I have a cheap SDR receiver and today I was trying to measure the noise in the system. I tried to disconnect the antenna and measure the power over the frequency range of the device. I also decided to take samples varying the gain from low to high at each frequency step.
After testing different (unsuccessful) attempts of RF shielding, I thought it would be interesting to hook different antennas and see their effect. I was quite surprised to learn that the power dropped drastically around some frequencies when I hooked an antenna for low gains, and raised a lot for high gains.
My question is what is causing the power to decrease when an antenna is plugged in?
Here are some plots of my measurements. The different curves represent different Gains. The units of the legend are dB. All plots are at the same scale.