Ladder line

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300Ω Twin Lead connected to a balun used for broadcast TV.
300Ω Twin Lead connected to a balun used for broadcast TV.

Ladder line or window line is a type of transmission line similar to twin-lead for balanced connection of antennas. Ladder line is constructed as a pair of evenly spaced wires with supportive plastic webbing holding the wires apart. The plastic webbing has windows cut in it to reduce its dielectric effect and reduce loss in the transmission line. The alternating webbing and windows gives ladder line is characteristic look and name.

Common ladder line types have characteristic impedances of 300Ω, 450Ω, or 600Ω with a velocity factor of 95%.

Ladder line is less expensive and has lower losses than other transmission line types, but is more sensitive to surrounding metal objects than coaxial cable.

[edit] Formulas for Calculating Parameters for Ladder Line

The formula to calculate the impediance of ladder line is:

Z_0 = \frac{120}{\sqrt{ _r}}cosh^{-1} \frac{S}{d}[1]

The formula to calculate the impediance of open (air dielectric) ladder line is:

Z_0 = 276 log_{10} \frac{2S}{d}[2]

The formula to calculate the distance between conductors of open ladder line is:

S\approx10^\frac{83-276 log_{10}d-Z_0} {-276}

Where:

  • Z0 = Impedience.
  • S = Center to center distance between wires.
  • d = Diameter of the wire.
  • r = Effective dielectric constant (Air = 1.00054).

The unit of measure for S and d are not critical as long as they are the same.

[edit] References

  1. ^ Balanced Transmission Line in Current Amateur Practice, ARRL Antenna Compendium, Volume 6. Wes Stewart, N7WS.
  2. ^ ARRL Handbook for Amateur Radio 2000, Pg 19.3.
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