Editing Electrical length (section)

=== Electrical lengthening and shortening ===
In many circumstances for practical reasons it is inconvenient or impossible to use an antenna of resonant length.  An antenna of nonresonant length at the operating frequency can be made resonant by adding a [[electrical reactance|reactance]], a [[capacitance]] or [[inductance]], either in the antenna itself or in a [[matching network]] between the antenna and its [[feedline]].<ref name="ARRL" />  A nonresonant antenna appears at its feedpoint electrically equivalent to a [[electrical resistance|resistance]] in series with a reactance.  Adding an equal but opposite type of reactance in series with the feedline will cancel the antenna's reactance; the combination of the antenna and reactance will act as a series [[resonant circuit]], so at its operating frequency its input impedance will be purely resistive, allowing it to be fed power efficiently at a low [[standing wave ratio|SWR]] without reflections.

In a common application, an antenna which is ''electrically short'', shorter than its fundamental resonant length, a monopole antenna with an electrical length shorter than a quarter-wavelength (<math>\lambda/4</math>), or a dipole antenna shorter than a half-wavelength (<math>\lambda/2</math>) will have [[capacitive reactance]].  Adding an [[inductor]] (coil of wire), called a [[loading coil]], at the feedpoint in series with the antenna, with [[inductive reactance]] equal to the antenna's capacitive reactance at the operating frequency, will cancel the capacitance of the antenna, so the combination of the antenna and coil will be resonant at the operating frequency.  Since adding inductance is equivalent to increasing the electrical length, this technique is called '''electrically lengthening''' the antenna.  This is the usual technique for matching an electrically short transmitting antenna to its feedline, so it can be fed power efficiently.  However, an electrically short antenna that has been loaded in this way still has the same [[radiation pattern]]; it does not radiate as much power, and therefore has lower [[antenna gain|gain]] than a full-sized antenna.

Conversely, an antenna longer than resonant length at its operating frequency, such as a monopole longer than a quarter wavelength but shorter than a half wavelength, will have [[inductive reactance]]. This can be cancelled by adding a [[capacitor]] of equal but opposite reactance at the feed point to make the antenna resonant.  This is called '''electrically shortening''' the antenna.