A while ago I was using a software radio receiver to perform some antenna analysis. I left the spectrum display on while disconnecting the cable and noticed that when the cable was disconnected I could see a very large spike in the frequency component at the center frequency of the display (where the receiver was tuned). I changed the center frequency several times and noticed the same spike appeared regardless of where I tuned.
Some time later I reconnected a cable and accidentally tuned to a frequency that was completely "out of spec" for the antenna I was using. The same spike appeared at the center of the display; I tuned around to several nearby frequencies and found that the spike stayed put. It disappeared when I moved back to a frequency at which the antenna was designed for operation.
What is the nature of that spike and why does it appear when tuned to a frequency at which the antenna is not resonant, as well as when the antenna is disconnected?
EDIT: Here are some screenshots of the spike I'm referring to:
This is an FFT of the received spectrum through an antenna tuned for the 2M band. Everything is nice and flat.
This is an FFT of the spectrum through the same antenna but at a completely unworkable operating frequency. The middle is a bit noisy but still shows the spike at the center frequency well.
Here's the FFT at the same frequency, but with no antenna connected. The spike is clearly visible here, and this is what I'm interesting in learning about.