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
Element Element position Element length
Reflector 0 mm 2980 mm
Dipole 510mm 2930mm
Director 1 965mm 2776mm
Director 2 2180mm 2574mm
Gain 6.2dB d
Bandwidth 2 MHz
Front/Back 24 dB
Boom length 2.2 M

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