"Steve" wrote in message
om...
I have a span of approx 32 ft. to use for a horizontal dipole. It will
be about 40 ft high.

I want to use 2 parallel wires to cover 40M, 20M, and 15M. Ladder line
is not feasible here, so I need to tune the antenna for min SWR to
minimize coax feedline losses.

I have had good results with one 40 meter dipole (a little coil loading to
shorten it shouldn't hurt), a 20 meter dipole suspended (by nylon string)
below it (about 3 ft below the 40m wire at the 20m wire ends) and a 15 meter
dipole suspended below the 20 meter dipole. All three dipoles were
connected in parallel at the center, and fed by 75-ohm (RG-59 or cable
coax). Never found a satisfactory length that would work well on 40 and 15
using one wire, so the extra 15m dipole solved the problem. Did not find
much, if any detuning caused by the extra dipoles, although the antenna
starts to look like a spider's web.

I also found no need for a balun when my coax came straight down to ground
level and then was run into the basement. The way to test to see if a balun
is needed is to check the resonant points on the three bands without a
balun, then put a balun in and retest. If the resonant points don't
noticeably increase in frequency when you insert the balun, then the balun
isn't needed. (i.e. the coax isn't radiating as if it's part of the antenna)
Fifty ohm coax (RG 58) will work also, but the SWR at resonance will be
somewhat higher - especially on 15m and 20m because of the height above the
ground. (75 ohm coax matches best when the dipole is higher than a half
wavelength over the ground.)

Good luck & 73, Andy K4YKZ