Twin-lead feeder (antenna to TV receiver) — typical inductance per metre What is the approximate inductance per unit length for a common twin feeder used to connect an antenna to a TV receiver?

Introduction / Context:
Balanced twin-lead (e.g., 300 Ω twin-lead) was historically used to feed TV antennas. Its per-metre inductance L and capacitance C determine the characteristic impedance and propagation velocity. This question asks for a typical L value for such twin feeders.

Given Data / Assumptions:

• Typical consumer twin-lead geometry and polyethylene dielectric.
• Order-of-magnitude engineering estimate rather than an exact spec.

Concept / Approach:
For transmission lines, Z0 ≈ sqrt(L/C). Common twin-lead has Z0 around 300 Ω with C around 60–70 pF/m. Rearranging gives L ≈ Z0^2 * C. Using C ≈ 67 pF/m and Z0 ≈ 300 Ω yields L on the order of 0.6–0.7 μH/m, matching handbook values.

Step-by-Step Solution:

Verification / Alternative check:
Transmission-line tables list L near 0.6–0.7 μH/m for 300 Ω twin-lead, consistent with the back-of-the-envelope calculation.

Why Other Options Are Wrong:

• 0.2 μH/m: too low for typical 300 Ω twin-lead parameters.
• 1.6 μH/m or 10 μH/m: too high; would imply very different C or geometry.

Common Pitfalls:
Confusing twin-lead with 50 Ω or 75 Ω coax values; mixing units of pF/m and μH/m.

Final Answer:
0.6 μH/m