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This is my construct of a 4 element Yagi
for the 6 metre amateur band,
a DK7ZB design found here.
PHOTO: ready for final assembly
PHOTO: in situ, ready for some testing ?
Note; antenna hardware is re-cycled commercial VHF TV antenna; boom, elements and clamps etc
A 'co-axial' sleeve balun (called Bazooka in U.S.) is used to match the unbalanced 50 ohm coax feedline to the 28 ohm impedance, balanced dipole of the yagi array. A λ/4 length of metal tube is tradionally used. The coaxial cable is passed thru the tube; at one end the braid is bonded to the tube while at the other end no connection is made. The DK7ZB variant of the sleeve balun is to use two parallel'd λ/4 section's of coax. This is an easy and elegant solution, no metal tube required, and 'balun' being flexible coax, it can be 'coiled-up' The impedance of this dual λ/4 section is reckoned to be 75/2 = 37.5 ohm From the well known ability of a λ/4 section to match impedances at each end, it follows; 37.5 x 37.5 = 50 x X therefore X = 28 ohm; which according to the aerial designer is the "load" impedance of the aerial array. So this is both a balun and and impedance matching device! link to yagi antenna; design notes (magazine scans) 5 pages |