2 metre 7 element yagi beam homebrew project
Boom made from 25mm square aluminium. A flat piece of aluminium to form a shorting clamp between the DE & the Gamma Rod. A right angle piece of aluminium to mount a SO-239 panel mount socket.A piece of wire. Nut,bolt and washer. Elements are made from 12 x1mm wall round tubing & 10 x 1mm wall round tubing. Gamma Match made from 12mm & 8mm or 6mm tube (could be ROD for inner section) with insulating dialectric which can be heatshrink or outer coax cable sleeve. Make sure they slide inside each other freely. Pop rivets. Zinc Plated Outdoor type Steel screws. Perspex block. U bolt mounting clamp.
Spacing: REF to DE 16 inches, DE to DIR01 16 inches, DIR01 to DIR02 18 inches, DIR02 to DIR03 20 inches, DIR03 to DIR04 22 inches, DIR04 to DIR05 24 inches.
Boom Length 10 feet or 120 inches long so there will be 2 inches on each end free. You could put 2 plastic end caps at the end of the boom.
Element lengths: SSB Reflector 104cm - Driven Element 98cm - All Directors 93cm
Element lengths: FM Reflector 102cm - Driven Element 98cm - Directors 93cm to 89cm ( Tapered for greater bandwidth )
Gamma Match: 12mm OD outer tube 7 inches long. 6 to 8mm ID inner tube 10 inches long. Heatshrink and/or coax outer sleeve.
Spacing Between Gamma Rod and Driven Element centre to centre is 1.5 inches.
What I want to build is an adjustable antenna to change frequency and get good bandwidth. Workout from the lengths above how much 12mm OD tube length the main part of each element with 10mm OD tube at the ends to adjust the length of each element. You can either screw to secure the two parts together or if you desire cut a split in the bigger tube and use very small hose clamps.
I'm using a gamma match as I find it easy to use, however more bandwidth could be got by going to a folded dipole. The Spacing is designed to give the best gain for the boom length. What you want to do is drill through the boom to slide in the elements then you want to secure them with pop rivets at the top so the elements are close to the top of the boom ( NOT mounted dead centre ). Then it is time to setup the gamma match. Plexiglass, Perspex or other UV resistant insulator block - drill four holes. One for the DE to pass through and one for the gamma rod and two screw holes. Slide the block 3" long x 2" wide onto the driven element with the top hole above which will be for the gamma match outer tube. When the block meets the boom, secure it with two self-tapper screws which will be under and to each side of the driven element. Make the gamma match using the smaller 10" long rod/tube and fit the dielectric ( heatshrink etc )
Then slide it into the larger 7" long tube to make a reasonably firm but easy to slide fit. Put it in the top hole facing out along one side of the driven element. Now get a section of flat aluminium sheet cut it to a strip about 1" wide and around 6 to 8" long. ( use your own judgement ) Bend the strip around the DE and Gamma tubes so that the ends meet in the middle. Drill a hole in the middle to secure it with nut, bolt and washer. That is your shorting strap. This strap will be about halfway along one side of the DE roughly 10" from the booms' centre point. Make a right angle from the aluminium flat sheet big enough 2" x 2" to fit the chassis mount SO-239 socket. Drill out and file/ream a hole big enough to put in the SO-239. Secure it by whatever means. Drill two holes on the other bottom part and screw mount it to the beam's boom near the gamma match. Solder the inner of the SO-239 with a wire ( keep short ) and solder on a spade lug on the other end which you attach to the outer tube - a small bolt, nut and washer is what I used. A bit of Selley's Silastic clear is used to waterproof it. Find the balance point of the antenna somewhere near the middle and drill, file and install two holes for the " U bolt mounting clamp. " The Yagi beam is now ready for testing. I temporarily mount it on a PVC non metallic mast and attach a short RG58U jumper cable to my Antenna Analyser or SWR meter and radio if you don't have an analyser. First thing to do is find the centre resonant frequency of the beam which should be where the SWR is lowest. Then slide in or out your gamma rod to lower the swr. Also try moving the shorting clamp to get the lowest swr.
I'm getting 2Mhz bandwidth 2/1 SWR or 1Mhz bandwidth 1.5/1 SWR.
You now are ready to screw on the coax ( RG-213, 9913, 9914 ) PL259 to PL259 or N Plug depending on what you need. A run of 10m to 15m would be OK.
See Martins' WWARG on Talkforce, Zello and Echolink.
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