Hundreds Of Dollars Worth of Hollow “Scrap” Metal?

Take a close look at the picture above. How much do you think this piece of metal cost? The “thing” shown above illustrates a hollow metal piece and for the majority of people, it is considered as a mere piece of hollow “scrap” metal. However, as the saying goes, one man’s trash is another man’s treasure, this hollow metal piece is indeed valuable to a certain group of people. It might be absurd, but this piece of “scrap” metal actually cost hundreds of dollars.

Here is the fun fact. Big industries such as Nokia, Huawei, Broadcom, and so on are willing to spend billions of dollars to produce or purchase these “scrap” metals. Now the question is why are these “scrap” metals so important and sought after by these industries? What makes this “scrap” metal so unique and valuable?

The Waveguide Filter

Waveguide structures and its equivalent circuits. Source:

This piece of hollow metal is known as the Waveguide Filter. They are designed to “guide” desired high-frequency EM waves with minimal energy loss in the process while at the same time blocking other undesirable EM waves from passing through. In communications and radio-frequency (RF) engineering, waveguide filters are used to propagate or attenuate electromagnetic (EM) signal for broadcasting, mobile communications, radar, remote sensing, surveillance, etc.

How It Works?

The EM waves in a waveguide (metal-tube) can be visualized as traversing down the “guide” in a “zigzag” pattern via repeated reflection between two opposing walls of the guide. It is fascinating that by just altering the dimension of the internal hollow area of the conductor tube (usually the width), it is possible to transmit different signal frequencies from one end to the other.

Irises and Cavities in a waveguide filter. Source:

Since a waveguide structure only consists of a single conductor, it can only propagate two different modes i.e., TE (transverse electric) mode and TM (transverse magnetic) mode. Waveguide filter can be in any shape but typically rectangular and circular shapes are in favour. Inside the hollow region of the waveguide, there are several obstructions (depending on the filter order) known as waveguide irises.

Waveguide irises provide capacitance or inductance throughout the waveguide structure. By altering the obstruction region, capacitive and inductive elements can be introduced inside the structure. The capacitive element can be introduced by placing the iris at the edge of the broad wall while placing an iris at the edge of the narrow wall will introduce the inductive element. Without these structures, a waveguide is just a mere piece of hollow scrap metal.

The irises divide the waveguide structure into several smaller compartments or cavities known as resonators. A resonator will oscillate with greater amplitude at certain frequencies (called resonant frequencies) than at other frequencies. This phenomena allows a resonator to isolate frequencies from the EM waves to pass through. Another interesting point about the waveguide filter is that it is possible to excite multiple modes at the same time so that multiple resonators can be introduced in a single waveguide cavity.

Advantages of Waveguide Filter

The size of the waveguide is proportional to the wavelength of the signal and thus, the smaller the operating frequency, the larger the waveguide structure. Therefore, waveguides are seldom employed in low-frequency applications and will only be used in low-frequency applications that required low energy loss or high-powered applications or both. As the size and weight of waveguide filters decrease with increasing frequency, waveguide filters are exceptionally useful when employed in high-frequency applications (typically > 3 GHz) such as radars and satellite communications. This is even more important because most of the high frequency applications demands miniaturization of components to fit into size-sensitive equipment.

Besides that, waveguide filters are an alluring option for performance sensitive requirements as the filters have very low loss and can handle high power. With the rapid deployment of 5G base stations, waveguide filters will play an integral role in communication systems and it is predicted that more waveguide filters will be employed in the foreseeable future. Various subtractive manufacturing (CNC machining) and additive manufacturing (3D printing) have been utilized to produce waveguide filters.

One thing is for sure. We will definitely be seeing more waveguide filters in the future in the telecommunication industry.

Footnote: If you are one of those minority that is in desperate need for a good filter to meet certain specifications, don’t worry we are here to help. At FILPAL, we provide a complete customised RF filters solution from the design to the hardware fabrication to the measurement of the filters. We have multiple experiences in filter designs and had provided multiple filters to several industries such as Keysight, Telekom, Petronas, Stride, etc. Please visit our website FILPAL for information on our filters and feel free to drop a message for any inquiry.