Editing Superheterodyne receiver (section)

===Local oscillator radiation===
{{further|Electromagnetic compatibility}}

It can be difficult to keep stray radiation from the local oscillator below the level that a nearby receiver can detect. If the receiver's local oscillator can reach the antenna it will act as a low-power [[Continuous wave|CW]] transmitter. Consequently, what is meant to be a receiver can itself create radio interference.

In intelligence operations, local oscillator radiation gives a means to detect a covert receiver and its operating frequency. The method was used by MI5 during [[Operation RAFTER]].<ref name="Wright_1987"/> This same technique is also used in [[radar detector detector]]s used by traffic police in jurisdictions where radar detectors are illegal.

Local oscillator radiation is most prominent in receivers in which the antenna signal is connected directly to the mixer (which itself receives the local oscillator signal) rather than from receivers in which an RF amplifier stage is used in between. Thus it is more of a problem with inexpensive receivers and with receivers at such high frequencies (especially microwave) where RF amplifying stages are difficult to implement.