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A basic 1/2 wave wire antenna can be figured by dividing 486 by the frequency in mHz. (in feet/inches) The center of the FM broadcast band is ~98 mHz. A 1/2 wave length of wire is 4.96 ft.
Put 4 horizontal 1/2 wave radials at the base of a 1/2 wave vertical wire and the impedance is ~75 ohms. Put a 1/2 wave counterpoise vertically under the active element and the impedance will be ~300 ohms. A 1/2 wave folded dipole will be ~300 ohms..
Most of those ElCheepo xmtrs. have an unbalanced antenna connection. 300 ohms seems to work the best..
A folded dipole will be somewhat directional “through the loop”. A “groundplane” configuration will be omnidirectional using a horizontal or conterpoise ground return..