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Old 01-04-2011, 02:32 PM
DaveInDenver's Avatar
DaveInDenver DaveInDenver is online now
Hard Core 4+
Join Date: Jun 2006
Location: Larimer County
Posts: 8,447

I don't think balanced feedline would be necessary and for VHF open 450 ohm ladder would be bad, more so if the air was moist. Using 300 ohm TV feeder would be an option, though. In particular to avoid tons of loss at >144MHz the conductors would have to be close, others it's no longer balanced since they don't couple and just act like independent wires.

Probable that I'm missing something, what are you trying to solve using the balanced feed here Brian? If you feed an unbal 50 ohms into a 9:1 balun into 450 ohm ladder, which you connect to a high impedance, you still get a huge reflection and essentially make the ladder line the radiator. So yes, it's keeping you from blowing the radio finals but doesn't solve the feedpoint inefficiency.

I 100% agree that solving a full wavelength dipole problem is not worth the time and doing a matching stub or changing to a 5/8 radiator makes a lot more sense. :-)

BTW, teachable moment... If you look at the construction of this antenna, you'll see the author wrapped the coax several times at the end. He's making a 1:1 current balun to solve the balanced-to-coax issue. In this case being a correct Hertzian dipole the feedpoint impedance will be ~72 ohms, so you don't need the impedance transformation that Brian and I are discussing.
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Last edited by DaveInDenver; 01-04-2011 at 03:14 PM.
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