[Antennas] Ladder Line

[Robert Lay]

Dear Mike, et al,

This looks like a good place to expound on one of my pet peeves - the myth
about ladder line impedance.

For some reason, many hams are leery of ladder line because of its
characteristic impedance. Notice that I snuck in the word "characteristic".!

Apparently, many believe that since the characteristic impedance is 450,
that you have to have a 9:1 impedance transformation at the point where the
ladder line is connected to a 50 ohm (characteristic impedance) coax. That
simply is not case, in general.

In general, the impedance transformations that are necessary where the
transmission line joins the antenna or where the transmitter or a coax join
a balanced line are transformations that depend on the "impedance seen
looking in the direction of power flow at that point". That impedance will
be a function of many aspects of the system.

Without conducting a tutorial on the overall subject, let me just give one
simple numerical example of a case where all that is needed is a simple 1:1
balun.

Imagine an antenna with a feed point impedance of 50 ohms - that's not
unreasonable, although your antenna may be different. Now imagine a 450 ohm
ladder line coming from the feedpoint down to the shack. Let's make that
exactly 1/2 wavelength at the frequency of operation. That means that the
impedance terminating the transmission line is 50 ohms, and because the
transmission line is 1/2 wavelength long, you will see exactly 50 ohms
looking into that transmission line. Put your 1:1 balun on there and connect
your 50 ohm coax to that to serve as a lead-in to the shack, OR bring the
ladder line and balun into the shack and connect directly to you transmitter
output. You have a perfect match.

Don't get me wrong! I'm not advocating this as a practical system - all I'm
saying is that there are a lot of misunderstandings about impedances in
transmission lines and how to get the overall system into a matched
condition.

Bob Lay (W9DMK) in Dahlgren, VA
http://www.qsl.net/w9dmk