Need DIY method for RF calibration

[Conrad Hoffman]

Is there any sneaky DIY way to establish field strength or calibrate a meter (if I had one) at around 100 MHz? Is there a reciprocity method that's practical? Thanks!

[Conrad Hoffman]

Man, 150 views and no replies. This isn't looking good. Maybe I need to start building vacuum thermocouples or thermistors?

[tggzzz]

Part of the problem is that you know more about metrology than most people on the forum

A search for "electric field probe" throws up things like
https://ieeexplore.ieee.org/document/1143126 Electric field probes--A review
https://www.theemcshop.com/emc-test-equipment/rf-field-strength-probes-meters/electric-e-field-probes-up-to-40-ghz/

[Conrad Hoffman]

I know less than you think and negative zero about RF stuff. I wish that the old Linear Technology LT1088 was still made- it went out to 300 MHz.

[fourfathom]

You might want to look at what my friend Glenn Elmore (N6GN) is doing with calibrated short broadband antennas (a field probe).  Don't know if this comes close to what you're looking for, but Glenn knows his stuff...

http://www.sonic.net/~n6gn/OSHW/FP/FP.html

[iMo]

Are you talking RF voltages or RF fields ?? - that is a big difference.

With RF voltages I was/been using the AD8307 which is a 0-500MHz logarithmic amplifier - you get dB on 50ohm for example with pretty good numbers out of it easily (you get a DC voltage out of it which is dB value after a simple math conversion).
BTW I built myself a thermocouple before I got the AD8307 (there are newer chips, sure).

Fields - that is quite difficult as you would need a calibrated antenna, or something like that, and it depends on polarization, distance, surrounding objects geometry, etc. You would also need a Faraday cage with a special ferrite material covering the walls, etc..
There are the EMC labs which mess with it and such a lab would cost you beefy 6 digits USD.

The AD8307 is the first logarithmic amplifier made available in
an 8-lead SOIC_N package. It is a complete 500 MHz monolithic
demodulating logarithmic amplifier based on the progressive
compression (successive detection) technique, providing a
dynamic range of 92 dB to ±3 dB law conformance and 88 dB
to a tight ±1 dB error bound at all frequencies up to 100 MHz.

[Conrad Hoffman]

Thanks, very interested in what Glenn is doing. I'm interested in both voltages and fields and the 8307 is also interesting. Yes, I know how much EMC labs spend on facilities but hobbyists have to be clever and beat the system! Six figures is five figures out of budget.

[mawyatt]

For small uses Dr Steve Cripps developed some clever probing techniques and devices.

https://www.semanticscholar.org/paper/A-novel-microwave-non-contact-current-probe-with-Al-Ziayree-Cripps/5ae8aabc3cda2813d77f829d2e69c0e1d7b1a7d9

Best

[iMo]

FYI - my 8307 contraption made around y2k..

[Conrad Hoffman]

I understand inductance and loops but I still have trouble getting used to cramming things together as tightly as needed. 😐

[ElizatronicWarfare]

As previously mentioned you will need a calibrated antenna and shielded area for tight results, but it may not be necessary depending on your needs - Australian airports get their field strength meters calibrated without the use of a shielded room, just a loop antenna that is set on a table a fixed distance from the DUT. The resulting figures are used in some arcane way by ground crew engineers to determine if the meter is fit for use. All the points with limits are done using a direct coaxial connection to an auxiliary input.

[tggzzz]

I understand inductance and loops but I still have trouble getting used to cramming things together as tightly as needed. 😐

Frequency-dependent losses (in PCB tracks) and resonances of various kinds.

If you haven't already, have a look a microwave filters and other components (e.g. the rat race). You aren't in Kansas anymore - for a start "ground" becomes a nebulous concept

[ingkarlgauss]

Hi, there are several methods, depends on the required uncertainty in dB.

For simple stuff, calibrated horn antennas pairs and simple RF math (yes in the 100MHz there are ones) are a easy DIY solution

A little higher in the food chain are TEM cells and GTEM cells with sensors on the end and reference generators on the input side

For more advanced things, field sensors like this https://www.montena.com/en/products/measurement-tools/derivative-sensors-and-integrators/b-dot-and-d-dot-field-sensors, work very good

Even more advanced (non-metallic) all-fiber sensors exist from company Kapteos in France.

There are even more advanced stuff for that on my domain (with green uniforms) that help calibrate fields, but dont ask please.

Best