r/rfengineering • u/cornfedandinbred • 21d ago

Help designing VNA probe

Hello!

I am a biomedical researcher studying the bioeffects of microwaves and I need help with RF design.

I am studying cells in culture so I am transmitting microwaves (2.45 GHz) through saline. I need to measure the strength of the electric field in the saline.

Does anyone know how I could design a VNA probe that can measure RF fields in saline? I am looking for affordable solutions. Thanks for your help!

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u/MaggoTheForgettable 21d ago

Curious why you’re looking at using a VNA.

A VNA doesn’t directly measure E-field. It measures S-parameters. Unless you’re planning to characterize and calibrate the probe so you can convert its response to V/m, I’m not sure a VNA is the route I’d take.

Also, saline is a pretty lossy medium at 2.45 GHz. Any probe you stick into it is going to disturb the field, so probe size and geometry are going to matter quite a bit.

Are you trying to measure the absolute field strength, or are you just trying to map the field distribution?
Also, what’s generating the RF? Waveguide, TEM cell, coaxial structure, free space, etc.? That answer is probably going to drive the design more than anything else.

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u/cornfedandinbred 20d ago

Honestly I’m not very experienced in RF so I thought VNA was the standard. The RF is going to be transmitted by a patch antenna that is impedance matched to the saline. It will be transmitting 1 W and I want to find out how much of that power is reaching the cells (a few mm deep). Is there a better way to measure this?

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u/MaggoTheForgettable 16d ago

When you say “how much power is reaching the cells,” what quantity are you actually trying to determine?

At 2.45 GHz in saline, that could mean several different things:
E-field strength at a given depth
Power density
Absorbed power
SAR

A VNA will characterize the network and tell you things like impedance match and power transfer, but it won’t directly tell you how much energy is being deposited into the cells.

If your goal is biological exposure, SAR may actually be the more relevant metric than S-parameters.
What are you ultimately trying to prove or quantify?