Your wife is correct to use a copper stick in the ground, but there are other less expensive ways you could create an antenna, like using a wooden stick instead with copper wire coiled around the outside (as shown with the antenna next to my wall strawberries).
A decent overview of this subject can be found here: https://peakd.com/hive-140635/@samstonehill/electroculture-a-synthesis-of-free-energy-and-agriculture-how-to-grow-massive-vegetables
Have fun with your ongoing investigations and please don't forget to post your results in this group :)