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I was wondering what happens when you directly electrically couple the transmitter antenna to the receiver antenna? Particularly I’d like to know what happens for FM radio broadcast.

Since I don’t have an FM transmitter I cannot try this at home.

My intuitive guess is that the receiver will not get the intended signal. Is this correct? Why?

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If your transmitter emits enough power to be heard more than a few meters away, the likely result will be destruction of components in the receiver and possibly in the transmitter as well.

Receivers are designed to amplify, detect, and decode signals in the range of a few to a few tens of microvolts; transmitters as weak as a few watts will put tens of volts of RF onto their antenna. The latter signal, applied to the receiver's components, will at a minimum overpower and desensitize the receiver, resulting in no reception at all. It's very likely that this level of power will damage components (transistors or integrated circuits) in the receiver RF amplifier or detector stages.

Further, the receiver (at least after components start to burn) will appear as a grossly mismatched impedance to the transmitter, and the reflected power from that mismatch might well damage the final amplifier stage in the transmitter (burn out transistors, pop capacitors, etc.).

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    $\begingroup$ Um, your GSM phone's receiver can pick up things at -120 dBm. Assuming 50Ω line impedance, that'd be 5·10⁻¹⁴ V, or 50 femtovolt. That's 50 billionths of a microvolt, not a few microvolts :) I think your receiver – although possibly not as sensitive as the GSM spec would demand if it was a GSM phone – is probably a bit more sensitive than you admit here :) $\endgroup$
    – Marcus Müller
    Jul 30, 2019 at 21:55
  • $\begingroup$ @MarcusMüller -120 dBm is about 7 uV in 50 Ohms. $\endgroup$
    – tomnexus
    Jul 31, 2019 at 10:01
  • $\begingroup$ hm, let me recalculate that, @tomnexus: I was wrong, but with P= U²/R I get a little less than a µV. $\endgroup$
    – Marcus Müller
    Jul 31, 2019 at 10:52
  • $\begingroup$ Common BCB superhet receivers are commonly listed as between 35 and 50 microvolts sensitivity. $\endgroup$
    – Zeiss Ikon
    Jul 31, 2019 at 11:23
  • $\begingroup$ @MarcusMüller ah yes. -120 dBW is 7 uv, -120 dBm is 0.2 uV. Also not thinking. $\endgroup$
    – tomnexus
    Jul 31, 2019 at 13:25

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