2 Meter Mag-Mount
I had picked up a cheap 2 meter mag mount at a ham fest a few years ago, and used it with an old cookie sheet as a ground plane. I finally decided to break out my NanoVNA to see how well (or badly) it performed!
I thought about trying five different orientations with two matching cookie sheets. The sheets are "WonderBake" brand and well used. They are 13" by 19" with a lip around them. By flipping them upside down, an 11.5" by 17" flat surface can be used.
First I did one cookie sheet with the antenna in the center with the sheet right-side up. This did not perform great, giving an SWR of 4.029 at 160MHz. One theory I have is that the lip moved the tuning up in frequency, as the SWR appears to drop as the frequency goes up.
Next I flipped the cookie sheet upside down and got much better results: 1.797 at 147.400 MHz. This is the configuration I had been using. These results still aren't great, and this is one reason I am looking for a new 2m indoor base antenna.
Next I tried a few combinations with both cookie sheets. The first was the two sheets upside-down pushed together along the long side with the antenna in the middle of the two. This performed a bit better than one, giving a minimum SWR of 1.714 at 146.200 MHz.
Next I pushed the two sheets together along their short side. This performed worse, giving a minimum SWR of 1.967 at 147.700 MHz.
The final setup had the two cookie sheets meeting at their corners. I thought this would perform the best as it would give the largest ground plane under the antenna, but I was wrong. It had a minimum SWR of 1.948 at 146.500 MHz.
The mag mount is not working great as the ground plane is not big enough. These antennas are meant to be on top of a car roof, not a small cookie sheet. At 144Mhz, the wavelength is 82 inches (~2 meters). These cookie sheets are only 19in in the longest dimension, just under 1/4 wavelength. Having a larger ground plane would make this antenna work much better.
I stopped using this antenna when I got a 2 meter loop antenna. You can read more about it here.