A <a href="https://en.wikipedia.org/wiki/waveguide" class="extiw" title="w:waveguide">waveguide</a> <a href="https://en.wikipedia.org/wiki/antenna_feed" class="extiw" title="w:antenna feed">feedline</a> for a typical military <a href="https://en.wikipedia.org/wiki/radar" class="extiw" title="w:radar">radar</a>.   This type is called a keyed-oscillator transmitter.  The <a href="https://en.wikipedia.org/wiki/magnetron" class="extiw" title="w:magnetron">magnetron</a> vacuum tube oscillator <i>(lower left)</i> produces short high power pulses of microwave energy, which are conducted up the waveguide line to the parabolic antenna.  The TR switch is turned off to protect the delicate receiver circuits from the high power output pulse.  Between the pulses the weak radar return from the target is received by the antenna and passes down the waveguide.  The TR switch is turned on, admitting the return signal to the receiving circuit which consists of a mixer diode.  A microwave signal from a <a href="https://en.wikipedia.org/wiki/local_oscillator" class="extiw" title="w:local oscillator">local oscillator</a> is also applied.  The two frequencies mix in the diode and create a lower <a href="https://en.wikipedia.org/wiki/intermediate_frequency" class="extiw" title="w:intermediate frequency">intermediate frequency</a> which is applied to the radar receiver.
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Retrieved July 4, 2014 from <a rel="nofollow" class="external text" href="http://www.rfcafe.com/references/electrical/NEETS-Modules/NEETS-Module-18-2-11-2-20.htm">NEETS  Navy Electricity and Electronics Training Series, published by Naval Education and Training Command, US Navy, 1984, NAVEDTRA 172-18-00-82 Module 18: Radar Principles, p. 2-14, fig. 2-8</a> on <a rel="nofollow" class="external free" href="http://www.rfcafe.com/">http://www.rfcafe.com/</a>