A High Power, Broadband, Ka-band Extended Interaction Klystron

Communications & Power Industries Canada Inc. (CPI)
Georgetown, Ontario, Canada

Today’s emerging Ka-band satellite communications (SATCOM) systems are designed to support the enormous commercial telecommunications market. These SATCOM systems require high power amplifiers for their earth terminals. The mm-wave amplifiers must be efficient and highly reliable, and provide sufficient power and gain in a small package.

The VKA2400 series mm-wave extended interaction klystrons (EIK) are designed for commercial SATCOM uplink service and meet the application’s stringent performance and reliability demands at Ka-band frequencies. The klystrons’ design concept provides more efficiency than a standard klystron or helix traveling-wave tube and feature lower beam voltages, broader bandwidths and are smaller and lighter than standard klystrons. In addition, the klystrons’ wide instantaneous bandwidth eliminates the need for mechanical tuning and allows for remote, unattended operation such as is required at the antenna hub. Locating the tube at the antenna hub avoids the high losses associated with long waveguide runs at these frequencies. The tubes’ small size also allows multiple EIKs to be collocated in a multifeed arrangement. The elimination of mechanical tuning increases reliability by eliminating moving parts.

One of the first tubes in the series, the model VKA2400S02 EIK, is designed to operate from 27 to 31 GHz with 400 MHz of instantaneous bandwidth. The tube provides 600 W of CW power and 43 dB of gain. Figure 1 shows the klystron’s power output vs. frequency for several input drive levels. In this case, the tube is operating with a 6.8 kV beam voltage and 328 mA beam current at a gain of 58 dB. At an output of 400 W, the 1 dB bandwidth is 410 MHz. The tube also can be operated at a reduced beam current of 300 mA to produce 200 W at saturation with approximately 350 MHz of bandwidth.

The tubes are specified to operate using a 6.7 kV beam voltage and 440 mA of beam current. Typical heater voltage and current are 6.3 V and 1 A, respectively, with a 5 A maximum surge current. Beam focusing is accomplished using an Sm2 CO5 integral permanent magnet and the tube is capable of operating in any orientation.

The new Ka-band klystron is air cooled and operates over an ambient temperature range of –20° to +50°C. Body cooling is separate from collector cooling. This configuration allows the body temperature to be better stabilized, thus increasing useable bandwidth. The required collector flow rate is 700 lb/hour at standard temperature and pressure, while the required body flow rate is 200 cfm at sea level.

The coolant temperature affects the klystron’s body temperature directly, which in turn shifts the tube’s center frequency and associated gain response. The tube is adjusted to meet its specifications at the center frequency within the required body temperature range. A temperature sensor is attached to the tube’s body to allow the user to provide temperature (frequency) stabilization. Other versions of this design have been built that are liquid cooled and produce over 1.2 kW CW at 28 and 35 GHz for plasma heating applications and over 1.5 kW pulsed, 250 W average at 35 GHz for pulsed amplifier use. These versions require higher beam voltages and currents.

The tube’s physical outline is shown in Figure 2 . The RF input and output connections are WR34 waveguide and mate with type UG-1530/U cover flanges with aligning pin holes. Maximum waveguide pressurization is 200 kPa and the required load SWR is 2 maximum. The klystron weighs 23 lb.

This Ka-band klystron is the first in a series of high frequency SATCOM klystrons designed to cover a wide range of uplink applications, providing CW power from 1000 W at Ka band to 100 W at W band. Due to their high efficiency, small size and light weight, the tubes are suitable for both fixed and mobile requirements. The Ka-band klystron is available currently at a production rate of four units per month with higher rates possible subject to a six-month lead time. Contact the manufacturer for price information.

Communications & Power Industries Canada Inc. (CPI),
Georgetown, Ontario, Canada
(905) 877-0161.