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Which feed line should an antenna use?

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A feed line is the transmission line between a radio and its antenna. The main choices are coaxial cable (unbalanced, shielded, 50 or 75 Ω), open-wire or ladder line (balanced, 300–600 Ω, very low loss), printed microstrip and coplanar waveguide on PCBs, and waveguide at microwave frequencies. They are compared by characteristic impedance, loss per meter, power handling and velocity factor.[1][2]

How is coaxial impedance set?

For coax, Z0 = (60/√εr)·ln(D/d), where D is the inner diameter of the shield, d the center conductor diameter and εr the dielectric constant.[1] The velocity factor is 1/√εr: about 0.66 for solid polyethylene, roughly 0.8–0.88 for foamed dielectrics.

How does a line transform impedance?

A line of length l terminated in ZL presents Zin = Z0(ZL + jZ0tanβl)/(Z0 + jZLtanβl). A λ/4 line inverts impedance (Zin = Z0²/ZL) and a λ/2 line repeats it. These properties are the basis of quarter-wave transformers and phasing lines.[1]

How much loss is acceptable?

Cable loss rises with frequency roughly as √f (conductor) plus a term proportional to f (dielectric). At 2.4 GHz and above, thin cables can lose several dB in a few meters, so radios are often placed near the antenna. Mismatch increases line loss because of standing waves; ladder line tolerates high VSWR with little added loss, which is why it is used with tuners on multiband HF antennas.[2]

What about printed feeds?

On PCBs, 50 Ω microstrip or grounded coplanar waveguide carries the signal from the radio to the antenna; width depends on substrate height and permittivity. Keep feeds short, avoid unnecessary vias, and route them over continuous ground.[1][3]

Frequently asked questions

Why 50 Ω coax?

A compromise between minimum loss (about 77 Ω for air coax) and maximum power handling (about 30 Ω).

Why 75 Ω for TV?

It is near the minimum-loss impedance and matches folded dipoles via 4:1 baluns.

Does feed-line length matter?

For loss and impedance rotation; with a matched antenna, length mainly affects loss.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

  1. “Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
  2. “The ARRL Antenna Book for Radio Communications”. ARRL. multiple editions.
  3. “Antenna Selection Guide (SWRA161)”. R. Wallace, Texas Instruments. application note.
Categories: Feeding and matching