Monday, May 6, 2019

The off centre fed (OCF) dipole

Most amateurs know about the half wavelength dipole. Two wires, each a quarter wavelength long, connected to a coaxial cable feedline in the middle. It's an efficient no-fuss antenna that provides good performance on a single band.  Variations include bending the wire elements down to form an inverted-vee (so you only need a single high support), adding a 1:1 balun or adding open wire feedline to allow operation on multiple bands. 

What happens when you move the feedpoint away from the centre towards one end? The answer is that the impedance increases. When you take it to its extreme you get an end-fed half wavelength wire with impedance in the thousands of ohm. At an intermediate point, about 1/3 the way along the wire, or 1/6 wavelength from the centre, you might get an impedance of 200 or 300 ohm. If you want to retain coax feed then you'll need to transform the impedance presented. That is typically done with a balun. 4:1 or 6:1 baluns are quite commonly used for this purpose. 

An attractive characteristic of this arrangement is that not only is the antenna suitable on the frequency that it's a half wavelength for but also even multiples. For example a 3.5 MHz OCF can work on 7 and 14 MHz. That's handy if you want to cover several popular bands from just the one wire.  

Want to learn more? Here is some more reading on OCF dipoles:

* DJ0IP OCF dipole
* K8JHR OCF dipole (pdf)
* W8JI on OCF dipoles
* VK2DQ OCF dipole (pdf)

Note that you will sometimes see reference to the 'Windom'. From a distance it looks the same as an OCF dipole. However it uses only a single wire feedline. That was suitable for the transmitters of the time where the wire could be tapped onto the output coil. Windoms now are rarely if ever used and what some people call a windom is likely to be an OCF dipole.

PS: Want to read more about antennas and other amateur radio topics? These books could be of interest. Available in both electronic and paperback.

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