Archive for September, 2009
The Off Center Fed Dipole – by Dan, N5AR
The Off Center Fed Dipole – how they work
Several companies sell a multiband dipole. One is called The Carolina Windom and another is called the Buckmaster OCF Dipole.
These are basically similar but vary in small details.
1. A conventional center fed dipole has the lowest feed point impedance at the center, 50-75 ohms depending on height above ground. If you attempt to feed it at some frequency other than resonance this will be higher, as much as several thousand ohms in the worst case. Obviously this is a terrible mismatch for the 50 ohm coax feedline.
2. It is possible to feed the ½ wave dipole off center and the feed point impedance will be higher. If you feed at the point where the impedance is around 200 ohms an interesting thing is that the impedance for several different bands is actually is in the area of 200 ohms creating the possibility of multiband operation.
3. If the antenna height is varied and possibly the antenna is tilted or bent at one or more points the antenna can be tweaked to improve the SWR in several bands.
4. If you now connect a 200 to 50 ohm wide band transformer at the feed point you have created an antenna that is somewhat near a 50 ohm match on several bands.
5. You have now solved the multiband matching problem if you copy the exact configuration and height which was determined by trial and error by the designer and if your ground characteristics are the same and there are no nearby towers or antennas.
6. How effective is this antenna? First, it is a cloud warmer with a high angle of radiation due to the low height. Second, the coax shield will have some current on the outside due to the fact that the feedline has unequal induced currents from the unequal antenna half lengths. It will have a bit of low angle radiation due to the radiation from the vertical feedline and possible problems with RF in your shack. A good sleeve balun at the ground end of your feedline will help the shack RF problem but will not help the radiating feedline.
7. I would consider a fan dipole myself instead of one of these.
73,
Dan, N5AR
