Research Article  Open Access
L. A. Haralambiev, H. D. Hristov, "Radiation Characteristics of 3D Resonant Cavity Antenna with GridOscillator Integrated Inside", International Journal of Antennas and Propagation, vol. 2014, Article ID 479189, 6 pages, 2014. https://doi.org/10.1155/2014/479189
Radiation Characteristics of 3D Resonant Cavity Antenna with GridOscillator Integrated Inside
Abstract
A threedimensional (3D) rectangular cavity antenna with an aperture size of 80 mm 80 mm and a length of 16 mm, integrated with a fourMESFET transistor gridoscillator, is designed and studied experimentally. It is found that the use of 3D antenna resonant cavity in case of small or medium gain microwave active cavity antenna leads to effective and stable power combining and radiation. The lack of lateral cavity diffraction and radiation helps in producing a directive gain of about 17 dB and radiation aperture efficiency bigger than 75% at a resonance frequency of 8.62 GHz. Good DC to RF oscillator efficiency of 26%, effective isotropic radiated power (EIRP) of 5.2 W, and SSB spectral power density of −82 dBc/Hz are found from the measured data. The 3D antenna cavity serves also as a strong metal container for the solidstate oscillator circuitry.
1. Introduction
The miniaturization in antennas is of great significance for the modern mobile and portable aircraft and spacecraft wireless electronic systems. However, in addition to the structural, technological, power, and other limitation factors valid for all electronic devices, the miniaturization in antennas is inherently restricted by the space electromagnetic radiation mechanism.
Great antenna size reduction is achieved by using the resonant field phenomenon. The resonant antenna is usually commensurable in size with the design wavelength. If the outer resonant antenna surface is excited in resonance, the antenna is termed exoresonant. Examples of such antennas are the resonant dipoles or monopoles, loops, and slots. By analogy, if some antenna comprises a cavity tuned in resonance it is named endoresonant or simply cavity antenna (CA).
Depending on the configuration and inside field mode structure the passive singlecavity antennas are divided into two basic classes: closed or 3D cavity antennas [1–5] and open or beamwaveguide (FabryPerot) cavity antennas [6–13]. The classic optical or quasioptical FabryPerot resonator is a laterally open, onedimensional (1D) resonant cavity with a big transverse dimension and length , where is the resonance cavity design frequency and is an integer number. Frequently in practice, the microwave FabryPerot or 1D cavity antennas do not obey the above quasioptical size condition. The above referenced 1D cavity antennas are based on fundamentalmode cavity resonators with a smaller transverse size of about several wavelengths and a length near to half the wavelength. As a rule, this compromise is on account of the antenna radiation properties. Most of the 1D passive cavity antennas have relatively low aperture radiation efficiency. For example, the radiation efficiency is 15–53% in [8], 41% in [9], 43% in [10], and 30–40% in [11]. An exception is claimed in [7], where a maximum efficiency of 75% is predicted.
Both 3D and 1D cavity antennas have partially reflecting surfaces (PRS) named also electromagnetic band gap (EBG) structures that serve as antenna radiation apertures. An arrangement of a 3D passage cavity and 3D or 1D radiating cavity forms a doublecavity antenna [3].
Due to the cavityfield resonance the passive 3D and 1D antennas are narrowband, with a typical frequency bandwidth from about 1 to 3% and a gain of 15–20 dB. For these bandwidth and gain ranges the cavity antennas are much smaller in length, volume, and weight compared to the equal in aperture nonresonant antennas.
Along with the development of compact highpower communication, radar, and radionavigation equipment the need of a largescale integration of electronic devices like microwave/mmwave detectors, amplifiers, oscillators, phaseshifters, and so forth with the antenna structures is taking place, and thus, the antennas become active devices. The active antennas belong to the socalled functionally small antennas, a term that applies where additional performance is being added without increasing the antenna size [14].
An efficient power combining in a free space or in a cavity antenna volume is possible using a distributed (array) oscillator approach. The oscillating solidstate devices (Gunn or IMPATT diodes, MESFET or HEMT transistors, etc.) are mounted in a grid array. The gridoscillator arrays act simultaneously as power combiners and antennas.
The open cavity antennaoscillator (CAO) approach for combining the output powers of multiple microwave devices has been first studied by Mink in [15]. There, a large relative to the design wavelength FabryPerot cavity of two reflectors, plane total reflector and perforated (PRS) radiation shell, with a spherical curvature is applied. Such a large relative to wavelength 1D cavity is more suitable for millimeter and submillimeter (quasioptical) antennaoscillators. In [15], the planar array of IMPATT or Gunndiodes is placed in a small distance from the total plane reflector.
Intensive and fruitful work on efficient 1D cavity power combining devices is described later on in [16–19]. However, because of the bigger diffraction loss and inner cavity field distribution change, their radiation characteristics are inferior compared to those of the corresponding passive cavity antennas.
For small and medium aperturesize microwave and longmmwave passive and active antennas the use of 3D antenna cavities has some advantages.(i)There is no diffraction loss as in the case of the laterally open quasioptical cavities, and thus better radiation characteristics and higher gain and efficiency are possible.(ii)The metal closed cavity is a robust container for the solidstate electronic circuitry placed inside.
Early work on the power combining in 3D round cavity antennaoscillators is found in [20]. These are Xband singlecavity and doublecavity antennaoscillators containing Gunndiodes integrated in halfloop feeds, where power combining efficiency is bigger than 80% and aperture radiation efficiency is 40–50% for the twocavity and bigger than 60% for the singlecavity antenna oscillators.
In this paper an effective 3D cavity antennaoscillator comprising an array of MESFET oscillator elements and radiating through a rectangular slottedarray aperture is contrasted to a similar in shape and dimensions 3D passive cavity antenna.
2. Profiles and Resonant Features of Passive and Active Cavity Antennas
2.1. Profiles of Passive Cavity Antennas
Profile crosssections of 3D and 1D passive cavity antennas are sketched in Figures 1(a) and 1(b), respectively. Each antenna cavity is fed in the middle of the back reflector wall (BW) by a flanged (F) hollow waveguide (WG) through an endwall iris (I). The 3D cavity antenna (3D CA) is laterally closed by a side wall (SW), while the 1D cavity antenna (1D CA) is laterally open. A3 and A1 are the transverse profile cavity dimensions, while L3 and L1 are the lengths of 3D cavity and 1D cavity, correspondingly. Both cavity antennas, 3D CA and 1D CA, have partially reflecting aperture wall (AW).
(a)
(b)
2.2. Profiles of Active Cavity AntennasOscillators
Figures 2(a) and 2(b) illustrate the profiles of 3D and 1D cavity antennasoscillators.
(a)
(b)
The passive cavity antennas illustrated above (Figure 1) are converted here into the active (antennasoscillators) sketched in Figure 2. It is supposed that these antennasoscillators have the same cavity shapes and dimensions as the corresponding passive ones but are fed by inner (integrated) autonomous gridMESFET oscillators (GOs).
2.3. Mode Spectrum Density in Lossless Cavities
In the 3D or closed cavity a number of natural field modes can be excited. Their resonant frequencies form a discrete spectrum in the cavity volume with a spectrum density given by the asymptotic RayleighJeans formula [21] where is the number of field modes for a unit frequency increment and is the freespace light velocity. Obviously, the mode discrete spectrum density becomes bigger with the volume and frequency increase ().
In the 1D or FabryPerot cavity the spectral density does not depend on the frequency but on the cavity length only Or, evidently, the resonant frequencies form an equidistant (constantdensity) spectrum.
It is concluded that as a result of the lateral opening in the 1D cavity there are no transverse field reflections and transverse cavity modes. Thus, the latter are selectively radiated and only the axial standingwave modes are left inside. This is the physical explanation for the frequency spectrum thinning effect.
Like the cavity resonator the passive or active cavity antenna is supposed to operate in a singlemode regime; that is, in a specific frequency band only a single characteristic mode exists. Most often this is the fundamental mode with the lowest resonant frequency, which is achieved by choosing a proper cavity size and field excitation and suppression (filtering) of unwanted modes.
3. Passive and Active 3D Cavity Antenna Designs
3.1. Passive Cavity Antenna Design
The passive cavity antenna (Figure 1(a)) comprises a 3D rectangular cavity. The antenna is designed for a resonant frequency of 9.0 GHz (or resonant wavelength of 33.33 mm). The cavity is excited by means of mode rectangular waveguide through an endwall capacitive iris 23 mm 4 mm in size.
Our experience with the passive cavity antennas has shown that the fundamental cavity field mode very much prevails over the nearest higherorder TE and TH modes if the characteristic transverse cavity size is less than , where is the cavity mode resonance frequency [2–4]. The cavity aperture wall (AW) is chosen square in shape with dimensions as follows: aperture side (or ) and cavity length (or ).
For a lossless and nonradiating rectangular cavity the resonance frequency of the fundamental mode is easily found by The corresponding cavity wavelength is
This passive cavity antenna radiates through a slotted (partially reflecting) aperture printed on a copperclad dielectric substrate of thickness 1.5 mm and permittivity and loss tangent equal to 2.3 and 0.005, respectively. The aperture slot array consists of equal slots, with a size of each.
3.2. Cavity AntennaOscillator Design
The passive cavity antenna described in Section 3.1 is converted here into the cavity antennaoscillator sketched in Figure 3. This active antenna has the cavity of the passive one but is fed by an inner grid of MESFET transistor oscillators, similar to those described in [22, 23]. Each MESFEToscillator element has the configuration, dimensions, and electric data shown in Figure 3(a). The grid plate is centered on the cavity wall opposite to the slotted aperture wall (Figure 3(b)). A feedback coupling occurs between the basic cavity field mode and the oscillator array dipole elements. This phenomenon leads to an injection phaselocking and high efficient power combing in the closed cavity volume. Like the semitransparent slotted aperture plate, the MESFET grid oscillator circuit is printed on a copperclad Teflon substrate. The detuning effect of RF induced biasing caused by the cavity field mode to the MESFET oscillator action is not considered initially. The gridoscillator is designed for an action in free space. As expected, after its insertion in the antenna highQ resonant cavity, the oscillator phase noise and stability performance have been improved. Small adjustments of the carrier frequency and radiation efficiency have been done with a change of the cavity and dipoles’ lengths.
(a)
(b)
The MITSUBISHI MGF 1303B MESFETs are employed in the grid oscillator. Within the Xband each transistor provides about 16 mW CW power.
3.3. Experimental Results for Passive and Active Cavity Antennas
Figure 4 illustrates the copolarization (solid lines) and crosspolarization (dashed lines) radiation patterns of the studied cavity antennaoscillator measured at 8.62 GHz in the main antenna planes: Eplane (right curves) and Hplane (left curves). Evidently, the copolarization Eplane radiation pattern has narrower beamwidth and lower maximum sidelobes, while the maximum Hplane crosspolarization is smaller compared to the Eplane one.
The frequency spectrum of cavity antennaoscillator measured in antenna farfield region is shown in Figure 5. The carrier oscillator frequency is 8.62 GHz and the measured SSB spectral power density is −82 dBc/Hz at 100 KHz off the carrier frequency. Overall DC to RF efficiency of 26% and effective isotropic radiated power (EIRP) of 5.2 W are computed from the experimental data. Further increase of DC to RF efficiency and EIRP is possible by further diminishing of mutual coupling in the griddipole array and material loss in the oscillator circuitry and antenna cavity.
In Table 1 the measured radiation characteristics of the passive cavity antenna at the resonance frequency of 8.779 GHz and active cavity antennaoscillator at the carrier oscillator resonance frequency are contrasted. These are beamwidth BW and first sidelobe level SLL of the Eplane and Hplane radiation patterns, fundamental resonance frequency in GHz, antenna directive gain in dB, radiation aperture efficiency , in %, where is the antenna aperture surface, DC to RF efficiency in %, EIRP in W, and SSB spectral density in dBc/Hz.

As it is chosen in the design text above, both passive and active cavity antennas have the same structures and dimensions and differ only in their field sources: narrow capacitive iris (I) fed by a rectangular waveguide in the case of passive antenna and gridoscillator (GO) integrated within the cavity on its back cavity wall RW.
From Figures 4 and 5 and Table 1 several important comments follow.(1)The basic radiation parameters (BW, SLL, , etc.) of the cavity antennaoscillator reasonably deteriorate compared to those of the original passive cavity antenna. These effects are due to the inevitable field distribution change because of the presence of the gridoscillator circuitry.(2)The antennaoscillator directive gain and efficiency compared to those of the passive cavity antenna are falling down by only 0.4 dB and 3.6%, respectively.(3)Both passive and active 3D cavity antennas enjoy high aperture gain efficiency of about 75–80%. In contrast, the 1D cavity antennas for the same relatively small cavity and aperture dimensions typically have about 1.5–2.0 times smaller radiation efficiency.(4)Insertion of gridoscillator circuitry printed on a dielectric substrate diminishes the cavity resonance frequency, from 8.779 GHz to 8.623 GHz, but it could be corrected simply by changing slightly the cavity length and optimizing the mutual coupling between the oscillatorarray dipole elements.
4. Conclusions
An undersized 3D cavity antenna integrated with a fourMESFETtransistor gridoscillator power combiner is designed and studied experimentally. Very good DC to RF oscillator efficiency of 26% and effective isotropic radiated power (EIRP) of 5.2 W are obtained from the measured data. It is proved that the use of 3D antenna resonant cavity instead of 1D parallelplane one in case of small and medium aperturesize and gain microwave passive and active antennas has an effective power combining action. The lack of diffraction loss due to the lateral cavity opening leads to a higher directive gain of about 17 dB and radiation aperture efficiency bigger than 75%. In addition, the 3D cavity serves as a strong metal container for the solidstate oscillator circuitry inside.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgment
The authors acknowledge the Chilean Science Agency CONICYT for the support within the Anillos Project ACT53 and Fondecyt Project 1120714.
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Copyright
Copyright © 2014 L. A. Haralambiev and H. D. Hristov. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.