This design is based on calculations from a program created by Walter Lau KK1CW and is available with his permission on our website in the Support Downloads tab under Antenna Design Links. Most people still believe a Windom MUST be fed at the 13 – 23 point.

The Off Center Fed Dipole Revisited A Broadband Multiband Antenna

### K1POO 4 Band Off Centre Fed Dipole 40-20-15-10m.

**Ocf dipole design**. The conventional Guanella design modified by Sevick is the conventional choice but I always found common mode problems even when using type FT140-43 toroids. The OCF or Off Center Fed Dipole antenna is much like a conventional dipole antenna in that the formula you use to calculate the length of it is the same. This is the length of a standard dipole but instead of dividing the result in half use a 3664 offset to determine the feedpoint.

The picture on the right above shows an OCF Dipole with the feedpoint positioned at the same feedpint of the classical WINDOM antenna fed with 50 Ohm coax through a 41 or 61 balun at a point 13 of the distance from one end. It works on 40 20 10 and 6 meters. Does anyone have any ideas.

After its initial publication on the Internet in 2007 the CL-OCFD was subsequently described in the following publications. I have put up many dipoles windoms and skyloops over the years in different locations all home brew. The Ruthroff type voltage balun with a 11 in series was better but I wanted even better common mode without using bigger toroids.

I Use an OCF dipole with a 41 balun and find it works. For an 80-10 meter OCF dipole antenna divide the 12 wavelength standard of 468 by the lowest operating frequency of 36 MHz and you arrive at an overall length of 130. Design Parameters For an 80-10 meter OCF dipole antenna start with a total length of 13585 feet of insulated wire.

DJ2KY modified the VS1AA antenna design by replacing the single feed wire with balun and a coax feeder. We shall see how many choose to either use a 41 or 61 balun for an OCF dipole. An example this is the 63 Diamond W8010 80-10 meter trap dipole.

For this design we look at a resonant dipole for the lowest band desired and feed it off center with a balun. Multiband HF antennas part 3 Windom and OCF dipole. The antenna was described in QST and a commercial version was available at time of writing.

This is the length of a standard dipole but instead of dividing the result in half use a 3664 offset to determine the feedpoint. An OCF at its fundamental frequency performs similarly to a center-fed dipole of the same length. I have horrific RF feedback in the shack on 40 meters.

Centre- fed dipole in free space has a theoretical radiation resistance of 732 ohms However practical measurements have shown that this value is not a constant. The rationale for the design is that it is a short dipole not requiring. Given your length requirement an 80-10 meter 80 40 20 15 10 solution is a trap dipole.

The classical feedpoint of the windom will give you the best SWR curves for a few of the bands but if you are willing to accept a slightly higher SWR it is possible to cover additional bands. An example of a 40-6 meter OCF antenna is the Buckmaster BMT-DX-OCF-40. 468 Frequency Mhz will give you the overall length of the antenna.

In fact the only difficult part of an OCF dipole is the balun and I will be explaining how the balun works and also how you can make your own. W2AU Balun in place. The modern common Windom antenna is not an original single-wire fed Windom but rather an off-center-fed or OCF two wire feed dipole.

Design Parameters For an 80-10 meter OCF dipole antenna divide the ½ wavelength standard of 468 by the lowest operating frequency of 36 MHz and you arrive at an overall length of 130. The 7 band OCF Dipole antenna was erected at 25 to 30 feet on a hill side using a military style push up mast with a few customization on it. An antenna that will operate well on all Ham bands with minimal assist from the tuners included in many modern transceivers.

There seems to be some minor variances of this particular design too a foot or to longershorter etc. Here are some analysis results for two forms of an HF multi-band dipole antenna for 160 m and up. The short end was placed at about 40 feet off the ground tied to a tree limb using about 10 feet of insulated rope.

The graph on the right from Antenna Theory Design Vol 2 by HP Williams published by Pitman shows this variation for both vertical and horizontal dipole radiators. The key to obtaining more bands from any Windom or OCF is the positioning of the feedpoint. A commercial antenna system the Buckmaster 7-band OCF Dipole exploits this property.

Off Center Fed Dipole Notes. In other words an 80-meter Windom 137 feet long would have one leg 45667 feet long and the other leg 91333 feet long. 160-6 meter Double Off Center Fed antenna layout using a double tuned stub for better 2040 meter performance Here is the 160-6 meter SWR graph using 200 feet of RG213 feed line and an average height above ground of 35 feet.

But this time was a fiasco. The normal Windom is described as having a 13 length leg and a 23 length leg. The OCF dipole does require a balun.

Here are some typical OCF dipoles that are successful designs. The half-size version of the FD4 is known as the FD3 402010m and is the subject of this article. The picture above on the left shows the classical Dipole which is usually fed with 50 Ohm coax through a 11 balun.

The 220 ft W8JI multi-band dipole is based on the design guidelines presented in this reference on page 7. There seems to be 2 versions of a 10-80 meter OCF dipole floating around. A center-fed half-wavelength wire antenna cut for 715 MHz about 673 long has a maximum gain of about 76 dBi at 70 up with an elevation angle of maximum radiation of 27 degrees above the flat uncluttered terrain assumed by NEC programs.

It is 68 long with leg lengths of 23 and 45 ft. For a conventional antenna I would normally split this wire in half and place a feed point in the center. The center-loaded off-center-fed dipole is the original invention and work of Serge Stroobandt ON4AA.

I just put up an OCF dipole using an excellent 41 balun the OCF Dipoles 15 54MHz 5kW 4115ocf from Balun Designs. This became known as the Fritzel FD4 80 402010m but is more correctly described as the Off Center Fed Dipole OCFD. The big advantage of this design is that you can make an antenna that does not require an antenna tuner by utilizing the constant impedance of the happy convergence that happens at.

The OCF short vertical dipole for HF has become popular particularly disguised as a flag pole for low impact installations and encouraged by claims of outstanding performance. One of them is built from a 135 ft dipole with one leg being 45 ft and the other 90 ft. The off center fed OCF dipole analyzed here is based on a commercial product with total length of 270 ft with one leg of 180 ft and the other of 90 ft.

Most commercial versions like the Buckmaster seem to follow this design.

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