40 Meter Gamma Feed Antenna

How it came about …

I could write this and use a lot of math and modeling software and such, but that just isn’t the way it happened. This antenna is part math, part intuition, and part junk box, a portion of luck, and a wife (W8AMA) that knows how I love to fabricate things whether it is an antenna or a meat smoker. When UPS delivers several boxes about six feet long my wife’s first question is” What frequency is this over engineered pile of aluminum going to be resonate at ”? I do not buy what I am sure I can build, even if it costs a few more bucks. The antenna was inspired by two things, the need for a 40 meter antenna because the 160 meter off center feed antenna I was loading on 40 iced up and came down, and a lash up gamma fed antenna for 40 meters put together by Jack K8HFX. The vertical antenna worked for me as I have very few trees on my lot to hang antennas from, and the backyard is mostly a fresh water wet lands so I figured I would have better than average soil conductivity. I have tried trapped antennas with moderate to poor results and limited bandwidth so the gamma fed vertical looked like a good direction to proceed. I have a pretty good library of amateur radio books, the ARRL hand book, antenna book and ON4UN’s low band antenna book and others, so I started digging through the available material with a gamma fed vertical antenna to be the end result. I scrounged the web looking for info as I didn’t want to reinvent the wheel, if there was one out there I could copy that was a goal also. I couldn’t find exactly what I was looking for so I decided to experiment and improvise. I found the plans for a gamma fed 20 meter beam and decided to double the dimensions of the gamma fed portion and proceeded from there. The result has proved to be very satisfactory. This antenna is aluminum tubing from DX Engineering. The aluminum plate is .250″ thick, the angle aluminum is 1″x .125 thick. It is TIG welded to ensure good conductivity and strength. 40 Meter Gamma Fed Vertical at my QTH is located in wetlands, and has 8 ground radials laying in water. The ground rod at the base of the antenna is 10 foot long and extends into a flowing water table. The photos show the antenna as it sets at the back of my yard.The following graphs were created with a RigExpert antenna analyzer.

Antenna relay switch and 40 meter vertical as it sits on the back of my yard.
The base showing the ground, capacitor housing, and gamma insulator.
The SO-239 location and current balun.
The gamma short at about 10 foot.
SWR Chart 7.000 – 7.300 Mhz.
Smith Cart 7.000 – 7.300 MHZ.

 

SWR Chart 5.000 – 10.000 MHZ.

I am going to start the manufacture of the base section of this antenna in the near future. I will sell them through my corporate identity Duir Life Skills Training LLC. I am working on a materials cost structure to determine a selling price. If you are interested email me at duirlst@gmail.com and I will keep you informed.