|Antenna 007: Ladder Dipole
Author: Frederick R. Vobbe, W8HDU
February 1, 2008
|When you walk around a hamfest, you can always find someone selling Ladder Line. Ladder line is typically a 450-ohm line, however some people also consider 300-ohm line in this catagory. Personally, I group 300-ohm line into the arena of transmission line used for FM and TV. So let's stay focused on 450 ohm line.
Ladder Line, (seen at the left), is a 450-ohm transmission line with two 16 AWG, strand copper-clad steel parallel conductors spaced apart, and encased in and separated by a dielectric material. The dielectric material is provided with a plurality of spaced apart rectangularly shaped cutouts that yield the 450-ohm impedance. Ladder line can handle power up to 1,500 watts.
In the figure below, this antenna is cut for 15 meters (21 MHz), and 10 meters (28.4 MHz)
The way you match this antenna is to construct the antenna longer than you need, then trim back the wire to give a good match on your VSWR meter. Below is a chart showing "aproximate" line lengths.
In some cases, where you might have high winds, the line may flap around and even twist. The way to solve this is to just attach some weight on the bottom wire. The way I did it was to use some fishing line and a few lead sinkers. If the string hangs down at least 12", it will keep the wire from flipping around.
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