I'm putting up a multiband dipole in some tall trees. (Perhaps it's more correct to call it a "dipole-doublet"). The situation I'm imagining looks like this:
The legs of the antenna are each 100 feet long. They are fed in the middle by ladderline. This ladderline runs 150 feet to a 1:1 current balun (e.g. the MFJ-918). This balun is fed with 150 feet of high quality coax (LMR-400) to an antenna tuner inside the shack.
Unfortunately, I can't run the ladderline all the way to the shack, because the transmission line has to be buried under a lawn for about 15 feet or so.
My questions are:
- Are the losses inside the coax between the tuner and the balun going to be ruinously high?
- What will the impedance of that leg of the circuit be, assuming the tuner feedpoint is 50 ohms, and the dipole and ladderline are something realistic?
- Is a 1:1 current balun the right choice?
- Is 300 ohm ladder line the right choice? Would 400 ohms or 600 ohms be better? Does it matter?
UPDATE This design is flawed, because the tuner does nothing to tune the impedance mismatch at the balun. As Mike Waters notes below, my drawing is basically a G5RV, with an separate wound 1:1 balun at the ladderline feedpoint. The G5RV looks like this:
What I want is this, the random-length multiband dipole.
So it appears in conclusion that I do need a remote tuner at the base of the ladderline feed, or I need to bring the ladderline into the shack. Or, an alternative would be to go with Phil Frost's suggestion and use a folded dipole and operate it at its resonances. I think I probably want wider-frequency coverage than the resonances of the folded-dipole, so it sounds like the tuner + simpler wire setup is more optimal for my situation.