Ve6fi Amateur Radio

 

 

 Beam or 4 Square

My plan was to have a two-element 80-meter beam that was tunable across the complete band. I did have design and construction pictures on this web site previously. I will go through my learnings on this project.

My first design and construction was with two elements that had a coil in them about half way down the element. This was to shorten the element. Then further down the element I had a tunable tape, which was adjustable under computer control, such that a person could tune any frequency in the 80-meter band as well as reverse the antenna pattern without turning the antenna. The elements with the coils installed were about 70 ft long with the last 10 feet being adjustable with copper tape (similar to Step IR) Well we had all this working and tested in the air at the 20 ft mark. Everything worked but the voltage along the element (at the tuning location) was rather high. Yes as you move away from the center of the element (boom) the RF voltage goes up. Well I was at 10 ft from the end of the element. With 10 watts excitation I had about 100 volts RF at the tuning section. This was too high especially when you consider that we would be running 1500 watts. The stepper motors and their signal would not handle this. Also some of this voltage would work its way down to the controller.

I had bought a commercial controller to control the stepper motors (two per element) and I did have some problems with them failing even without any RF present. I also had problems getting the stepper pulses to the stepper motors as the cable detuned the antenna. I initially ran the control wires through the center of the loading coil. Not good enough I had to run the control cable inside the copper tubing that I had wound the coil with now just figure out how easy that was.

So I abandoned the first design.

My second design got rid of the stepper motors and the copper tapes as well as the loading coils. In this design/construction I built a variable coil and installed them at the low impedance point at the center of each element. Of course on the driven element I had to put a coil on each side. This was a 2 to 5 micro henry coil that I could expand or contract and control with the stepper motors used in the first design. I found that putting inductance at the center of the element shortened the element length to about 50-ft on each side of center. For the reflector element I only needed one variable coil of 5 to 10 Uh to tune it. So changing from a movable tape scheme to a variable coil had merit as now all the tunable components were at the center and by tilting the structure one could get access to them.

I was still using the commercial stepper controller and having trouble with it. I also had trouble with the stepper motors having enough power to compress the coils. Also I was getting to much software to control the commercial controller.

So I abandoned the second design.

My third design continued to use the variable coil but used an Ameritron SDC 102 antenna controller as the controller. I also used a nice DC motor to drive the coils. The SDC controller had 12 stops programmed into it so I could have a stop every 50 kHz on 80 meters. I had to add some components so that the SDC controller had a reference point to start from. So with DC as the drive for the motor on the elements things were quit reliable. I had the anchors in the ground for a 100 ft 24 inch tower that I was going to install the antenna on. I had the bearings and rotator all assembled and ready to install.

But I had not tested the array in the air and time was pushing me to decide whether I was going to put the 80-meter yagi array up. We had a rental crane on site and it was time to return it to its owner after having it three years to handle our station move. So I did not abandon the third design but did not use it. Things do not always work out like you plan them!

So I decide to simplify 80 meters and installed a four square antenna instead of the 2 element beam. I had the 67 self-supporting poles from my previous four in line array so I just had to pour footings for the four square and we were in business on 80 meters. Of course it is not as simple as that as I had to put 60 radials per vertical and connect them together, put in a controller and a way to monitor the dumped power.

Denis June 22, 2014

 

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