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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.

Power distribution with no antenna Power distribution with small monopole Power distribution with home-made dipole

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  • \$\begingroup\$ What do the different colored lines represent? \$\endgroup\$
    – Bruce Abbott
    Jul 4, 2021 at 6:55
  • \$\begingroup\$ @Bruce Abott different gains, I tried to make it more clear now. Thanks \$\endgroup\$
    – edmeme
    Jul 4, 2021 at 8:32
  • \$\begingroup\$ Replace the antenna with a 50 ohm resistor to see the noise of the device itself. \$\endgroup\$
    – user16324
    Jul 4, 2021 at 11:38
  • \$\begingroup\$ Plot the two different antennas on the same axis, one gainsetting per plot. Those look relatively close, but the plots make it hard to extract the differences between the two. It's possible that your sdr is changing its input impedance as the gain changes, which would change the antenna match as the gain changes. \$\endgroup\$
    – rfdave
    Jul 4, 2021 at 13:07
  • \$\begingroup\$ @user_1818839 thanks! I will try when I'm back home. \$\endgroup\$
    – edmeme
    Jul 4, 2021 at 16:09

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