The WB0CMT 3 Element 2 Meter Yagi
Using PVC boom
This 3 element 2 meter Yagi is nothing more than
an "adjustable match" half-wave dipole antenna (the driven
element) using direct feed 50 ohm coax, mounted between the other two
elements which are the reflector and the director. It will give you about
"7db" gain according to the author.
that provide a snug fit to the elements approximately 1/4 inch or so from
the boom ends. Epoxy the director and reflector in place after entering
them in these holes or add screws to secure them to the boom. A pair of
holes spaced 1/4 inch and centered 16 inches from the reflector hold the
two-piece driven element. The short ends of the element halves should
extend about 1/4 inch through the boom. Solder the 50 ohm feed line
to the driven element as shown in the drawing.
You may want to turn the boom "T" 90° for horizontal polarization.
Typically the reflector is 5% longer and the director 5% shorter than the driven element. By spacing elements about 0.15 to 0.25 wavelengths from each other you end up with a beam antenna with about 7dB of gain. (According to the author) To figure out the correct length of the driven element the following simple formula for a half-wave antenna above 30 MHz is used :
feet = 475 / Frequency (MHz)
Construction: 3/4 inch PVC plumber's pipe is recommended for the supports. Two 18 inch pieces become the boom where the elements are installed. A 36 inch piece is used for the mast. (Although not shown in the original drawing, it is assumed the the T is about 2 inches long.)
The boom and mast pieces are held together with a PVC T joint. Three small holes are drilled in the T as well as the boom and mast pieces. (Look at the T in the drawing for the small black dots. These are the locations of the screws).The boom and mast are inserted into the T and rotated until the holes are aligned. Then the boom is secured with screws...or you may use bolts also if you prefer.
Screws or bolts are used to secure everything in place. Put them just to the side of the elements so as to press against them when tightened. The antenna in the drawing is vertically polarized which is best for FM work.
The director and reflector elements are cut as
according to the lengths in the drawing above.
To build the driven element the boom is measured and marked at a distance of about 16 inches from the reflector. Two holes are drilled 1/8 to 1/4 inches apart. Two rods each around 20 inches in length are pushed through the holes until 1/4 inch protrudes from each side of the boom. THE DRIVEN ELEMENT AND MATCHING DEVICE HAS JUST BEEN MADE. Also see "Tuning" below for more info.
The 50 ohm coaxial cable is soldered directly to the protruding stubs. Screws or epoxy is used to secure the rods to the boom but only after swr tuning is done.
Electrical tape or a silicon compound is used to weather proof the solder joints.
It is better to have a little excess length for the
driven elements. The antenna should now be hoisted to an elevation of at
least 5 feet from the ground and kept away from nearby metal objects or
the reflector can be against the ground with the director aimed straight
up while braced to prevent it from falling over while tuning. Keep the
coax away from the driven element while tuning. See tips
After lowest swr tuning is finished use the screws
or bolts to secure the elements.
The 2 meter
beam can be made very portable by removing the 2 18" boom
sections or use your own method to disassemble the
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