Ve6fi Amateur Radio

Two Element 80 meter Beam

Initially I was thinking about full sized elements but found that the 130 - 140 foot elements poised some interesting challenges. I then decided to shorten the elements to 100 feet by use of a coil.

As such I would use the Eznec modeling program to determine the dimensions of the antenna and then did  a taper schedule and a mechanical design to see what was practical versus what we had in stock for material. 

My thoughts were to make the last ten feet of the element on each side adjustable and as such the last 10 ft of .75 inch tubing would be made of fiberglass.  One alternative for the adjustment was to use a rotatable ½ inch diameter Al rod screw driven by a DC motor with an encoder on it and controlled downstairs by a PIC (programmable controller).  The other alternative was to use a stepper motor and control a 5/8 inch flat tape with a PIC as well. In this matter I could tune across the 80 meter band with an SWR of less than 1.5:1 

Well the above thought were from an year or so ago but I think I am learning while I build this antenna.  I have decided to move the coil and the stepper motor element adjustment apparatus to the center of the element for the following reasons:

    Two Element 80 meter beam with 30 ft spacing with Coil loading and using a Reflector. 

In this application I am using a driven element/reflector application so as to increase the F/B ratio.  I am also using a loading coil to shorten the elements to a practical size. I have also increased the boom length to 30 feet so that I could utilize four tower sections for the boom.

 THIS IS THE DESIGN THAT I DECIDED TO BUILD.

The graph below shows the performance at 3.5 MHz

The graph below shows the performance at 4.0 MHz.

Notice that only the ends of the elements are adjusted as one changes frequency. I may have to do more modeling on this array to lower the F/B ratio.

The graph below shows the performance at 3.8 MHz. Notice that only the ends of the elements are adjusted as one changes frequency.

 The graph below shows the performance at 3.8 MHz when the antenna is reversed 180 degrees without rotating it. The pattern is not quit as good as in the forward direction as the spacing between the driven element and the now Director is not optimized.

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