DIY 5 Element 70cm Yagi
Because.... why not??
If you haven't noticed already, I LOVE to build antennas. After building the log periodic, I thought I would try my hand at building a Yagi. I decided to make a 70cm Yagi since it requires less materials, so the cost would be less that a 2m Yagi, which is the ultimate goal.
A couple obstacles I had to overcome in keeping the cost down was: what material to use for the boom, what material to use for the elements (hollow or solid), how to attach the reflector and directors to the boom, and how to attach the driven elements (direct to the boom, or isolated from the boom). The latter would directly decide how I would feed the driven dipole elements.
The boom was easy; I already had several sticks of 1' aluminum square tubing in various lengths left over from other projects.
For the elements, I not only took into account the cost, but the fact that I get some pretty high winds where I live. So I settled on solid aluminum rod. I put my Google fu to use and found 4' long 1/4" diameter rods online that were fairly inexpensive even when shipping was added.
I know of many, many different methods DIYers use to attach their elements to the boom. After looking over dozens of designs, I decided that I would simply drill holes straight through the boom, centered top to bottom. The elements were cut to length, center was found and marked, then marks made 1/2" on either side of the center mark. The elements were centered in their respective holes, then staked into place in 4 locations around the rod using a sharp center punch. The punch was placed on the boom, VERY close to the rod. Once struck with a hammer, the punch deforms the aluminum slightly and presses it into the rod. Staked in several locations around the rod, this hold them in very firmly.
By placing the reflector and directors in the center of the boom meant placing the driven element directly in the center also. The easiest way to do this is to use a solid driven element, not 2 separate ones. It is then fed with a gamma match, essentially a series capacitor. This is a similar method to shunt feeding a tower to use as a vertical antenna. The gamma match is made by installing an flange mounted SO-239 to the boom 1' behind the driven element. Soldered to it is the center of a piece of RG8X coax (the center conductor and the dielectric surrounding it) and ran through a hole on the side of the boom. A piece of 1/4" diameter tubing that had sufficient wall thickness to snugly fit over the coax center section is then slid over the coax piece. A shorting bar was made from two pieces of 1/2" x 1/4" aluminum bar stock, bolted together, and drilled with two 1/4" holes exactly 1" apart. Resonance at the design frequency is found by moving the gamma tube, and the location of the shorting bar. Once a good match is found, the shorting bar is tightened, and everything is sealed up with clear RTV. I also slid on a 1/4" rubber cap over the end of the gamma tube.
The ends of the boom will be sealed with 1" square end caps once they arrive from Amazon.
Once it was tuned, I stuck a small piece of wood into the end of the boom and connected it to my HT. Holding the antenna out in front of me I was able to hit a repeater up in Estes Park, some 44 miles away from my house in Nunn, on .5 watt with a report of full quieting. Success!
My plan for the antenna was to mount it on the south side of my workshop and point it at my club's busiest repeater, and install my old Leixen VV-898 25 watt mobile above the workbench. The antenna was mounted in a vertical orientation with the gamma match on the top side.
Now I have a very efficient antenna connection me to the 447.275 machine on the 898's lowest power setting. I can use this rig while tinkering in the shop, and not forget my HT on the bench for the battery to run down and die anymore!
