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Open-wire and ladder line
Low-loss balanced feed lines.
Low-loss balanced feed lines.
Open-wire line, ladder line and window line are all balanced feed lines: two parallel wires held apart by insulating spacers, mostly with air between them. They carry RF with very little loss, even when the antenna is badly mismatched, which makes them the classic feed for multiband wire antennas.
The two wires carry equal and opposite currents, so their fields cancel a short distance away. That is why the line radiates little, as long as the currents really are equal; see Baluns and ununs.
The impedance is set by wire diameter and spacing, and by the insulation between them (see Characteristic impedance). With air in between and wide spacing you get a few hundred ohms, much higher than coax.
Loss is low for two reasons. There is almost no solid insulation to absorb RF, and the high impedance means less current for the same power, so less resistive heating in the wires. The result: even at an SWR of 5 or 10, the extra loss is often modest on HF, which is the whole point.
It shines on multiband antennas with a tuner. A dipole cut for 40 m presents a very different impedance on 20 m, 15 m and 10 m. Fed with ladder line, the line can sit at a high SWR on those bands and still waste little power, and an antenna tuner restores 50 Ω at the radio. The tuner must have a balanced output or be paired with a 4:1 or 1:1 balun.
It loses to coax when you need convenience. Compared with coax:
| Open-wire / ladder line | Coax | |
|---|---|---|
| Loss at HF, even with high SWR | very low | higher |
| Near metal and walls | must be kept clear | can touch things |
| Routing | careful: stand-offs, no tight bends | easy |
| Connectors | none; terminals or a balun | standard |
| Impedance | 300 to 600 Ω typical | 50 or 75 Ω |