How can S11 component measurement circuit be made on a pcb?

Question

I have a material whose dielectric properties need to be measured. I have done literature review on the physics & impedance based methods to determine the dielectric component.

It requires S11 to be measured. I can design circuits and PCB's along with embedded system development but dont have a great knowledge about RF, Can anyone suggest on how to proceed further?

I read about VNA's to measure S11, can that be done using a custom miniature circuit for 4.5 GHz (using any relevant IC like AD5933)

Answer 1

1. You don't necessarily need to measure \$S_{11}\$ to determine the dielectric constant of your material.

   You could, for example, make a capacitor out of your material, put it in series or parallel with an inductor of known value, and excite it with a swept-frequency source. Then you'd only need a rectifier or rf detector (basically the same circuit but named differently depending on operating frequency) and a voltmeter (or ADC) to find the resonant frequency.

   This would probably work best in the 1 kHz to 10 MHz frequency range.

2. You could make a dielectric resonator out of your material and again use a swept frequency source, rf detector, and voltmeter to determine the resonant frequency. This would likely work best in the 100 MHz to ~5 GHz frequency range.

3. If you do decide you want to do a reflection measurement, if you don't need phase information (so still not a full \$S_{11}\$ measurement), then you could use a VSWR-meter. This is much simpler than a VNA, but still requires a directional coupler to build one. This means some difficulty if you want the measurement to span an octave or more of frequency. You can google "VSWR-meter schematic" and get numerous design ideas.

4. If you don't mind doing the measurement manually, you can measure \$S_{11}\$ using an airline probe. See my question on VSWR for more on what this looks like. Measuring VSWR give the magnitude of \$S_{11}\$ and the position of the probes to find the minima and maxima gives the phase. The minimum frequency of this measurement is limited because the air line must be at least one quarter wavelength long, and the maximum frequency is limited by the precision with which you can position the probes (and the frequency response of the probes).

5. Another step short of building a VNA is a vector voltmeter. This allows an automated measurement of both gain and phase.

6. If you google, you will find a few published designs for homebrew VNAs, mostly targeted at frequencies used in amateur radio. Basically this means combining multiple vector voltmeters with directional couplers to get completely automated amplitude and phase S-parameter measurements.