Design Of A Miniature L-band High Power Directional Coupler

The HPM (high power microwave) radiation field measurement is one of thefundamental researches of HPM technology, and it is an important means to estimate theproper property and the radiation field effect of HPM system. A normal HPM radiationfield measurement system is composed of receiving antenna, attenuation part, detectorpart and etc. Considering that the HPM radiation field has a characteristic of high peakpower, so the antenna and attenuation parts usually select the waveguide devices, whichhave high power capacity. But for L-band microwave applications, the size of thewaveguide components is comparatively large, and the composed radiation fieldmeasurement system has large size and is heavy, which are not suitable for using. Theeffect between two measurement systems constraints the number of elements that couldbe used during the power measurement by array method, that increases the integral errorand decreases the radiation field measurement accuracy. So the study on miniaturizationof measurement system for L-band HPM radiation field measurement is significant.To resolve the problems mentioned above, a study on a miniature L-band high-powerdirectional coupler is proposed.1. On the basic of widely survey, research status about directional coupler wasanalyzed and the factors affect the coupler design were concluded, those were powerhandling capacity, frequency range and the size. The design principle aboutminiaturization is decreasing the cross-section area of the coupler under the condition ofensuring the power handling capacity and shortening the length of the coupler under thecondition of expanding the frequency range. On this principle, the structure of theminiature L-band high-power directional coupler was confirmed. A circular rectangularline (C-R coaxial line) with proper size was used as the directional coupler’s main lineto make sure the coupler has high power handling capacity and in the meantime, todecrease the cross-section area of the main line. As the vice line was not need to havehigh power handling capacity, so the CBCPW (conductor backed CPW) line which hasa more compact structure than that of the main line was used as the coupled line todecrease the cross-section area of it. It is a diamond aperture rather than a multi-hole array was used as the coupling structure to shorten the length of the coupler andexpanding the frequency range.2. The coupling and directivity of the coupler were analyzed via small-hole couplingformulas and infinitesimal method. The results show that the coupling C is lineardecided by the length of the short diagonal of the diamond aperture2s, meanwhile theflatness of the coupling and directivity D are decided by the length of the long diagonal2L. According to the relation of D and L/, the directivity could be better than15dBwhen2L values within the range of. The above conclusions were verified bysimulation results and the size of the L-band directional coupler could be obtained. Thesimulation properties of the coupler were that within the range of1GHz~2GHz, thecoupling is about50dB while the flatness is about1.5dB, the directivity is larger than17dB.3. Processing the miniature high-power coupler and test the parameters of thecoupling and directivity by a vector network analyzer. A miniature L-band HPMradiation field measurement system of L band was build up by the miniature high-powercoupler, antenna with coaxial feeder, detector, etc. The power handling capacity of theminiature coupler was verified by comparing the waves receiving respectively from theminiature measurement system and normal waveguide measurement system in the sameradiation field.The results show that within the range of1GHz~2GHz, the VSWR of input port isless than1.2, the coupling is about47dB while the flatness is about2dB, the directivityis larger than17dB and the power handling capacity is not less than1.4MW. In a word,the coupler could be used in L-band HPM radiation field measurement. The size of thecoupler is150mm×46mm×55mm that is tenth-value of a L-band waveguide coupler.The miniaturization of the L-band high-power directional coupler is achieved.