The Truth About Spiral Shield Cables

Normally when a spiral shield is used on a cable it is for one of three reasons, increased flexibility, ease of termination, or reduced manufacturing costs.

Let's consider the differences between a spiral shield cable and an ideal, solid, homogeneous shield cable. In the solid homogeneous shield cable the shield current is longitudinal along the axis of the cable and the magnetic field, produced by the shield current, is circular and external to the shield.

In the case of a spiral shield cable the shield current 'I' follows the spiral and is at an angle ø with respect to the longitudinal axis of the cable, as shown in the figure below.

The total current 'I' in the shield can be decomposed into two components. The longitudinal current I(L) along the cable axis is equal to I cos ø, and the circular current I(c) perpendicular to the cable axis and around the circumference of the cable is equal to I sin ø. This is shown in the figure below.

The longitudinal current I(L) acts the same as the shield current on a solid homogenous shielded cable.

The circular current, however, forms a solenoid (coil of wire, or inductor) along the axis of the cable and produces a magnetic field along the axis of the cable. This magnetic field will increase the inductance of the shield.

In the case of an infinitely long cable (an approximation to a long skinny cable) the continuous circular current sheet will produce a longitudinal magnetic field 'H' inside the shield and no magnetic field outside the shield, as shown in the figure below.

This is just the opposite of the magnetic field produced by the longitudinal shield current, which exists outside the shield with no magnetic field inside the shield.

Spiral Center Conductor

If the center conductor is also spiral wound around an insulating center core, it acts the same as explained in the above discussion for the spiral shield. Therefore, the circular component of the center conductor current will produce a longitudinal magnetic field component in the insulating central core, increasing the inductance of the center conductor.

For the video cable in question, the spiral of the center conductor is wound in the opposite direction to that of the shield spiral. Although the longitudinal components of the shield current and the center conductor current are in opposite directions (and produce canceling magnetic fields), the circular components are in the same direction as shown in the figure below. Therefore, the longitudinal magnetic fields produced by the circular currents on the shield and the center conductor are in the same direction and add, further increasing the inductance of both conductors.

As a result of this increased inductance, the characteristic impedance of the cable will be larger than what you would predict by the dimensions alone.

AC Resistance

The next question to ask is, are their any other advantages/disadvantages to this type of spiral construction of both the shield and center conductor? The answer is, yes.

Both the shield and the center conductor are tubular, with spiral current flow; hence a longitudinal magnetic field exists inside the tube. Due to the magnetic field inside the tube there will be current on the inside of the tubular conductors. This means that the current will flow through more of the conductor cross section, thus reducing the skin effect and decreasing the AC resistance of the conductors. The skin effect is related to eddy currents within the conductors, hence the construction will reduce the eddy current loss, but at the expense of increased inductance.

Therefore, the cable behaves as a normal coaxial cable (as a result of the longitudinal current component) with additional inductance and reduced ac resistance in series with both the shield and center conductor (as a result of the circular current component).

Shield Transfer Impedance

The shielding effectiveness of a coaxial cable can be expressed in terms of the shield transfer impedance [1]. The lower the transfer impedance the more effective the shielding. This means that the signal stays on the inside of the cable and any noise present remains on the outside of the cable, and "never the twain shall meet."

At low frequency the transfer impedance is equal to the dc resistance of the shield. As frequency increases the transfer impedance of a solid homogenous cable shield decreases, due to skin effect, and the shielding of the cable increases. Skin effect causes the noise currents to remain on the outside surface of the shield and the signal currents to remain on the inside surface of the shield.

For a spiral shield cable, however, the transfer impedance contains two terms, one due to the longitudinal component of the shield current that decreases with frequency (which is good), and the other due to the circular component of the shield current that increases with frequency (which is bad). The net effect is that the shield transfer impedance increases with frequency above about 1 MHz. The high frequency transfer impedance is a strong function of the pitch angle, the larger this angle the larger will be the transfer impedance and the less shielding effectiveness the cable will have.

Normal braided shield cables also have shield transfer impedances that increase at high frequencies [2], but the increase is much less than that for the case of a spiral shield cable.

Conclusion by Dr. Henry Ott

The cable in question, with a spiral wound shield and spiral wound center conductor, will have increased inductance and increased characteristic impedance. It will also have reduced ac resistance and increased high-frequency shield transfer impedance (less shielding effectiveness).

Does the advantage of a lower AC resistance overcome the disadvantages of increased inductance and increased shield transfer impedance when used for video signals? The answer is clearly, NO! Spiral shields (as well as spiral center conductors) are in essence inductors (coils of wire), which make them inappropriate for use at video or any other high frequencies. In addition, the increased shield transfer impedance will reduce the high- frequency (above 1 MHz) shielding effectiveness of the cable. Video signals are especially susceptible to such HF-interfering signals if coupled into the cable.

Spiral shields are basically coils of wire exhibiting inductive effects and large high-frequency shield transfer impedances. Therefore, spiral shield cables should not be used in high-frequency applications (such as video), and their use should be limited to low frequency applications (such as audio).

A good discussion of spiral shields is the book by Anatoly Tsaliovich [3]. I often refer to this book as containing; "more than you ever wanted to know about cable shielding."

Henry W. Ott
President
Henry Ott Consultants

References
[1] Ott, H. W., Noise Reduction Techniques in Electronic Systems, Second Edition, John Wiley & Sons, 1988, page 55.
[2] Ott, H. W., Noise Reduction Techniques in Electronic Systems, Second Edition, John Wiley & Sons, 1988, pages 62-63.
[3] Tsaliovich, A., Cable Shielding For Electromagnetic Compatibility , Van Nostrand Reinhold, 1995, pages 187-194.

Article Epilogue

The poor quality of shield in this exotic cable only tells part of the story behind the signal loss and ghosting I experienced when using it for HDTV applications. Characteristic impedance is the next issue we will examine with this cable example and/or other exotic cables in the industry touted as being ideal for both audio and video applications. The sad reality is the cost of materials that make up this cable are less than even most free OEM patch cords. Yet the unwary customer at a high-end storefront may never know this since they observe a $1200 price tag for a 2 meter cable. Toss in a snazzy salesperson spouting the manufacturer's pseudo science and snake oil claims (and the wonders of what this cable can do for their precious home theater system) and you can see why we desire to educate the masses.