Transmission lines: coax and open wire

A feedline (transmission line) carries RF between the radio and the antenna. Its job is to deliver power with minimal loss and without radiating on its own.

Coaxial cable

Coax has a centre conductor inside a shield. It is convenient, weatherproof, and unbalanced (the shield is at ground). Common types are 50 ohm (RG-58, RG-213, LMR-400). Loss rises with frequency and cable length, so for long microwave runs you want low-loss cable or none at all.

Open-wire and ladder line

Two parallel conductors spaced apart form a balanced line with very low loss, even at high SWR — which is why the G5RV and many multiband doublets use a ladder-line matching section. The downside: it must be kept clear of metal and is harder to route indoors.

Characteristic impedance

Every line has a characteristic impedance (50 ohm coax, 300–600 ohm open wire) set by its geometry. Matching the line to both the radio and the antenna is what keeps power flowing forward instead of bouncing back.