When the transmission line portion is nearly a quarter wavelength long,
it has very little effect on the feedpoint impedance, so the resonant
length will be about the same as it would be if the transmission line
didn't exist. In the case of TV twinlead, though, if shorted at the
ends, the transmission line can make a noticeable impact on the
feedpoint impedance. It would take only a few minutes with EZNEC, using
the method I described earlier of modeling a folded dipole or monopole
as an unfolded dipole or monopole with a transmission line stub in
parallel with the source, to see how great the impact is. With this
model, you can give the transmission line whatever velocity factor or
length you choose, independently of the "antenna" portion of the model.
If you use the same twinlead to make antennas for different bands, the
impact of the transmission line will be different on each band because
the transmission line isn't being scaled with frequency. Therefore, any
conclusion you reach about the impact of the transmission line on the
antenna length will be quantitatively correct only at one frequency
(and, of course, only the assumed type of twinlead).

I don't have the time right now to do the modeling, but if you're truly
interested, you won't mind taking the time to do it yourself.

Roy Lewallen, W7EL

Jerry Martes wrote:

Roy

Thanks for the clear and meaningfull response to my post. I'm not
qualified to enter this discussion. I'm in the learning mode. I did wonder
if the dielectricly loaded "shorted stub" transmission line (1/2 the folded
dipole) wuld have a shortning capability for determining antenna length. As
I read it, the VP of the twin lead does effect the folded dipole's length to
aceive resonance. And, the amount of "shortning effect" is somewhere
between 20 percent and 3 percent in your example of twin lead with VP=0.8
.Please correct me if I'm wrong, but, I'd expect the "shortning effect" to
be much closer to the 3 percent end of the scale for reasons that would be
too confusing for me to try to explain. ( besides, I'm probably wrong in my
thinking)

Jerry