Comparing Coax and Waveguide

Blog Post created by benz on Sep 15, 2016

Originally posted Jun 20, 2014

Making the choice for microwave and millimeter connections

I haven’t used waveguide very much, but it’s been an interesting technology to me for many years. I always enjoyed mechanical engineering and I’ve done my share of plumbing—everything from water to oil to milk—so waveguide engages my curiosity in multiple domains.

Coaxial cables and connectors are now readily available at frequencies to 110 GHz and at first glance they seem so much easier and simpler than waveguide. I wondered why waveguide is still in use at these frequencies, so a couple of years ago, while writing an application note, I spoke to electrical and mechanical engineers to understand the choices and tradeoffs.

It’s perhaps no surprise that there are both electrical and mechanical factors involved in the connection decision. At microwave frequencies, and especially in the millimeter range and above, electrical and mechanical characteristics do an intricate dance. Understanding how they intertwine is essential to making better measurements.

Coaxial connections: flexible and convenient. Direct wiring, in its coaxial incarnation, is the obvious choice wherever it can do the job acceptably well. The advances of the past several decades in connectors, cables and manufacturing techniques have provided a wide range of choices at reasonable cost. Coax is available at different price/performance points from metrology-grade to production-quality, and flexibility varies from extreme to semi-rigid. While the cost is significant, especially for precision coaxial hardware, it is generally less expensive than waveguide.

Coax can be an elegant and efficient solution when device connections require some kind of power or bias, such as probing and component test. A single cable can perform multiple functions, and the technique of frequency multiplexing can allow coax to carry multiple signals, including signals moving in different directions. For example, Agilent’s M1970 waveguide harmonic mixers use a single coaxial connection to carry an LO signal from a signal analyzer to an external mixer and to carry the IF output of the mixer back to the analyzer.

All is not lost for waveguide. Indeed, loss is an important reason waveguide may be chosen over coax.

Waveguide: power and performance. Power considerations, both low and high, are often the reasons engineers trade away the flexibility and convenience of coax. In most cases, the loss in waveguide at microwave and millimeter frequencies is significantly less than that for coax, and the difference increases at higher frequencies.

For signal analysis, this lower loss translates to increased sensitivity and potentially better accuracy. Because analyzer sensitivity generally declines with increasing frequency and increasing band or harmonic numbers, the lower loss of waveguide can make a critical difference in some measurements. Also, because available power is increasingly precious and expensive at higher frequencies, the typical cost increment of waveguide may be lessened.

On the subject of power, the lower loss in waveguide comes with high power-handling capability. As occurs with small signals, the benefit increases with increasing frequency.

As you can see from the summary below, other coax/waveguide tradeoffs may factor in your decision.

Comparing the benefits of coaxial and waveguide connections for microwave and millimeter frequency applications.

Comparing the benefits of coaxial and waveguide connections for microwave and millimeter frequency applications.

Mainstream technologies are extending to significantly higher frequencies and I have already wondered if you can push SMA cables and connectors to millimeter frequencies. In some cases, however, the question may be whether cables of any kind are the best solution, and whether it’s time to switch from wiring to plumbing.

Several application notes are available with information on measurements at high frequencies, including Microwave and Millimeter Signal Measurements: Tools and Best Practices.