40 Meter Vertical

(Patent Applied For)

    40 Meter Gamma Feed Vertical .... The 40 meter vertical antenna was my first HF vertical project. One evening on 2 meters I was talking with Jack, K8HFX and we were talking about all the countries he was working on 40. I asked him what he was using for an antenna and he told me a home brew gamma feed vertical. I went to Jack’s QTH and got a look at his antenna and the wheels started to turn. He had his mounted in a hay field next to his house and had it guyed, which was not an option for me as I live on a lake with no adjacent fields for guying so my antenna had to be free standing. He also had the room to put out 24 ground radials, again that was not an option for me, but I did have something he didn’t and that was a freshwater swamp behind the house or as I like to call it, The Natural Ground Plane. I used the good old formula 234/Frequency for a ¼ wave antenna and determined that I needed about 34 foot of antenna. I got on line and ended up at the Texas Towers web site, www.texastowers.com , and ordered 6 ' lengths of aluminum tubing starting at 1.75 to .75 inches. Because it was to be free standing I nested 2 foot inside of each tube and secured them with self-tapping sheet metal screws except for the top section which was attached with a stainless steel hose clamp so I could adjust it for resonance. On the bottom section I welded two pieces of ¼ “ aluminum stock to use to mount the antenna on a treated 4X4 and the feed point and gamma tube. The gamma tube is spaced 11” from the main element and is two 6 ‘ lengths of tubing .625” and .500” isolated with a 6” length of .500” plexi-glass rod. At the top of the gamma tube I used a 1/8” aluminum rod and stainless steel hose clamps to short it back to the main element. I feed the RF to the Gamma tube through a 140pf-7.5 KV door knob capacitor. I drove a 10 ' copper clad ground rod at the base next to the 4X4 and grounded the base of the antenna directly to it. (See Photos)
 
    OK the building part is done now comes the adjusting part. I adjusted the vertical element using the above formula to 7.150 Mhz. or 32’-8” and erected the antenna. The capacitor at the feed was what I had in the scrounge box, so I was hoping to do all the remaining adjustment with the gamma tube. I started moving the gamma tube up in 6” increments. I checked the SWR with my Autek VA-1 antenna analyzer with each move until I got the SWR under 2:1 at 7.150 Mhz, then I moved it back and forth until I got the SWR to 1.4:1. By using the larger tubing in construction I was hoping for more bandwidth and it paid off, the antenna was 1.8:1 at 7.00 Mhz. and 1.7:1 at 7.300 Mhz. This was with no ground radials. The SWR went up slightly in the shack but stayed below 2:1. I added one ground radial. I feed this antenna directly and do not use an antenna tuner. I have run it at full legal limit with no issues. Since building and installing it last November 2009 I have worked 40 plus countries and had loads of QSO’S in the US. I lowered the antenna this spring and fine tuned the length and gamma tube and dropped the SWR to 1.3:1. I also added and tuned 3 more ground radials but it did not change the SWR, I guess "The Natural Ground Plane" does the job just fine.  (Patent Applied For)

40 Meter Antena


Gamma Match


Gamma Short