Design and development of precise high power wide band attenuator cum dummy load for HF and VHF frequencies

Particle accelerators utilize high power radio frequency system to provide necessary accelerating field to charged particles for acceleration. The development of RF power sources over wide range of frequencies is in progress for various applications in accelerators. In this paper the design and indigenous development of precise low cost wide band attenuator cum dummy load has been discussed. The precise wide band attenuator/ dummy loads not merely used for absorption of RF power for testing of high power sources without any remarkable reflection but are also used for broad ranges of measurements and calibration of both active and passive RF components and devices. The ideal requirement of an attenuator/load is invariance of its resistance within specified temperature range and over wide range of supply frequencies as well as absence of any parasitic reactances. Although no practical device can meet these ideal requirements. However proper selection of design parameters and minimizing the inherent parasitic reactance and making them distributed in nature above requirements can be met satisfactorily. In order to achieve all the above requirements we have used tapered transmission line to keep both sending and receiving end impedance equal to 50 Ohm. The techniques to meet necessary requirements and practical method to realize them have been discussed in detail. The discussion about difficulties encountered and their solutions are also recommended to meet practical requirement. A 40 dB attenuator cum 50 ohm load capable of handling 1 kW input power has been already developed and tested. In this device heat is removed by natural convection. The design and development of 5 kW, 60 dB attenuator is taken care by forced convection cooling to manage thermal issue. Theoretical simulation of aforesaid attenuator had been performed with freely available software RF SIM 99 using multiple sections of lossy transmission line. To realize real practical situation hybrid analysis has been carried out using lump cum distributed parameter analysis. (author)