EM wave and electrical signal relation

Vilius_Zalenas

Joined Jul 24, 2022
27
Hello,

I am a second year electronic engineering student just starting my way in the industry. This time I will ask for theory clarification that simply is missing to make the ends meet in my understanding ofRF physics.

Last semester I did a deep dive into RF world at university. I was learning basics about transmission lines, waveguides, antennas etc. Lets say we have a microstrip type transmission line (50 ohms). In order to match the impedance, we have to connect both conductors with a 50 ohm resistor (standard impedance matching procedure.) On the other hand, the EM wave can only propagate (without immense attenuation) in the dielectric material (In this case, lets say its a standard 1.6mm thickness FR-4 PCB). So my question is, how these two laws are related? I mean from my point of understanding, that 50 ohm resistor is just a physical component, how can it affect the EM wave propagation in the dielectric?

I have another side question which is very closely related to the first part. I did a laboratory work with a VNA at the university. I was measuring S parameters of the the antenna amplifier circuit. I do not understand how the VNA does signal acquisition. At some point that instrument has to threat the signal as an electrical one in order to do all the ADC, signal processing and other difficult processes, but the VNA is acquiring S parameter signal in the EM wave form. So my question is what is the principal of EM wave to electrical signal conversion?

Like I mentioned, I am a complete beginner in the RF world and I wrote this based on assumptions coming from my understanding. If I am completely wrong, please explain to me, I will be very grateful. I just feel like I can not continue learning without understanding this and I have not yet found a good enough book to explain all this. I thank you in advance.

Vilius

Papabravo

Joined Feb 24, 2006
19,578
As to the first question: when you look at the PDE (Partial Differential Equation) for a transmission line you notice the importance of the boundary conditions. When you encounter an impedance discontinuity a couple of things happen. Part of the wave is transmitted through the discontinuity and part of the wave is reflected. The condition where the reflected wave has a negligible magnitude is when the source impedance of the source matches the characteristic impedance of the transmission line, or the characteristic impedance of the transmission line matches the load impedance. This is very easy to model with LTspice, where you can see the effects of stepping the various impedances and measuring the magnitudes of the transmitted and reflected waveforms.

As to the second question: there are circuit configurations called directional couplers and power dividers that facilitate the sampling of RF signals for measurement and characterizations. See the following for more details:

https://en.wikipedia.org/wiki/Wilkinson_power_divider
https://en.wikipedia.org/wiki/Power_dividers_and_directional_couplers

nsaspook

Joined Aug 27, 2009
10,683
A short video with IMO one of the best visuals of boundary discontinuities and their effects.

Another great product of the Cold War space-race.