K7ITM wrote:
. . .
But a better way to do a broadband vertical collinear is to feed
several dipoles, stacked end-to-end (with some gap from one to the
next), each fed with the same electrical length of feedline, with the
far ends of all the feedlines paralleled. If the gap from one dipole
to the next is enough that the mutual impedances among the dipoles are
all small, then each dipole will have current very nearly in phase
with the others and the radiation pattern will be perpendicular to the
axis of the dipoles. It's a messier feed arrangement, but it's much
better for keeping the antenna currents in phase along the whole
antenna across a relatively wide frequency range.

You can avoid the problem of different feedpoint impedances due to
mutual coupling by using lines of an odd number of quarter wavelengths
to feed the elements. If you use lines of those lengths all going back
to a common point, the currents in the elements will forced to be equal
in amplitude and phase regardless of differences in their feedpoint
impedances. There's more about this in Chapter 8 of the _ARRL Antenna
Book_. It's become known as the "current forcing" method.

Roy Lewallen, W7EL