Open Wire and Ladder Line

Before coaxial cable became affordable and widely available after World War II, practically every HF amateur antenna was fed with open-wire feeders — two parallel bare conductors held apart by ceramic or Bakelite spreaders, running from the antenna through a window lead-in down to a transmatch or antenna tuner in the shack. That old technology has never been surpassed for one specific application: feeding a multiband HF dipole across a wide range of frequencies with minimum loss.

Open-wire feeders and their commercial descendant, window-type ladder line, achieve their remarkable low loss because they have almost no dielectric between the conductors — the gap between the wires is mostly air. Where coaxial cable might lose 2–3 dB per 100 feet at HF, a well-installed run of ladder line loses about 0.1 dB over the same distance. That is twenty to thirty times less loss. For a 100-watt transmitter, this means 98 watts reaches the antenna system, not the 60–75 watts that might arrive through a long run of standard coax.

Three balanced parallel transmission lines compared. The wider the spacing and the less plastic between the conductors, the lower the loss and the higher the characteristic impedance. Open-wire feeder (far right) achieves the lowest loss of all three.

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Why Open-Wire Feeders Have Low Loss

Recall from the coaxial cable lesson that feedline loss has two components: conductor loss and dielectric loss. Open-wire feeders dramatically reduce both:

Dielectric loss is almost zero: In a coaxial cable, all the field energy is stored in solid polyethylene or foam between the conductors. Even foam-dielectric cables are perhaps 40% air, 60% plastic. Open-wire feeders hold the two conductors apart with very small spacers — often just a short piece of plastic or ceramic every 12 inches or more. Between the spacers, the dielectric is air. Since air is essentially lossless, the dielectric loss of open-wire feeders is negligible even at VHF.

Conductor loss is reduced: Because the characteristic impedance of open-wire feeders (typically 450–600 ohms) is much higher than coaxial cable (50 ohms), the current in the conductors for the same power level is much lower. Power = V × I and V/I = Z₀, so for the same power at higher impedance: I = √(P/Z₀). At 450 ohms versus 50 ohms, the current is √(50/450) = 0.33 times as much. Since conductor loss goes as I²R, the resistive loss is (0.33)² = 0.11, or about one-ninth as large as it would be in a 50-ohm system with the same conductor size. This factor, combined with the absence of dielectric loss, is why open-wire feeders are so dramatically more efficient than coaxial cable.

The penalty is that open-wire feeders are balanced, unshielded lines. They cannot be routed next to metal objects, cannot be coiled without changing their impedance, and require a balanced-output antenna tuner or a balun at the tuner to connect to the unbalanced 50-ohm world of your transceiver. These practical complications are real — but for the operator who runs a full-size dipole or long-wire on multiple HF bands, the low loss of open-wire feeders is hard to match.

Types of Parallel Transmission Lines

Window line (ladder line)

The most common form of open-wire-style feeder sold today is 450-ohm window line, also called ladder line. This cable consists of two parallel conductors (typically #18 AWG tinned copper) held apart by a polyethylene ribbon with rectangular windows cut out. The windows keep most of the space between the conductors as air while the small plastic bridges maintain a consistent spacing.

Standard 450-ohm window line is about 1 inch wide, flexible, easy to work with, and available from most amateur radio suppliers. It handles surprisingly high power — the manufacturers typically rate it for 600–1000 watts at HF — because its high impedance means the current is low even at high power levels.

Wet weather has essentially no effect on ladder line loss because the dielectric is mostly air. Even with rain coating the plastic, the loss barely increases, unlike 300-ohm twin-lead (where the solid plastic becomes lossy when wet).

True open-wire feeder

The original open-wire feeder uses two bare copper or copperweld wires spaced 4–12 inches apart by ceramic or plastic spreaders placed every 12–18 inches. With a 6-inch spacing between AWG 14 (2 mm) conductors, the impedance is approximately 600 ohms. True open-wire feeders have the lowest loss of any practical transmission line, but their construction requires more effort and they must be kept well clear of obstacles. They are rarely installed new today, though they remain in service on many older stations.

300-ohm twin-lead

300-ohm twin-lead was the standard feedline for TV antennas for decades. It uses two #22 AWG conductors embedded in or separated by a solid polyethylene ribbon approximately 8 mm wide. Its solid dielectric causes significantly higher loss than window line when wet, and it has largely been replaced by coaxial cable in TV applications. It is occasionally used in amateur radio for portable VHF antennas where its flat profile is convenient.

Comparison table

Characteristic Impedance of Ladder Line

As you learned in lesson M13B, the characteristic impedance of a parallel two-wire line is:

Z₀ = 276 × log₁₀(2S/d)

For commercial 450-ohm ladder line, the conductors are typically #18 AWG (1.02 mm diameter) spaced about 28 mm (1.1 inches) center-to-center:

Z₀ = 276 × log₁₀(2 × 28/1.02) = 276 × log₁₀(54.9) = 276 × 1.740 = 480 Ω

Close to the nominal 450 Ω — the slight reduction from the ideal is due to the plastic bridges between the conductors, which slightly increase the effective dielectric constant.

The exact impedance of commercial ladder line varies between manufacturers and is not always critical. For antenna tuner operation (where the tuner matches whatever impedance the line presents), the exact Z₀ value is less important than the fact that the line is balanced and low-loss.

Ladder Line and High SWR: Why It Does Not Matter

Here is one of the most important conceptual points about open-wire feeders: it is completely acceptable — indeed, intentional — to operate with high SWR on ladder line. This confuses many beginners who have learned that high SWR is always bad.

The key is that SWR-induced additional loss is only significant when the feedline already has high matched-loss. When the matched-loss of the feedline is very small (as it is for ladder line), even a very high SWR adds very little additional loss. Consider a 100-foot run of 450-ohm ladder line at 14 MHz:

This is why the classic "G5RV" and "ZS6BKW" doublet antennas — and indeed almost any center-fed wire antenna of arbitrary length — can be operated across all HF amateur bands by feeding them with ladder line and using an antenna tuner in the shack to match the line's varying impedance to the transceiver's 50-ohm output. The tuner sees whatever impedance the ladder line presents at its input (which varies with frequency and antenna length), and transforms it to 50 ohms. The feedline loss is so low that the system works efficiently regardless of the antenna's electrical length at any given frequency.

Installation Rules for Ladder Line

Open-wire feeders are more demanding to install than coaxial cable, but following a few rules ensures proper operation.

Keep it away from metal objects

The electric and magnetic fields of an open-wire feeder extend outward from the conductors into the surrounding space — unlike coax, where the fields are confined inside the outer conductor. Any conducting object placed within a few inches of the line will distort the fields and change the line's impedance locally. As a rule, keep ladder line at least 6 inches (15 cm) away from metal gutters, downspouts, mast pipes, and conduit. Keep it at least 2 inches away from wooden siding, fascia, and brick. Wet wood has significant conductivity that also affects nearby balanced feeders.

Do not coil it

Coiling a balanced feedline creates a coil of mutual inductance that acts as a common-mode choke and changes the line's behavior. Excess ladder line length should be zigzagged in a back-and-forth S-shape, not coiled. The turns must be separated to avoid coupling between adjacent runs.

Do not allow it to touch itself

The two conductors of ladder line must maintain their spacing throughout the run. Where the line touches itself (from wind-blown contact), the impedance is short-circuited at that point. Keep the line under moderate mechanical tension or support it at intervals to prevent sagging and contact.

Support it at intervals

Ladder line can be supported every 6–8 feet using non-conductive spreader sticks (wooden or fiberglass dowels) to keep it taut and away from nearby objects. Plastic cable ties can clamp the line to a non-metallic support structure, provided they do not clamp both conductors so tightly that the spacing changes.

Route it in a straight line as much as possible

Right-angle bends in ladder line cause local impedance changes and increase radiation from the line. Where bends are unavoidable, make them gradual curves rather than sharp corners. The minimum bend radius should be about 4–6 inches for standard 450-ohm ladder line.

Bringing Ladder Line into the Shack

This is the trickiest part of a ladder line installation. You cannot simply run balanced open-wire feeders through a window hole alongside the metal window frame — the metal will distort the fields. Several approaches work well:

Feed-through insulator: A dedicated ladder-line lead-in insulator mounts through a small hole in an exterior wall, with the two conductors maintaining their spacing through the insulator body. This is the cleanest solution.

Window-entry plate: A weatherproof panel of polycarbonate or PVC with two binding posts provides a convenient feed-through for any window opening. The panel replaces part of the window sash and keeps rain out.

Short coax transition: Some operators convert the ladder line to coax via a 4:1 or 6:1 balun outside the shack, running a short coax pigtail through the wall to the tuner. This adds the loss of the balun and the short coax section but simplifies the house entry. For short transitions (under 10 feet), this works well.

Once inside the shack, connect the two conductors of the ladder line directly to the balanced input terminals of the antenna tuner. Keep the shack section as short as practical.

What Kind of Antenna Tuner You Need

You cannot connect ladder line directly to a standard T-network or L-network antenna tuner designed for unbalanced coaxial cable — those tuners have an unbalanced output that would force unequal currents in the two conductors of the balanced feedline, causing the line to radiate instead of containing its fields.

For ladder line, you need one of the following:

• Balanced output tuner: A tuner with a built-in balun or a true balanced output (such as the MFJ-949E or MFJ-998RT with balanced outputs, or the Johnson Matchbox and similar vintage designs). This is the preferred solution.
• External 4:1 balun at the tuner input: Mount a current 4:1 balun on the outside of the shack, connect the ladder line to the balun's balanced terminals, and run 50-ohm coax from the balun's unbalanced output to a standard coax-input tuner. The balun must be rated for the power levels involved and for use with high SWR.
• Current choke balun + standard tuner: A W2DU-style current choke at the tuner output, followed by a short section of 50-ohm coax to a balanced tuner terminal block, can also work.

The important principle is that the ladder line must see a balanced termination — equal and opposite voltages on its two conductors relative to ground. Any imbalance drives common-mode current on the outside of the feeder, which radiates and can cause RF in the shack.

Coax vs. Ladder Line: When to Use Each