Why is there a central spike in frequency plots from many software defined radios?

From Ted Cline 9/9/2025:

The spike at frequency zero is just a consequence of the fact that your signal does not have an average of zero.

The value at “frequency zero” is the constant DC component of the input.

Interesting history, a gift from 2 guys who were bored at a conference meeting.

IBM could not patent it so it is free, and changed the world, since only 1965 (with the help of digital computers).

I was taught the FFT method about 1977, and I only later realized the excitement was because it was so new, and solved so many problems.