High Power Filters
By Manny Assurian, Ray Hashemi and Jim Assurian, Reactel, Incorporated

High power RF and microwave filters and diplexers are an integral part of many signal transmission systems. Typical applications include base stations for commercial wireless systems, high power amplifiers to prevent the transmission of harmonics, medical processes such as MRI machines, and a variety of military functions. No matter what the ultimate application is, high power filters and diplexers require significantly more care and understanding than units at nominal power levels.

Careful consideration must be given to the design and construction of a filter or diplexer which will see power levels into the kW range. Aside from the electrical specifications needed to meet the performance requirements, there are significant power issues to be dealt with: circuit selection, selection of housing material, selection of connectors and internal components, and method of construction, among them. The following paragraphs will touch briefly on each of these aspects.

Circuit Selection
Much of this selection process is determined by the electrical specifications themselves. We feel that the optimal circuit design to operate in a high power environment is a Chebychev design. This is the least complex of all filter circuits. A Chebychev design allows for a reasonable amount of selectivity while offering the lowest loss. Having a unit with low loss is key, as additional loss will turn into heat which will need to be dissipated through heat sinks or some sort of external cooling mechanism. A Chebychev design will also keep component count to a minimum, helping to reduce loss as well. In instances where a Chebychev circuit will
not provide the desired out of band attenuation, we have developed a lowpass circuit with an elliptic-like response that offers high attenuation while withstanding power levels up to 2 kW.

Housing Material Selection
Now that we have selected a circuit which will satisfy the electrical requirements, our thoughts turn to materials and how to realize the design. As noted earlier, whatever signal loss there may be will eventually turn into heat, and that heat needs to be dissipated. We find that manufacturing a housing for the filter which has heat sinks as an integral part of that structure works most efficiently. Weather the housing is rectangular or tubular, there are techniques and methods for integrating heat sinks into the housing. The typical choice for housing is aluminum, as it has the ability to transfer heat rapidly. Aluminum is an easy metal to work with, and machining the heat sinks into the housing is a snap.

Component and Connector Selection
With the material and housing issue solved, our focus becomes the actual components which will comprise the inner workings of the filter. All components must be of extremely high quality and procured from reputable manufacturers. There is a school of thought that, with lowpass filters, lower frequency units are better suited for an LC design, and higher frequency units are limited to tubular designs, while with bandpass filters one would include cavity designs in the discussion. No matter which design is chosen, the point of selecting high quality components which are properly rated for the power is paramount. For an LC design, there are a variety of high voltage capacitors available from a number of reputable suppliers. It is, however, important that the supplier has a wide variety of capacitance values to select from. LC inductors can be either air-wound or wrapped around some sort of Q-inducing material such as an iron power core. With a tubular design, material selection is generally limited to brass, wire and Teflon, though there are critical construction methods used to ensure safe passage of the power through the filter, avoiding the creation of Corona. Cavity filters are similar to tubular filters in that there are few choices to be made material-wise, but important choices to be made in its construction.

Connectors pose a different challenge in that not all RF connectors are suitable for high power applications. It is the opinion of most that 7/16 DIN, Type N, TNC, or EIA connectors are advisable.

Method of Construction
With all materials gathered, build method is the next to tackle. With an LC design, the arrangement of components in the housing can help minimize the amount of internal heat retained by the filter. While it is true that heat generation is a function of the power applied and signal loss, a thoughtful layout of the Ls and Cs can help the heat from accumulating inside of the housing, which could lead to premature degradation. While a tightly packed layout may work well at nominal power levels, it is not, however, advisable and could be catastrophic in a high power environment and should be avoided at all costs. If the design choice is a cavity filter, specific care should be taken to the resonators and tuning elements. Cavity filters are a bit more involved in that the spacing between resonant posts and tuning elements must be sufficient enough to prevent any arcing within the filter. Extensive work goes into calculating the voltage present in each cavity as the amount varies from cavity to cavity in the filter. The results of those calculations are then converted to the important spacing between resonant posts and tuning element mentioned above. We feel that the addition of some safety margin is advisable. Tubular filters offer a limited variety of build methods. These are mainly frequency dependent and center on a high-low impedance transmission line vs. standard tubular build method.

Multiple Channels, Multiples of Power
While designing a filter at a particular power level is a challenge, that task multiplies when the unit needed is a multiplexer. It is important that a firm understanding is reached between the manufacturer and customer about power distribution and if the specified power is an “overall” power level for the unit or a “per channel” power level. In the case of an “overall” power level, the power at the input is then split into the available output channels. Thus, if the input power to a diplexer is 1000 Watts, each channel will see 500 Watts. In this instance, the design will be based on an overall power level of 1000 Watts. In the case of a “per channel” power level, the power at the input is the addition of each channel’s power. Thus, if the each channel of a diplexer is 1000 Watts, the input will see 2000 Watts. In this instance, the design will be based on an overall power level of 2000 Watts. This is an important distinction, as the possibility exists for designing a unit which will be severely underrated for the potential power it may encounter. In many circumstances, the multiplexer may operate without issue, but once one of the channels sees an open, all of that channel’s power will get reflected back to the input and potentially cause the unit to fail.

Conclusion
You may have gathered after reading this article that selecting the appropriate method for the design and construction of a high power filter or diplexer is a difficult task. There are a number of factors, often competing against each other, that stand between the paper design and a working unit. We at Reactel have years of experience in the design and manufacture of high power units and these types of challenges are something we face on a regular basis. We invite you to visit our website at www.reactel.com, or better yet, give us a call to discuss your specific requirements.

Reactel, Incorporated
www.reactel.com
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