[Antennas] Loading of dipoles and off center loading of dipoles

2003年10月31日 15:51:50 -0600

I am using a 66' dipole, center loaded with 57.5 feet of 450 Ohm ladder 
line fed with a 1:1 ferrite bead balun of RG58. and about 20 more feet 
of RG58 to the transmitter. I built this from instructions without 
questioning anything about the construction. It works fine. 
But now I am thinking of changing to an off center fed dipole so I can 
run 40, 20, 15, and 10 meters instead of just 40 meters. In my reading 
I find that feeding it at the 2/3 by 1/3 point the feed impedance will 
be 200 to 300 ohms and I can use the same ladder line and a 4:1 balun to 
feed it from the RG58.
In the off center fed dipole I can see the ladder line as matching the 
high impedance of the feed point and the 4:1 balun to match the ladder 
line to the coax.
But here comes the question............... What the hell is the ladder 
line for on the center loaded dipole? I assumed it was to match the 
impedance of the center fed dipole to the 50 ohm coax, but I now find 
that the center of the dipole is ALREADY 50 ohms or there abouts, so 
what is the ladder line for?