Thanks for the replies, folks.
I think I will proceed with the open field approach. I have a couple of options nearby. One, a soccer field, should be big enough to have many wavelengths, even at 2m on all sides. However, there is a freeway on one side of the field and the San Francisco Bay on the other, so maybe those are confounders. Another site, a bit inland, it a mostly flat grassy field in a park, surrounded by trees, which can also be a confounding variable.
It looks like I'll need an assistant to do this project (someone to read the meter or key the radio) so it might not happen for awhile. I'll report back.