If this page reads similar to the DIY 5 Element 70cm Yagi page, it's because it's practically the same build, just bigger. With this build, I wanted to be able to get on some of the local and regional 2m SSB nets. While it can be done (and I have done it) on a vertical antenna, signal strength is significantly lower than using horizontal polarization like everyone else. Like, -117dB to -93dB lower!
Materials were selected for the same reasons too; cost, availability, strength. The boom was easy; I already had several sticks of 1' aluminum square tubing in 4' lengths. Four foot was the perfect length to build a 4 element version.
The elements were cut from 4' long 1/4" diameter rods. These made for a sturdy build that should hold up to the ridiculous winds we get here.
Similar to the 5 element 70cm Yagi, all of the elements are mounted directly to the boom through the center of the boom. 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.
To tune this antenna I mounted it on a 5' piece of 1-1/4" PVC pipe that slides right over my Home Depot light stand tripods, and set it up in my back yard. I set the capacitor tube and shorting bar to "meh, that looks like a good place to start" positions. Upon turning on the MFJ-259 analyzer and setting it to 144.200 MHz, I was shocked to find the only tuning I needed to do was... NOTHING! It was dead on, so I just snugged up the tuning bar and called it a day.
Now it was time to order another set of 24" Rohn wall mount brackets, buy a couple more 10' sections of chain link top rail tubes, and wait. Once those arrived, I promptly mounted the brackets to the house, mounted the antenna to the now 20' mast, and put it all up in the air. The bottom of the mast is about 4' off the ground, so the Yagi is up at 24' AGL. I do not have the rotator installed yet, but once the snow melts off the roof and the weather get warmer, I will get it installed. The rotator is a simple RCA TV antenna rotator. Since the Yagi it is smaller than most large TV antennas, and the wind load on the motor should actually be very little.
I can now happily join in on the morning 2m SSB nets. Using the Yaesu FT-991A with 50 watts, signal strength reports have been good. Two regulars on the Beamspinners net that are 86 miles and 98 miles away almost always give me 59 to 59+10 over reports. There is a station in Nebraska, and one in Kansas that I can easily talk to while out in the Pawnee with the log periodic pointed at them, but I cannot hear them off the side of the Yagi, and I cannot point it their direction for now. But, that also speaks to the side rejection abilities of this antenna. Now I'm anxious to get the rotator installed and try to make contacts with them again.
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