The final installment of this three-part series offers practical applications for TM mode propagation along the surface of a single unconditioned conductor, such as a power line.
Earlier parts of this article (see May and June 2009 issues) described TM mode propagation on a single unconditioned conductor. The mode has many practical applications for last-mile information transport and distribution, for example by using the existing worldwide power grid, and can be applied as a lightweight transmission line for aerial antennas.
An obvious and promising class of applications for this transmission mode involves employing existing overhead electric power lines for last-mile information services. The low attenuation and broadband nature of the mode operating on the preexisting power-line infrastructure can provide a basis for low-cost information transmission in much of the populated world. Because the underlying hardware, rights of way, support and maintenance for power grids are already in place, the addition of high-capacity information transport can be quite inexpensive, particularly compared to other transmission methods such as digital subscriber line (DSL), community-accesstelevision (CATV), fixed or mobile wireless systems, or fiber-optic cable.
Previous examples of TM propagation have involved relatively small conductor diameters. Common power distribution and transmissionline conductors range in diameter from about 4 to 25 mm or even 50 mm. Modern power conductors are often constructed by winding multiple bare aluminum or copper wires around a central steel wire carrier that provides the resulting multistrand cable with extra strength and resistance to stretching under tension. Two or more individual cables are then placed under tension and supported by separate insulators mounted on periodic supports to form multispan segments of overhead power line. In much of the world, these supports are wooden "power poles" and may be 10 to 20 m tall and spaced 30 to 100 m apart. It is not uncommon for a single system of poles to provide support for multiple sets of lines, with a higher-voltage distribution line near the tops of the poles, possibly in conjunction with a step-down transformer. A second set of supports is positioned lower down the pole for lower-voltage lines that provide delivery to residential or business end-use sites located adjacent to the line. These lines are good candidates for last-mile information delivery systems. Because of the capability for very large bandwidth and low attenuation of the TM mode it is useful to examine the characteristics of practical TM mode power-line systems. These RF/microwave transmission systems using the TM mode that utilize overhead power transmission, distribution or delivery infrastructure have been dubbed "E-Line."
An example of a special slotted launcher adapted to mount on an existing power conductor is shown in Fig. 8. This launcher has a special triaxial section to allow coupling between a coaxial line and the surfacewave mode propagating along the aluminum power conductor without requiring any modification of the line conductor itself. The coaxial section is connected to bidirectional amplifiers, which are solar powered in this example, located behind the launcher and directly above a mechanical clamp that attaches the entire assembly to the power-line conductor. In this case, the launcher does not include any dielectric compensation to improve the impedance and mode match between the coaxial and TM modes.
Figure 9 shows a measurement of GAmax and S21 parameters for a pair of launchers like those shown in Fig. 8, mounted on approximately 18 m of #4 stranded copper power conductor. The bandpass nature of the coupler is made evident by the transmission response centered at approximately 2 GHz.
A second incidental response that is highly attenuated exists at about 500 MHz. The impedance match of this second response is very poor and results in a great deal of mismatch loss. The degree of this mismatch can be appreciated by comparing the GAmax measurements to the S21 response. As is apparent by comparing the plots, of the 7 dB total system insertion loss at 1900 MHz about 3 dB is due to port mismatch. Approximately another 3 dB is due to radiation loss from modal discontinuities of the uncompensated launcher and the remaining 1 dB loss is due to Ohmic losses in the 18 m of copper conductor.
While these particular launchers are not ideal, the measurements are useful for developing an appreciation of system characteristics. Of course, in power-line transmission and distribution systems, other factors contribute to attenuation, reflection, and radiation. Table 1 lists some common impairment factors and their characteristics at 2 and 5 GHz. Insulators normally account for no more than a few dB of extra attenuation. Tap lines that connect to a conductor and lead directly away from the line, such as those at a step-down transformer, interrupt the field lines in one plane and only cause about 3 dB of extra attenuation. In general, impairments located close to the surface of the conductor tend to have more influence than those even slightly removed. This is to be expected since this is the location of the largest fields. The effects of line bends generally depend a lot on the detail of the conductor and insulator close to the bend itself. A small radius bend is more influential than a slower bend having a larger minimum radius of curvature. Most of these impairments have a uniform effect versus frequency and produce rather low group-delay perturbation of the transmitted wave.
To use overhead power lines for transport of RF and microwave information- bearing signals, a link budget analysis can be made in much the same way as for other wired or wireless systems. To examine the capabilities of E-Line, it is useful to compare the underlying transport ability, in terms of bandwidth, attenuation, and distance, with other methods. Figure 10 presents information capacity as a function of distance for several existing last-mile transmission methods compared to that for E-Line. These include HF-BPL, which is highfrequency (HF) transport using two power-line conductors; xDSL, using twisted-pair copper telephone lines; a free-space wireless radio with completely line-of-sight propagation; a wireless radio in a typical suburban environment; CATV using low-loss distribution cable; and E-Line using TM mode on single-conductor overhead power lines.
This approach calculates information capacity as a function of distance by use of Shannon's equation
For each method, the associated spectrum was subdivided into 100 segments and the information capacity for each segment was calculated based on distance, segment center frequency, signal power, and noise power. The information capacities of each of these subsegments were then summed to produce an associated maximum capacity. These results describe the maximum information rate possible if a perfect encoding and protocol is used. No allowances or margins for variation have been included. These results are therefore the upper bound rather than a description of practical systems. Unless noted, a source power of 0 dBm (1 mW) and information bandwidth of 100 MHz have been selected. Other relevant attributes are as shown in Table 2.
In addition to the plots for each of the methods, a limiting value for SNR of 30 dB is shown that corresponds to the maximum information capacity possible in 100 MHz bandwidth if such a limit were imposed by the protocol or hardware used to implement the system. This is an arbitrary limit but is similar to the necessary and useful carrier-to-noise (C/N) parameter for systems such as IEEE 802.11a, 802.11g, WiMAX, Long Term Evolution (LTE), and other communications standards.
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In order to transport information over very large distances, all of these methods require periodic amplification, possibly accompanied by demodulation and remodulation. Figure 10 reveals the maximum distance allowable between such amplification for a specific information rate. For E-Line installed on typical distribution lines with pole spacings of
100 m, amplification every few poles is necessary to maintain the majority of the maximum possible capacity. Additional source power can allow increased spacing while the requirement for system link budget margin will decrease it. Practical systems have been built with five to ten amplifiers per mile of line that have supported more than 2 Gb/s information capacity using less than 100 MHz information bandwidth.
Figure 11 depicts an E-Line system capable of providing both highcapacity end-to-end information transport as well as information distribution to end users near the line. Simultaneous usage of power poles as sites for "nanocells" to provide access for adjacent users while enabling near line-of-sight (free-space) radio paths, allows very high user data rates along with small user antenna aperture and low transmit power. An E-Line distribution system can easily include both an antenna and active circuitry at selected poles in order to tailor a coverage footprint along and in the vicinity of the power line system. In a situation where the communications system is already frequency division duplex, such as for mobile telephones, this can be as simple as bidirectional amplification and filtering. In this way a single E-Line installation can provide backhaul (transport), fronthaul (distributed antenna feed) and access (distributed antennas) for end users at 3G and 4G speeds.
The relatively low attenuation of E-Line allows simple amplification to suffice at each amplifying node, rather than requiring demodulation, remodulation and the attendant delays (latency) produced by these processes. The result is that simple RF and microwave electronics periodically included with pairs of launchers placed along a power line can inexpensively provide and maintain both transport and distribution of high-rate information services and content. Because the system follows the electric distribution grid, it can simultaneously be used to provide "Smart Grid" communications with end-use locations for real-time power management and billing.
Because of the extreme simplicity and relatively small dimensions of a low attenuation and high bandwidth TM mode system, use as an antenna feed line between ground-located communications equipment and high-altitude antennas is attractive. For many practical terrestrial communications systems, coverage may be severely limited by the presence of hills, buildings, foliage, and other similar obstructions that are relatively close to the earth. The impact of these impairments can be appreciated by comparing the free space attenuation with that of the suburban environment attenuation of radio signals. As much as 40 to 60 dB of excess attenuation is commonplace for many practical and desirable path lengths. However, by locating at least one antenna well above the impairments, the total radio path loss rapidly falls and allows much higher carrier/noise ratios, performance, and coverage.
To further illustrate this application, a lightweight biconical antenna was fabricated and integrated with a small forward-horn type launcher. A small helium-filled balloon was used to lift the antenna and the entire assembly was tethered by means of small gauge copper wire that also served as a lightweight TM feedline for the antenna. Although the 15-in. antenna was too short to operate optimally at 100 MHz, the balloon was allowed to support the antenna at about 2 m above ground and a reference measurement of a distant commercial FM broadcast signal was made. The transmitting antenna was approximately 150 km away and there was considerable intervening obstruction. As a result, the signal was at or near FM threshold and could not be fully demodulated by a standard FM stereo receiver. The copper wire tether was then allowed to play out and the balloon rose to approximately 60 m. At that elevation, the antenna was well above local foliage and clutter. The received signal amplitude rose by more than 30 dB. As the feed line and antenna were passive structures, the Lorentz reciprocity theorem applied and the antenna and feed line system could be expected to provide the same improvement at the distant location if used for transmitting rather than for receiving. Although balloon- and kite-lifted antennas have been in use for about a century, the lightweight and low cross-sectional area of suitable TM line conductor allows the antenna feed point to be located at high altitudes rather than at ground level. Thus, existing heavy communications equipment can remain ground-mounted but be easily used with temporary lightweight antennas located at elevation and the communications range and quality of common communications systems greatly increased.