I have a metal pipe sticking out the ground that an old tv antenna was attached to many years ago. How should I go about cutting it off where it doesn't show show up above ground? Pipe is around 4 inches wide, maybe 1/4-1/2 inch thick and filled with concrete up above being level with ground.
Look up "how to cut a lally column" for illustrations, as that's exactly what they are (steel tube full of concrete). Dig a hole, cut the steel with whatever tools you have (hacksaw, grinder, sawzall) hit the top with a hammer and it will snap easily. If you happen to have a large pipe cutter for steel pipe you can use that, but odds are you don't. There are also specialized tools for lally columns - again, you probably don't have convenient access to one.
The essence remains that once the steel is cut the concrete will snap easily.
One option is to remove the entire thing. I recently removed some wooden 4x4" fence posts with large concrete anchors poured around them. To do so, I clamped a couple pieces of lumber to the side of the fence post, then positioned a hydraulic floor jack to lift at the bottom edge of the wood. For a couple of the posts, there was no solid surface to set the jack on next to the post, so I placed a plank of plywood on the ground under the jack base so that it did not press itself into the ground. With the clamps tight enough, this lifts the entire post and anchor from the ground. It leaves a fairly large hole to refill, but in my case I preferred not to leave anything behind.
I made a video: https://youtu.be/nuBrL5Vyauk