In preparation for the construction of the Yagi, I have already made a box, where the 28 ohm dipole can be fixed in later. The box is a standard electric box normally used for final assembly of electric cables. You can buy it at a hardware store. In the box, I made an aluminum plate which one side has a raised edge at an angle of 90 degrees. See the picture for further details.
To adjustment the dipole from 28 to 50 ohms, you’ll use two pieces of 75 ohm coaxial cable in parallel with a length of λ/4. In practice, this length is calculated using the formula λ / 4 * V where V is the velocity factor of coax you use. In my case that RG59/U with a V of 0.66. Then the length (300/144.2) / 4 * 0.66 = 0,343. More generically, the formula is: “(C / f) * V” where C is the speed of light, f is the frequency and V is the velocity factor of a cable.
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