DIY FM dipole antenna or half wave antenna are the most used because is easy to build and cost a few bucks but has some drawbacks like little directionality, small gain.
The rf dipole antenna is build generally like in the picture but it is symmetric and the coaxial cable is asymmetric, that’s why we use a piece of coaxial cable, that it is used to symmetrize, the same we use to connect the dipole antenna to the transmitter. This piece of cable has the length shown in the picture.
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The total length of the dipole rf antenna is: L (cm) = 145 / frequency (MHz)
Radiation pattern and gain of fm dipole antenna
Dipoles have a toroidal reception and radiation pattern where the axis of the toroid centers about the dipole. The maximum gain of a λ/2-dipole is 2.15 dBi.
This is a fm antenna used generally for frequencies between 88 and 108 MHz.