Success!  After last weeks failed field test I took the new code out on the ground unit and tried again.  The SD card failed again (suspect bad card ) but my new code allowed the system to keep working as display only YAY!

Note: rolling shutter effect blocks some of the text at any given photo. 

I needed a place where I could guarantee some test subjects but they needed to be far enough away to simulate a lost person- 

Enter the beach where I live which even on a lousy windy day had a few dozen tourists enjoying Lake Huron :)  

I pointed the Yagi out and immediately the hits started climbing.  

In a few minutes I hit 98 clients and several APs detected.  Of note- iphones spam new/fake MACs so this number is likely high per the subject count but that is just fine- we only need to detect them.  And detect them it DOES! 

At this distance I would say this is wonderful! 

Video: https://photos.app.goo.gl/bdPG9w2YTUGLhg2e9

I have an LNA (low noise amplifier) I could add as a pre-amp from the yagi but according to this result- it is definitely not needed.  

For simplicity of build and keeping the BOM to a minimum I will leave it out for now and design a new ground enclosure and battery system in the next few weeks.  I also want to see if I can fix up the code to start a new file if the old .csv is corrupted somehow just in case it is not hardware glitches on the SPI line after all. 

Thanks to the beach-goers for contributing to future search and rescue success :)