VHF Folded Dipoles, including specifically for Ch2-6, Ch2-FM, only Ch6 and only FM, analyzed using 4nec2. Includes Omnidirectional Vertical Dipole for FM Band, but see OMNIDIRECTIONAL album for more info. Example 4nec2 files included for most antennas.
There is a Quasi-optimum Separation between Folded Dipole Elements of about two inches for larger diameters, one inch for moderate and 1.0 to 0.5 inch for small wires. But a 2:1 variation causes only small differences in SWR.
Optimized Folded Dipole sizes are: 63.0 x 2.0 inch - 0.569-in (Type M) Copper Tubing (Ch6, also OK for CH5 and FM Band) 64.0 x 3.0 inch - AWG12 Copper Wire (Ch6, but SWR too high for CH5 & FM Band). 65.5 x 0.5 inch - AWG12 Copper Wire (Ch6, but SWR too high for CH5 & FM Band). 66.2 x 0.5 inch - AWG24 Copper Wire (Ch6, but SWR too high for CH5 & FM Band). [Reduce length by velocity factor if using 1/2-in Twinlead.] 76.0 x 2.5 inch - 0.569-in (Type M) Copper Tubing (Ch3-6, although SWR high on Ch3 & Ch6) 82.0 x 2.5 inch - 0.569-in (Type M) Copper Tubing (Ch2-6, although SWR high on Ch2 & Ch6) 99.24 x 0.5 inch - AWG24 Copper Wire (Ch2 ONLY) [Reduce Length by Velocity Factor if using 1/2-inch Twinlead.]
Optimum size for an almost OMNIDIRECTIONAL VERTICAL Folded Dipole for just the FM Band is 53.0 x 8.0 inch using 0.569-in O.D. Copper Tubing. Gain away from feedpoint is 2.6 dBi and 1.4 dBi in opposite direction. A clamp can be attached to the center of the long element since it is at zero ground potential. Alternative size for Horizontal or Vertical FM Folded Dipole is 55-in x 4-in. Both use Feedpoint Gap=1.25-in.
Folded Dipoles for FM Band frequently come with a Receiver: http://kgnu.org/ht/helpfm.html Here's an on-line Folded Dipole Calculator: http://www.k7mem.com/Electronic_Notebook/antennas/folded_dipole.html Note that IF element diameters are the same, impedance does not change as you vary the separation. By adding a Reflector and a Director, you've got a simple 3-Element YAGI: http://www.wfu.edu/~matthews/misc/dipole.html
EDIT (22Nov2012): Moved Ch6 and Vertical FM Charts to this new Album from Hi-VHF Folded Dipole album. Added analysis of optimized sizes for Ch2-6 and Ch3-6 Folded Dipoles.