When the antenna has an input impedance of 300 ohms balanced line, the 4:1 coaxial balun easily transforms this to match with our 50 ohms unbalance transmission line. One example to this is the folded dipole whose input impedance is 300 ohms and is balanced at its feed point.
To start with the construction, first we need to know the operating frequency of the balun. Assuming at 145MHz, so we may have the formula below.
Solve for the wavelength at 145MHz;
wavelength (feet) = 984 / 145MHz
wavelength (feet) = 6.78 ft.
We can express our unit of measurement by "inch" for ease in the calculation;
wavelength (inches) = 81.43 inches
We need also to consider the velocity factor of the coaxial cable used which in this case for the RG-58 is 0.66 (dielectric is made of PE).
Solving for the actual physical wavelength;
wavelength (inches) = 81.43 (0.66)
wavelength (inches) = 53.75 inches
Solving for the required 1/2 wavelength;
1/2 wavelength (inches) = 53.75 / 2
1/2 wavelength (inches) = 26.87 inches
Given above formula, you can easily compute for the required length of the 1/2 wavelength at any operating frequency. --- 73 de du1vss
