20m 3el Full Size Yagi on a 20ft boom
The antenna on the top is the 20m 3el yagi. The bottom one is the 5 el 10m yagi. Photo taken on 2004-02-01 by 4S7AB
A little history and some comparisions
This is the first Yagi I made for Ham radio work. That was in 1994 . Initailly this was on the rooftop and worked for nearly 3 months as I didn’t have a tower. Even at that height, this wonderful yagi worked beyond the expectations with my QRP FT 7B @ 50W max. Then I thought of giving a little more respect and hoisted it up on a GI pipe mast which was 35ft above ground. Then I was able to understand what this wonderful yagi can truely do for me. The DX on 20 was a pice of cake even with QRP 50W. For comparisions I used a G5RV which I used for all the bands from 80m onwards. The signals which were not copyable with the G5RV were 55 with the yagi…. this is just a one example.
All the construction was done on the roof. I have big trees around my house and there is no way of fitting the elements at ground level. The 3 elements were constructed seperately as described in the below drawing and fixed onto the boom. Then the Gamma match and the boom to mast clamp was consctucted and fitted. The process of taking the antenna to the 40ft tower was quite a job as there are trees everywhere. This took nearly a one day and involved abt 10 guys from my village. Thanks to all…
The element structure (Taper scheme).
|Element||spacing from reflector||Length (‘c’) from aluminium tubing of 0.75″(3/4 “)||Length (‘b’) from aluminium tubing of 0.625″ (5/8 “)||Length (‘a’) from aluminium tubing of 0..5″ (1/2 “)|
Let me further explain the elements. Each element consists of three type of aluminium tubing namely 1/2″ , 5/8″ & 3/4″. These each section has its own length (regardless of the overlapping length which may me about 6 inches). These lengths are marked in the figure 1.
For eg consider the construction of a half of the driven element. It has 72″ of 3/4″ tube, then 66″ of 5/8″ tube and then 74.5″ of 1/2″ tube. The overlapping distance of each may be around 6 inches or a little bit more and it’s your choice.This driven element is then spaced 101.0″ far away from the reflector which is in position 0.00″. The director is also can be constructed in the same method .
Please note that the above figure may not be in the correct scale.
Initial tuning was done at the roof level at a horizontal level for 1:1 with the Gamma match. Then some fine tuning was done after installing the yagi on the tower. The SWR was 1:1 throughout the active region of the band from 14.000 to 14.250 and it was about 1:1.6 at 14.350 ..not bad. After 6 years, the SWR has come up slightly now and it is abt 1:1.5 at 14.200 but still it is acceptable.
The gamma match
For the gamma match I used 3/8″ or 0.375″ rod as the gamma rod. Then ;
Gamma spacing – 4.00″
Gamma rod length – 23″
Gamma capacitor – 330 pf (Max)
Feed point impedance – 50ohms.
A 500 pf variable air spaced tuning capacitor will fulfill the requirement. The gamma length may vary a little bit. I used 2″x2″ square boom 20ft. It has a very good strength and I cannot see any overshape or bending after using for 6 yrs !!.The brackets and connectors are upto your construction skills.
This a computer optimized yagi using ‘YO version 3’ and you can freely distribute this among your ham friends. I also like to see somebody getting some use from my efforts.
Finally I hope you enjoy the construction of this and will add some dBs to your stn.