Good video Calabash.
My thoughts based solely on my use of both coils.
Rehunting sites using round HF coil where LF 9" and or 11" coils were used,,,user of roundup coil,,smart money is to use 28khz as primary detecting freq.
Then use 54khz as a secondary.
Rehunting sites using elliptical HF coil where LF 9 and 11" coils have been,,,can use 14khz or 28khz , but if ground is medium mineralized or higher,,smart money is on 28khz.
Rehunting sites where round HF coil has been, if you use elliptical smart money is on. 28.8 kHz.
Now, both coils, talking of the HF here. Sometimes one can see things the other can't and vice versa.
The shape of elliptical gives it advantage and disadvantage.
Here is a test Calabaah, you should do.
Take a nail, and bury say a coin at7" deep like a dime.
Get yourself a wooden yardstick.
Lay nail over to the side of buried coin.
Use a golf tee for reference of coin location to take measurements.
Use both the elliptical and round HF coil,,to detect coin,,,see how close you can move nail when you use Reactivty 2.5 and just change freqs only.
This test will show some advantage for elliptical coil.
Sweep in direction of coin and nail.
Look for a good two way signal on coin.
This test will show,,,in a sweep of the coil odds are better here for elliptical to hear coin tonally,,with a much closer distance (nail to vertical plane of coin when looking down upon.
Remember though, a person can strike this same coin maybe so if approach angle to coin and nail setup is altered using round HF coil.
But I like to hunt in some sites where the odds are in my favor moreso,,,especially in a site I just get one shot to hunt.
Soil minerals play into proper use of these coils.
Folks watching videos of HF coils, remember this.
What you see in the videos may not be what you in your soil.