Add-On Parasitic Element

If you use an indoor folded dipole to receive broadcast FM, you can increase signal strength a few dB by placing a wire near the antenna. Pick one of the following designs based on the favored direction and mounting constraints. I included 300Ω mismatch loss when optimizing these designs to obtain the maximum forward gain possible without antenna rematching.

Broadside Parasitic Element

Placing a parallel wire a few feet above or below a horizontal folded dipole can improve bidirectional gain as much as 3 dB. The azimuth pattern is unchanged and the elevation pattern maximum is almost exactly at the horizon. The wire only slightly reduces antenna impedance. The lower wire is likely to be too close to ground if the antenna is on the ground floor. But if oriented vertically, the antenna will work well anywhere. Its azimuth pattern will be the elevation pattern shown above.

This shows free-space performance for several designs optimized with AO 9.66. The curves include conductor and mismatch losses. All designs use the folded dipole shown in black. It is resonant near 96 MHz with equal gain at the band edges.

Use these lengths and spacings for bare #14 copper wire:

         Length   Spacing
Low      62.125″   85.75″
Mid      58.375    77.625
High     55.125    70.75
Broad    67.375    57.625

Detuned Reflector

When detuned, a reflector can still provide worthwhile broadband gain. Place the wire at the folded dipole height on the side away from the favored direction.

Use these lengths and spacings for bare #14 copper wire:

         Length   Spacing
Low      63.625″  25.125″
Mid      57       27.125
High     54       30.25
Broad    63.25    23.75

Detuned Director

Place the wire at the folded dipole height on the side toward the favored direction.

Use these lengths and spacings for bare #14 copper wire:

         Length   Spacing
Low      59″      25″
Mid	 55       19
High     52       11.375
Broad    50.625   30.25

September 8, 201888108 MHz