31. Caring for Twins
or Handling Parallel Feedlines

L. B. Cebik, W4RNL

Call it twinlead, open-wire line, or parallel transmission line, the concept is the same: two wires with a constant spacing make up a very useful form of transmission line to convey power from the transceiver or antenna tuner to the antenna (and back) with minimal loss. We use parallel line whenever coax will not do the job, for example, with multi-band doublets and loops where the impedance at the antenna terminals may range from very high to very low.

We have all seen the handbook accounts of how to figure parallel line--at least when we have air as the dielectric between the wires. See Fig. 1.

The characteristic impedance (Zo) of the line depends on the wire diameter (d) and the center-to-center spacing (S) between wires:

But what does this mean in practical terms involving the range of wire to which we have access? In terms of AWG wire sizes and spacing between wires in inches, the table below gives a good sense of what impedance results from what wires and gaps.

           Parallel Wire Transmission Lines

Wire Size (AWG)         #8      #10     #12     #14     #16
Wire dia. (in.)         0.128   0.102   0.081   0.064   0.051
Spacing (inches)        Characteristic Impedance (Zo) in Ohms
0.5                     246.0   273.7   301.5   329.3   357.1
1.0                     329.0   356.8   384.6   412.4   440.2
1.5                     377.6   405.4   433.2   461.0   488.8
2.0                     412.1   439.9   467.7   495.5   523.3
2.5                     438.9   466.7   494.5   522.3   550.0
3.0                     460.9   488.5   516.3   544.1   571.9
3.5                     479.2   507.0   534.8   562.6   590.4
4.0                     495.2   523.0   550.8   578.6   606.4
4.5                     509.3   537.1   564.9   592.7   620.5
5.0                     522.0   549.7   577.5   605.3   633.1
5.5                     533.4   561.2   589.0   616.8   644.6
6.0                     543.8   571.6   599.4   627.2   655.0

Fig. 2 provides a graphic view of the same information. Note that the impedance increases rapidly as we move from 0.5" to 1.0" spacing, but then tapers off. By the time we reach a 3" spacing, the impedance climbs slowly, and we are within the most common region of parallel line Zo values: 400 to 600 Ohms.

These values apply only to wire pairs when air is the dielectric (insulation) between them. A few spacers along the way do not materially change the Zo. However, the impedance will undergo significant change if we place a different dielectric between the wires, as in the case of the flat twinlead shown in Fig. 3. The special vinyl has a dielectric constant higher than that of air (1.0) which modifies the calculation of the Zo. As well, the line acquires a Velocity Factor (VF) of less than 1.0 (which open-wire lines closely approximate) with a special consequence. An electrical wavelength of line is now shorter than a physical wavelength of line. To figure a precise electrical wavelength of line, we multiply the physical length of a wave by the VF. Common twinlead has a VF of about 0.8, although it varies with manufacturing quality. To raise the VF, makers invented tubular line, with more air between the conductors.

Remember that a transmission line operates by virtue of the very close coupling of fields in each wire that result from current in the line. The currents at any point in an ideal line are equal in magnitude but opposite in phase. Hence, we get no radiation, since the fields cancel, and the energy is simply guided to something, like an antenna or a receiver input, where it can be used.

In a parallel transmission line, the strongest field exists between the two wires, but field components surround the wires and the wire pair. In tubular twinlead, the strongest portion of the field has mostly an air dielectric, compared to flat twinleadþs vinyl. Hence, a difference in VF, and a slight differences in losses. If you use twinlead for your parallel transmission line, use the very best quality that you can obtain.

If you prefer open-wire or ladder line, you can purchase good quality line, or you can make it yourself. Fig. 4 and Fig. 5 show two common construction methods.

When using tubes, rods, or dowels, holes or slots, with a means of locking wires in position, create the most durable line. Almost any non-conductor can be pressed into service, from wood boiled in paraffin to polycarbonate rods (the best material). One can use rods cut from plastic coat hangers, CPVC tubing, and similar materials that are easy to obtain, but at a cost: the ladder-line rungs do create a slight loss, so the better the material, the better the line.

One key point is to ensure that the material used is UV resistant, or the separators may grow brittle and break up over time. For this reason, and for its high RF insulation properties, polycarbonate (trade name Lexan) may be among the very best materials to use.

You can press other UV resistant materials into service, for example, remnants of vinyl siding. By cutting slats and using the technique in Fig. 5, a very durable line can be formed. However, the more material between lines, the lower the VF.

We have been discussing ladder line (open-wire parallel transmission line, to put in all of the words) from the construction perspective. Our goal is to construct the most durable, low loss line that we possibly can. However, unless we exercise good installation practices, we can undo all of the low-loss promised by such transmission lines.

We have noted the key factor that governs the installation of any parallel line, whether twinlead or open-wire. Although the fields between the wires are very tightly coupled, they are not confined to just the area between the wires. The fields extended for a considerable distance around the wire pair, perhaps up to several times the space between wires. These fields must not be disturbed, lest we create an imbalance in the current magnitudes and phases between the wires. When the current balance is upset, the differential in current becomes a field that expands without limit, which happens to be the definition of an antenna. Any energy radiated from the line is energy not available to the antenna itself.

There are a series of common practices that we need to avoid when using parallel lines. Most of the bad practices arise from hasty or careless installation, and the users often claim that they work--often because they have not compared the results with a proper installation. Sometimes, you cannot know that bad is bad until you have something good with which to compare it.

Fig. 6 illustrates some of the worst of the common practices. Placing parallel lines across or along metal, such as gutters, downspouts, aluminum window frames, interior house-wiring or ductwork is an open invitation to couple valuable energy into objects that not only will leave less to be radiated by the antenna, but as well may create RF hazards or irritations in the home or shack. The RF can get into telephones or into the radio gear itself. It only requires a few millivolts to disrupt the operation of keying, VOX, and similar circuits. As well, the RF feedback can add an FM component to a usually clean VFO signal or show up as hum on an audio line.

The remaining bad practices are mostly loss sources. Paralleling a line too close to a tree or other support can create line imbalances of lost energy that may vary with the weather. Trees and posts are variable conductors, depending on dampness.

Running lines along the ground or under a house in the crawl space may not show up in RF problems in the shack. However, coupling of energy to the ground increases line losses, even if the line is run through a PVC pipe or a similar means of isolating the line from direct ground contact. A through-wall entry may be needed, but it should be as short as possible.

Let's finish up on a positive note by listing a number of good amateur practices associated with parallel feedlines. These practices apply to both twinlead and open-wire lines and are sumarized by Fig. 7.

First, keep line runs as straight and in the clear as possible. Straight, clear runs are as important indoors as outdoors. Straight is self-evident. Clear means as far from other objects as possible, and in no case less than several times the line spacing away from anything.

Of course, we must bring the line indoors. We can use a short through-wall pipe, perhaps with caps that have cuts to keep the line centered. Or we can use a wood or plastic plate with feed-through insulators. The difference in spacing and bolt size of the board relative to the line is not important: it may create a small impedance bump but will minimize losses.

Outdoor supports can be of two general types: rings or clamps. We can suspend non-conductive rings (slices of PVC or similar) from limbs and posts to support the line on its way to the antenna. As well, we can create non-conductive guides or clamps that extend outward from tree trunks, posts, or walls to route the transmission line. Be sure to use enough supports.

Wherever possible, keep direction changes shallow. Never let the line fold back upon itself or roll it in a coil.

At the junction with the antenna, use a strain-relief fixture. A simple insulator may keep the line from being pulled by the antenna wire. However, over a relative short time, the line wires will flex back and forth until they break. A fixture that minimizes the flexing at the junction itself will make the connections much more durable.

These general guidelines will tend to give you the most efficient parallel line installation possible. Combined with good line construction, they will provide an effective and durable feed system for most antennas, and especially for multi-band wires.

Updated 11-05-2003. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.

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