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Tune Around! Topics For
Technicians! |
The KJ4IIF
Multiband "FAN" Dipole for 160, 80 and 40 Meters
(Optional 20 Meter band addition also) (Using Techniques and modified formulas from SRI research with Fan Dipoles) I did not model this with any antenna
modeling software. I did it the old fashioned way. With formula's, cut and
prune, a cooler full of cold drinks and some friends to help pull that
heavy antenna up in the air.
Included at the bottom of this
article is a frequency vs, "X" and "R" plus swr
results chart for all amateur bands, 2 meters through 160
meters. There is also a link to a harmonic relationship for all bands
courtesy of and compilied by N5JNX.
All measurements are taken with an MFJ
259B analyzer.
Apex of the multi band dipole is at 58
feet, Ends of the 160 leg are at 20 feet above
ground.
The multi dipole has a 1:1
current Balun at the feed point.
In the
photo above, the legs of the multi band dipole are spaced 6 inches apart
vertically, four inches apart horizontally at the feed point. I used
14 inches of non conductive Lucite six inches wide as shown.
I spaced the dipole legs with 1/2 inch PVC, Six inches from the fed point where the legs come off the Lucite. I drilled 1/4 inch holes through the pvc and threaded the wire through the holes. This keeps the wires separated at the feed point. See photo above. The top dipole is cut for 160 with a center frequency of 1.9 mhz formula is 468/1.9 X .96 works out to roughly 118 feet per leg. The center dipole is cut for 80 meters with a center frequency of 3.85 mhz. The formula is 468/7.18 X 1.04 works out to roughly 63 feet per leg. The
bottom dipole of the multi band dipole is cut for 40
meters with a center frequency of 7.18 mhz.The formula is 468/7.18 X 1.04 works out to roughly
33' 10" per leg.
Note: Always arrange the dipoles with the lowest frequency band first, (on top), then the next higher frequency band under it and so on. You should end up with them in this order from the top: 160 on
top The
interesting thing here is if you look at the swr readings for
each design frequency in the chart below this article, you will
see that each band has a very low
swr!
I wrapped the 80 meter leg through the pvc and secured it with a small cable clamp. At the end of the 160 meter leg a piece of 1 inch pvc was used to secure the leg and dacron rope was used to a tie point, any available tree or post will do. I sealed all of the connections with electricians liquid tape. As you can see by the chart below, the harmonics work on 15 meters as well. Although the analyzer shows a good VSWR on two meters I doubt if it can access any repeaters. I never tried. Same goes for six meters although with that huge a capture area it may be a good receive antenna for that band. On the air reports have been good. I hope this will help anyone trying to experiment with this
antenna for the low bands. Remember that this fan dipole was designed for the 160, 80 and 40 meter bands. In the chart below you will notice a very low swr on each of these bands with red numbers! It also appears that many other bands may be used with a tuner.
Click here
for the Harmonic relationship chart for this antenna for all
bands by N5JNX.
Adding the 20 Meter addition as an option! Just as an experiment, I added a separate 20 meter addition to the fan dipole. I now have, 15/20/40/80/160
meters on one antenna
20 meters worked exactly as the
others.
Formula used : 468/freq X
1.04
End Result Band Width for the 20 meter addition
Spacing at feed point was identical to the 3 band
version, six inches apart.
Further experimentation.
Maybe later, I will try to add 10
meters but the FAN configuration is starting to get out of whack with that
many wire elements on one antenna. I may have to space the 10 meter feed
point 12 inches from the 20 meter feed point to try and make it at least a
45 degree dipole.
Side by side comparisions on recieve show a
10 to 12 db less than the Mosley TA33 at 75 feet, about two S units less
than the beam, so it is as it should be. So there you have it....Four bands, (5 if you add 20), one feed line, no antenna coupler required, and 15 meters as a bonus on the harmonics with the designed bands having very low swr! Try it! 73 - KJ4IIF
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