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High-power attenuators

Apr 1, 2008 12:00 PM, BY DAVE DISTLER

Figure 1. This illustration shows the oil load vs. forced air. The relative sizes of an oil load with a 3-1/8in EIA fl ange connector are shown on the left, and a forced air load is shown on the right. The right panel contains the same oil load with an optional cooling fan and shroud to extend the power rating above 5kW.

Most attenuators on the market rely on thick or thin film-resistive designs screen-printed or deposited on a flat ceramic, much of which is Beryllium Oxide (BeO). This has special handling, processing and disposal requirements. While the topology is suitable for lower power, extending attenuator performance to more than 1kW can be difficult and expensive.

There is another attenuator configuration option that provides high average power and high peak power capability at a cost-effective price, using standard, off-the-shelf products. This article explains how to configure a high-power attenuator system from 1kW to 200kW for use in high-power applications above 1kW with readily available components.

Selecting an RF dummy load

The first step is choosing an RF dummy load for the maximum rated peak and average power. (See Figure?1.) Oil dielectric loads are readily available for power levels up to 12.5kW. Alternatively, forced-air cooled loads, heat exchangers (self-contained water-cooled loads) or water-cooled loads achieve power levels of 125kW and more. This attenuation system will use standard dummy loads, and customization isn't required.

Specify the load's connector

The second step is to specify the load's connector as either 1-5/8in or 3-1/8in EIA flange. The attenuation system uses an intermediate rigid line section that is based on either a 1-5/8in or 3-1/8in line size. Choose the intermediate line size based on your power attenuation requirements. A 1-5/8in EIA flange connector can be used on a load for up to about 30kW and the 3-1/8in EIA flange connector for up to 75kW.

Identifying the flanged rigid line section

Shown here is a completed high-power attenuator assembly using a lower power 1-5/8in rigid line with EIA fl anged connectors and coupling section. Not shown are the 1-5/8in to N, LC, HN, 7/16 or between series connector and adapter that can be added to the line section input. The attenuated output is available from the top of the element in the center of the line section. An optional BNC attenuator can be used to achieve higher attenuation values.

The third step involves selecting either a 1-5/8in or 3-1/8in flanged rigid line section to attach to the load. Once the load's connector has been specified, it's time to attach what will become part of a coupler/sensor assembly: a coaxial rigid line section designed to mate to the dummy load. This line section can be supplied in a variety of configurations, such as single, dual or triple socket, but for simplicity, select a single socket rigid line section with EIA flange connectors. Between the rigid line and the dummy load connector a coupling assembly will be needed that includes all the bolts and mechanical hardware to mate the two flange connectors.

Choosing an RF sampling element

The fourth step is picking an RF sampling element to plug into the line section. An RF sampling element is typically a nondirectional device that has a broadband coupling loop or plate that parallels the center conductor of the rigid line. To be broadband and have negligible effect on insertion loss and VSWR, its mechanical dimensions are held to a tight tolerance. Once inserted into the line section, this portion of the assembly becomes a 2MHz to 1GHz nondirectional coupler, capable of operating up to the power rating of the load. The elements, called X-Tractors, have a variable set screw to allow for minor adjustments of +/-8dB from their preset coupling value of 49dB for the 1-5/8in size and 56dB for the 3-1/8in line. (See Figure 2.) The attenuated signal output is via a BNC female connector on the top of the element. In theory, it is possible to further reduce the signal level by attaching a standard BNC fixed attenuator to the element. The attenuator should be rated for 2W.

Deciding on an input connector

Figure 2. This drawing of a typical attenuator performance shows the typical fl atness of the variable attenuator from 2MHz to 1GHz. The attenuator is better suited for broadband use above 50MHz, while single frequency or narrowband operation is recommended below 50MHz.
Click image to enlarge

The next step is an optional one: choosing an input connector for the rigid line section. The rigid line is ready to accept a 1-5/8in or 3-1/8in coupler and line section, but for convenience, the input to the rigid line section can be more of a standard coaxial type, such as an N, HN, LC or 7/16 type, or a larger between series adapter can be specified up to 6-1/8in line size. Note that an additional adapter will be required for this portion of the assembly.

Summary

Specifying a cost-effective, high-power attenuator does not require custom products, long lead times, multiple vendors or nonrecurring engineering (NRE) costs. Standard, off-the-shelf products that are properly matched together can save project cost, time and space.

Dave Distler is the sales and marketing manager for Coaxial Dynamics.