What are the main trade-offs of RF microstrip vs waveguide in terms of cost, size, and performance?

In a mm waves materials usually used for microstrip are quite expensive. On the other hand there are technologies that make waveguide cheap in mass production. If your lines are long you should consider waveguides and a price difference will not be high .

When going for higher frequencies like mmwaves, waveguide has a clear advantage due to 0 dielectric loss, but one should always think about the transition from waveguide structure to PCB where all components are placed. This transition can be very costly, bulky and lossy.

RF microstrip, with its thin and flat structure, is often favored in applications where lower frequencies are prevalent. Its adaptability and smaller size make it suitable for these scenarios. Conversely, RF waveguide, despite its bulkier form, excels at higher frequencies, providing efficient signal transmission within that range but necessitates more meticulous planning for clearance and alignment to counter interference and reflection issues.

Yes waveguide are bulky and as Katerina noticed transitions can be lossy. But it should be noted that lots of IC manufacturers in mmwaves starting to integrate the transitions into IC eliminating the size consideration ,PCB material cost,losses ..and showing their preference of waveguides.

The points made above about sums it up. In my experience, at higher frequencies, waveguide filters have losses less than 1 dB, higher Q, and can be tuned for the design frequency, however, connector termination adds to the size problem. PCB Microstrip using PTFE dielectric may have an additional 1 to 3 dB of loss but is easily manufactured. One performance note using microstrip is the use of good grounding techniques and the PCB soldered/mounted onto a rigid surface. Depending on the application, I have used both methods in the same project.

It is important to note that within the frequency ranges where both waveguides and microstrip lines are utilized, the bandwidth of a microstrip line is generally larger than that of a waveguide. For instance, a WR-90 waveguide only covers the X-band (8-12 GHz), whereas a microstrip transmission line can, in practice, cover frequencies from DC to tens of GHz, depending on the geometry and substrate used. Higher-order modes, including TM and TE surface wave modes and waveguide-type modes in the line's cross-section, limit a microstrip line's bandwidth. In most practical applications, the dielectric substrate is electrically thin, leading to high cut-off frequencies for these higher-order modes.

With me, it’s all about performance. I’m a former Navy Precision Approach and Landing Systems Tech/supervisor. It’s about radar performance. When I left in ‘93, we used magnetrons, klystrons, and amplitrons for the final. We’re talking up to 1, to 5 Mwatts. Every dB matters, and counts. It’s even more critical on the receive side as the transmission line feeds minute signals to the receiver for interpretation.