Thursday, March 3, 2011

Off-Center-Fed (OCF) dipole for 3.5, 7 and 14 MHz


Fig  shows the off-center-fed or OCF dipole. It is not necessary to feed a dipole antenna at its center, although doing so will allow it to be operated with a relatively low feed-point impedance on its fundamental and odd harmonics. (For example, a 7-MHz center-fed half-wave dipole can also be used for 21-MHz operation.) By contrast, the OCF dipole of Fig, fed  1/3 of its length from one end, may be used on its fundamental and even harmonics. Its free-space antenna-terminal impedance at 3.5, 7 and 14 MHz is on the order of 150 to 200 Ω. A 1:4 step-up transformer at the feed point should offer a reasonably good match to 50 or 75-Ω line, although some commercially made OCF dipoles use a 1:6 transformer.   At the 6th harmonic, 21 MHz, the antenna is three wavelengths long and fed at a voltage loop (maximum), instead of a current loop. The feed-point impedance at this frequency is high, a few thousand ohms, so the antenna is unsuitable for use on this band.

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