of the circular ridged er to investigate the reflection characteristics eometrical parameters given above.
(a) ged aperture type (a) feeding waveguide (a = 22 and RA = 6.9), unit [mm] 1 versus frequency robe is seen to offer the advantages in both h ll spatial confinement of electromagnetic en een in Fig. 2, its low Q factor, in compariso nsmit short pulses and to achieve good tempor sected unsymmetrical transmission nant transmission cavity type using aperture in orated as a filter in a waveguide run. The iris er allow coupling into and out of the cavity, hole to the other side, and provided that th esonant mode will be set up which couples th cavity type nput and output coupling of the same ses form the closed ends of the cavity The magnetic field along one side of he length between the two irises is hrough to the output (free space) side, If the resonant transmission ca is incorporated in the wavegu established. That is, the x-com irises at z =g and z = 0 becom /2. So without change in the tra long by putting the capacitive field at at z =g /2 midway i configuration inside the bisect original symmetrical cavity.
Note that transforming the cap hole (corresponding to inductiv assumption of high Q resonant terms of the unsymmetrical tra geometrical parameters of the c in the sense that different coupl for both ends of the conventio versus frequency. mmetrical transmission cavity type (a) probe str b) double ridge type of capacitive iris (c = 4, d = avity type using aperture input and output co uide run, when maximum transmission oc mponent magnetic field (corresponding to ta mes maximum and y-component electric field ansmission resonance frequency, we can bise iris of Fig. 3(b) which can support the orig inside the original cavity, as shown in Fig.  ted transmission cavity (g /2 z 0) rem pacitive iris at z =g /2 through the length o ve circuit element) leads to the formation of p t circuit. From this, we expect the single ri ansmission cavity structure, as shown in Fi circular hole and capacitive iris. Here "unsym ing irises are used as in Fig. 3(a) instead of th onal symmetrical transmission cavity. Fig. 4 (b) ructure of bisected transmission cavity = 4.83, and t = 1), unit [mm] oupling of the same sized small holes ccurs, the strong resonant mode is angential magnetic field) on the two becomes maximum roughly at z =g ect the original transmission cavity g ginal maximum y-component electric . 3(a). Under the situation, the field mains almost the same as that in the of g /2 to the small coupling circular parallel LC resonant circuit under the idged aperture can be synthesized in ig.3(a) through some adjustments of mmetrical transmission cavity" is used he same small sized aperture coupling 4 shows the reflection characteristics Fig. 4. Scattering parameter S11 versus frequency for the bisected transmission cavity.
It can be seen from Fig. 4 that, also in the transmission cavity type of Fig. 3, impedance match can be achieved at 11.05 GHz, i.e., all the incident power from the feeding rectangular waveguide can be made to be radiated through the small coupling hole at z = 0. The difference between the circular ridged aperture type and transmission cavity type is that the transmission resonance frequency (10.872 GHz) for the former is somewhat lower than that for the latter and that the frequency selectivity Q for the former is much smaller than that for the latter as seen in Fig. 2 and 4, under the assumption that RA in Fig. 1 be the same as the radius R of the coupling hole in Fig. 3. As mentioned above, the transmission cavity type of Fig. 3 tends to show relatively larger Q than that for the circular ridged aperture type in Fig. 1. The Q can be, however, lowered to any desired value by increasing the radius of the coupling hole of one closed end of the transmission cavity or by decreasing the width of the vertical conducting strip with a gap which corresponds to the capacitive element of the other end of the cavity.

III conclusion
We have considered two types of near-field microwave imaging probe of a small coupling holes whose transmission efficiency can be significantly enhanced while the coupling hole size is maintained as small as possible for better resolution. One type of the coupling hole is a circular aperture with double ridge. The other is an unsymmetrical transmission cavity type whose closed ends comprise the small circular aperture ( obtained by removing the ridge structure from the above circular ridged aperture) and capacitive iris, located g /2 (half guide wavelength) apart from the small circular ridged aperture. This study may help to understand the working principles of filter and small aperture antenna as well as the near-field probe with main interest centering on the compromise between two conflicting physical characteristics. The two conflicting characteristics for the filter and antenna problems correspond to impedance matching bandwidth and frequency selectivity. On the other hand those for near-field probe design problem mean the transmission efficiency of the small coupling hole and spatial confinement of electromagnetic energy for the better resolution.