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I have built the following AM radio circuit : enter image description here

I was able to pick several radio stations. I fixed the variable capacitor so that I can only hear the 1230 kHz station which I identified from the FCC call letter. I connected the antenna directly to my oscilloscope and obtained the following FFT:enter image description here

This shows all the stations received by the antenna. However, I connected my oscilloscope between the diode and the variable capacitor in reference to the circuit ground and obtained the FFT: enter image description here

How is it possible that the largest peak was not attenuated while small peaks surrounding the station I hear were attenuated even though I can't hear the largest peak frequency? and why is the voltage increased?

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    \$\begingroup\$ looking at the plots I'd say that it's tuned to 950Khz, not 1230 \$\endgroup\$ – Jasen Mar 25 '18 at 5:33
  • \$\begingroup\$ it could be that the largest peak is not an AM radio station. \$\endgroup\$ – Jasen Mar 25 '18 at 5:35
  • \$\begingroup\$ Looks like adding the scope probe re-tuned the antenna circuit from 1230 to 900 kHz. \$\endgroup\$ – Brian Drummond Mar 25 '18 at 10:08
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Try putting the variable capacitor in parallel with the coil and you will better results. Having the variable capacitor in series with the antenna just alters the electrical length of the antenna. If memory serves me correctly (i'm an old guy..its been a while) when the variable capacitor is in parallel with the coil will create a resonant circuit that will allow you capture radio stations.

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