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
September 2010
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:
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There is trouble decoupling the control wires to the stepper motors when the motors are far out on the elements |
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It would be impossible to get access to the coil and the motors if they are far out on the elements. If they are at the center I could tilt the boom against the tower to access both the driven element and the reflector |
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Having the motors and coils at the center is the lower impedance portion of the antenna. |
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The coil at the center would now be smaller in value (3 to 6 uH) and this would allow one to tune the complete 80m band. |
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I could use the same software to run a stepper motor that would adjust the coil at the center of the element as I previously used to move the copper tape. |
Two Element 80 meter beam with 30 ft spacing with Variable Coil Tuning at the center and using a Reflector.
In this application I am using a driven element/reflector application so as to increase the F/B ratio. I