OCF dipole for the WARC bands
The antenna described here is of a Windom-type, off-center-fed hence the name OCF.
Windom-s come in different flavour but I thought I would try a different approach for a
multiband operation. To the best of my knowledge, the published Windom antennae have
no traps; well, this is what I tried out.
I used EZNEC to model the antenna and the following pictures show the actual simulation
results. By the way, as for traps, I used the VA3MP-traps described here.
The 3D model of the antenna is shown on the
left. The X, Y and Z axes illustrate how the wires
of the antenna run, 'Z' being the affore
mentioned antenna tower. The feed point is
shown by a red circle at the top of the Z axis.
The numbers identify the the wires; this antenna
consists of 4 wires.
Wire #1 = 9.6m
Wire #2 = 3m
Wire #3 = 0.85m
Wire #4 = 0.95m
The traps are located at the junction of Wire #1 and #3, as well as Wire #2 and 4 and are
shown as small red rectangles. The scrren shot below shows the data for the traps.
Since I have a large spool of 75 Ohm Coax cable, I optimised the antenna for a 75 Ohm feed line, but
you can trim the wires (in EzNEC...) to the usual 50 Ohm impedance as well. The screen shot below
shows the resonance frequencies of the antenna.
The center point of the antenna (The top of the Z-axis) is mounted on a piece of 1/2" metal water pipe
attached to my 40 feet tower. The short wire is tied to a tree while the longer piece is attached to the
other tree at the front yard. You may have noticed that the wires are slanted, just like with the
inverted vee and also in an angle with respect to the longer wire. You can model all these variables and
different soil conductivity with EzNEC.
The completed antenna was checked with Mike's MFJ259B and returned the same swr values! I also
checked it with my PC-based antenna analyser and got the same results.
10Mhz
azimuth
and
elevation
18Mhz
azimuth
and
elevation
25Mhz
azimuth
and
elevation
The following screen shots show the azimuth and elevation data from simulation
If you have any question about the antenna or the design, please contact Steve, VE3USP