Physical Layout of Reflector Element
Modified
Element to Boom Interface 80 Meter Beam Ve6fi
Since the element is approximately 90 ft long it must be supported to the
boom by the 1/8 inch plates shown below. They are 28 inches long so that
the interface is spread over a larger area. The square plate (not shown) which
is on the end of the boom will attach to these brackets.
Extension of the Element
The triangular portion of the element is 64 feet long. The brackets shown
below in the next two pictures will support the rest of the element which
consists of a short piece of 1.5 inch Al pipe, a 8 inch long 6 inch diameter
coil, another 120 inches of 1.25 inch Al pipe, a stepper motor assembly and
another 96 inches of fiberglass tubing.
Boom Assembly 2 element 80 meter bean Ve6fi
Below is the boom section of the 80m beam. It is 32 feet long. The pipe on the left is a torque tube and it will be placed in the center of the boom. The triangular tower section does not have to much resistance to twist. As such the torque tube which will be bolted on each end and in the center will resist the torque created by uneven wind forces or uneven ice loads on the 100 ft. elements. The torque tube is built out of 3 inch AL pipe Schedule 40.
Shown below is the center of the 32 foot boom. Shown is the cradle and the center support for the torque tube.
Below is the 80m beam without end pieces attached. It is installed at a temporary location until I finish the rest of it
and build the 150 foot tower to put it on.
The mast is 4.5 inches OD Schedule 40 steel pipe. The cradle that holds the Yagi is shown welded
to the mast. The cradle is made out of 1/4 inch steel and is 36 inches long.
Stepper Motor Assembly
Coils
will be air wound and installed at the center of the element
Coil Assembly
The beam will use two coils in each of the elements with 13.7 micro Henry inductance to allow the
complete Driven Element and Reflector to be shorted to a total length of about 100 ft.
I used 6 inch ABS pipe as the coil form. I cut backing rings that are glued inside the coil form and
support the end plates that are glued to the backing rings.
The loading coils shown below are used to reduce the element length from near 145 feet
to just less than 100 feet overall length. I was going to pull a CAT5 control cable through
the coil but I should have string the cable before I wound the coils. I could blow a string
through the coils but I could not use it to fish anything through - to much friction once you
have wound the coils. Oh well!
I wanted to make the coils weather proof so I wrapped them with 1.5 inch
strips of soprema (roofing membrane). That membrane really does stick
and is non conductive. Then I wrapped the coil with 1.5 inch PVC tape.
I really wanted to have low ohmic connections and a large diameter conductor
so as to keep the Q high.
Below the platform is shown on which the stepper motor and tape drive will be installed.
Below are Denis ve6aq and Ted Ve6kib working on the software to control the stepper motors
on the 80 meter Yagi. The software will enable both elements of the array to be tuned so that
the antenna can operate from 3.5 Mhz to 4.0 Mhz.
Below is the inside of box that contains the stepper motor, the tape and the brushes
The stepper motor will move the tape in or out of the fiberglass enclosure on the right.
Tape that is not used is just slide inside the Al portion of the element on the left of the picture.
Below is a screen shot of the graphics used to control the antenna
parameters. For example if you wanted to operate at 3.750 one would
tap the 3.750 button. Since the antenna is in 'forward fire' it would adjust the elements
into a Driven element/ Reflector combination. The graphic software talks to the
Controller at the base of the tower. The Controller is run by a PIC which controls
the four stepper motors on the antenna. Two stepper motors are utilized for the driven element and
two for the reflector/director. The reason one uses two stepper motors per element is
because the stepper motors are located ten feet from the end of the elements.
If one wishes to change antenna direction by 180 degrees without rotating the antenna
one would click on the 'backward fire' and the antenna would be adjusted into the Driven
element - director configuration.
On initial setup the lengths of the antenna at each frequency can be changed and saved
into the program. I will probably do that while monitoring the front to back ratio.
Setbacks - I lifted one element up to 40 feet and checked it for resonance. There was a sharp dip at resonance. Now I connected the CAT5 to the stepper motors and check for resonance again. The resonant point had moved and the dip was rather broad. The CAT 5 cable was becoming part of the antenna and had to be decoupled. I had the CAT5 taped to the element so I moved it away from the element and added #31 ferrites every 10 ft. The coupling decreased but the CAT5 was still affecting the resonant point of the antenna. It did not seem to be affected much by the portion of the CAT5 that was taped to the outside of the structural portion of the element.
Now I am thinking about running the CAT5 inside the element itself and through the center of the loading coil. The only problem is that the portion through the coil is 12 inches of solid fiberglass. Even if I could drill a hole length wise through the fiberglass rod, the hole would intersect the holes going through the fiberglass rod at the ends that make connection to the coils. What to do!
Coils and Stepper motors will be moved to the center of the elements.
Nov 2009