As I teach my college students: there is NO difference between Audio Frequency AC math and so called "Microwave" math. It all about the circuit models. Wavelength is the issue for what is important. In low frequency electronics, component size is extremely small compared to a wavelength - we ignore parasitic elements because their magnitude is much smaller than the lumped element value of the component so we use simple lumped models and ignore parasitics. At 10 kHz, who cares if the resistor leads have 10 nH of inductance.
However, at 10 GHz that same inductance represents a reactance of 62.8 ohms, significant if you are in a 50 ohm system. Its the argument of "Lumped" vs Distributed" that defines what is important at the frequency of operation. Not arbitrary designations. After I spent time in the MMW world working on systems for Uncle Sam, I would say "Everything below 1GHz is like DC, everything below 20 GHz is IF, and we get serious at 100 GHz". A lot of this thinking reflects availability of parts, the "make vs. buy" decision.
We have the Decimeter spectrum, the Centimeter Spectrum and the Millimeter - nice clear cut decades that define frequency ranges. Where do you think the terms UHF, SHF and EHF came from (look at the military designations). What can we ignore and whats important?
We when pass thru the arbitrary boundary of "Lumped vs Distributed" we leave the realm of simple lumped element circuit models that use voltages and currents to define the behaviors of our circuit elements and we enter the world of transmission line field concepts where we no longer look at I & V as carrying the "power", instead we consider field concepts like the Poynting vector and Maxwell-Heavyside mathematics. The fusion of math at this boundary is that of the EE and physicist - they have to provide the same (similar) answers. When lumped element thinking no longer adequately describes the behavior of your circuit elements, you have to go to alternative distributed field theory concepts (harder math) which, by the way, always works independent of frequency, albeit more cumbersome to solve.
So the frequency that was considered "FBM" over the years changed as our measurement technology changed. In my engineering days 1 GHz "sort of" was where microwaves started (3 GHz was technically the boundary for centimeter waves), and 30 GHz was clearly MMW. Waveguide would work fine at 100 MHz but would be too costly. However, I saw waveguide at 430 MHz for the planetary radar at Arecibo, when I visited in 1986, so, no, waveguide sizes vs frequency are not the defining issue for use, cost & simplicity are.
What was the argument about anyway? Sort of detoured from VNA's. Was it who defines what frequency range as magic vs. plebian ("microwave vs RF")? Depends on what the work is and who is doing the speaking. I have heard UHF TV broadcast engineers "pooh-poohing" AM Radio station engineers "You dont know how tough it is...".
Jeff Kruth
