Tech-tut.com

Robbie builds a 2 meter yagi from his scrap pile (Dec 1, 2011)

This has been an interesting, aggravating, and fun project all wrapped up in one package. For starters, I didn't want to spend a lot of money on this project. I had about 15 bucks that I could spend. It's all I had in my wallet. The only item that I had to purchase was the copper tubing. I asked for 1/8", but I think I wound up with 3/16". The outer diameter of the copper tubing is 0.190". It worked out better this way because the tubing was perfect for the gamma match. The foam on the coax fit perfectly in the copper tubing.

For the reflector and directors, I used 1/8" aluminum wire that I had coiled up in my shack. I don't have any idea where it came from, but I've used it several times for small receiving antennas for my scanner and FM radio. My driven element is made from the copper tubing. In this design it is a split element, but I joined the two halves together because I changed my design at what became the halfway point. I originally designed it to be used with a 1/2 wave coax balun, but the SWR was wacked out so I tossed the idea out for a gamma match. I later discovered that I probably didn't know how to use my swr meter. The 1/2 wave balun might have been good.

The boom is a piece of scrap 2x2 or something like it that I found at work. It was, most likely, a scrap from a rip from a larger piece of timber. The gamma shorting bar is a piece of 1" x 0.250" aluminum bar which was also a scrap yard special.

The tools used for this construction should be fairly common to any half-time tinker-er. I used a cordless drill and varying drill bits, a 10-32 tap, a dremel tools for making larger holes, a hot glue gun, soldering iron, and a cheap drill press. The frustration that I had tool-wise was with the drill press. It bought it from Big Lots, so I know that I got what I paid for. The wiggle factor of that drill press is just short of mind blowing. You can set the thing to make a hole in something, and when you go to drill the hole it will be anywhere from spot on to 60 thousandths of an inch off. At any rate, it sure beats trying to drill straight and accurate holes with a hand drill. I also used it to hold the tap while I manually tapped the holes on the shorting bar, and it did that very well.

My dimensions for the element spacing are derived from this site: The Yagi-Uda Antenna @ Antenna-Theory.com. While I did neglect the diameter of the elements, which are suggested to be 0.0085*wavelength, I did stick to the 0.2*wavelength spacing of the elements, and I used the suggested lengths from the Antenna-Theory.com website as a starting point. The final design was refined using Quick Yagi (WA7RAI). As far as the lengths of the elements, the driven element is 38-3/4" in length. The reflector (R) is 40-1/2" long. The first director (D1) is 36-1/2" long. The second director (D2) is 26" long. The spacing between the elements varies depending on which element you measure from since my driven element is split. (The spacing of my driven element is a little off because I originally had a split driven element. I later combined the two, but they are offset by 7/16". If I had to do it again, I would have made it according to my original calculations and kept it in one piece.) The length of my gamma rod is 7-1/2". Check the annotated drawing for the complete measurements.

Testing of the Yagi went well. It was a chilly 40 degrees at the time of testing. I taped the yagi to my 1.5" PVC test pole that I drove into the ground. I pointed it towards the South-West and fired it up to 144.570MHz (145.170 with a negative offset). This is the repeater that I have been trying to hit for weeks. I set the sensitivity on my MFJ-841 then switched it to SWR and found a wonderful 1.4:1. After the call sign and statement of my intentions, I released the key, and I heard the wonderful courtesy tone.

Remember, it's more than just a hobby, it's entertainment.

Photos

2 meter yagi with gamma match drawing
This is the drawing of what my antenna looks like
2 meter yagi with gamma match drawing with dimensions
This drawing has the dimensions of the antenna and Gamma match
Gamma shorting bar drawing
This drawing has the dimensions of the Gamma shorting bar
Gamma shorting bar
This is the finished gamma shorting bar
Yagi Gamma match
The Gamma match as installed on the antenna

PVC cap used to secure the antenna to the mast
PVC cap used to secure the antenna to the mast
BNC connector
The BNC connector is hot glued into the wood boom
SWR graph as measured
Graph of the measured SWR values
Final Analysis using Quick Yagi
Analysis of antenna as constructed using Quick Yagi
Quick Yagi plot of radiation patter (azimuth
Quick Yagi radiation pattern (Azimuth)
Quick Yagi plot of radiation pattern (elevation)
Quick Yagi radiation pattern (Elevation)
Quick Yagi Plo
Quick Yagi plot of F/B, Gain, VSWR across bandwidth