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Stability of Directive Broadcast Arrays

Author: Edmund A. Laport

In almost every modern application of a broadcast directive array the conditions that have to be maintained are rather critical. Once adjusted and performance-proved, it is essential that the system maintain its adjustment and its pattern with a very high degree of stability. Work on the several hundred broadcast directive arrays in North America, using from two to six radiators (and with more extensive arrays in prospect), has revealed some fundamental principles that should be observed to obtain stability.

The commonest causes of instability include the following:

1. Variable resistance connections between sections of steel towers. All sections should be bonded at all corners by welding.

2. Loose or faulty connections in the ground system.

3. Loose or faulty connections in the feeder and network system.

4. Corrosion of connections.

5. Destructive corrosion of the ground wires in some acid soils.

6. The presence of flexible conduits in the field of ground wires and circuits of the feeder system, which causes a parasitic variation in impedance sufficient to produce considerable instability in some cases. The safest practice is not to use any flexible conduit for any purpose near the antenna circuits.

7. Static discharges over the antenna guy insulators, sometimes sustained by a power arc, which may be intermittently blown out by the wind and rekindled by static-charge accumulations. The cure for this condition is not always the same, but static leaks in the form of needle gaps across the insulators with a large-value heavy-duty resistor in series have given some measure of relief.

An experienced directive-antenna engineer does not start any work on system adjustment until the self-impedance of each radiator has been measured and until the radiator resistance remains constant within 1 or 2 percent when the radiator is vibrated, the ground connections are pulled and vibrated, and all the various conduits and wiring in the coupling house are touched, shaken, and grounded. If there is no wind to shake the tower, it can be set into vibration by pounding a guy or by pulling on a guy with a rope. When the radiator resistance remains constant under these various tests, the adjusting procedure starts.

When the network adjustments have been made and confirmed by field-strength measurements, permanent connectors of rigid form are installed and the pattern measurements are again confirmed. All connections are then brazed or soldered and all variable elements locked in place. The phase monitors and remote ammeters, having been previously calibrated, are also bonded and sealed to prevent false indications of trouble. Finally, the equipment is locked up so as to be inaccessible to everyone except an authorized person. Tamperproofing is an important factor in array stability.

Until one has had the personal experience of adjusting a critical directive array, there is small significance to the innumerable points of technique that come into notice during measurement.

The intrinsic electrical stability of a directive array depends upon the design parameters of the system. In general, the greater the degree of radiation suppression, the more sensitive is this null to small variations in the phases and amplitudes of the various radiator currents. The latter are in turn responsive to changes in system or individual radiator impedances. As the specifications for pattern stability become more stringent, greater attention must be given to every detail of design and construction to ensure constancy of impedances under all weather conditions over long periods of time. The stability demands of certain modern medium-frequency directive arrays with extreme suppression over wide angles are indeed extreme and require great ingenuity in design and skill in adjustment.

FIG. 2.52. Aerial view of a high-power broadcasting station (Radio Station WJZ when it was located at Bound Brook, New Jersey), showing the station building, the 750-foot radiator, and the freshly furrowed ground where the ground wires were plowed into the soil. (Photograph courtesy of National Broadcasting Company.)

The intrinsic stability of a proposed pattern can sometimes be examined very simply and quickly in a way which will inform the designer of the

FIG. 2.53. Aerial portion of a 750-foot guyed broadcast vertical radiator used by Radio Station WJZ. (Photograph courtesy of National Broadcasting Company.)

critical parameters and the tolerances he must maintain. This is done graphically by plotting the vectors for a null or minimum point according to the equation on page 148. If, for instance, a three-element array requires a very deep minimum at some angle, this is the angle where the field-strength vectors due to the three radiators add to a zero or some relatively small value with respect to any of the three vectors. By drawing these vectors carefully to scale, with their correct relative amplitudes and phases for the null direction, one may then proceed to the graphic study of the effect on the scalar-value resultant field of varying the amplitudes and phases of the individual vectors. If the resultant field must be kept under a certain limit, one can determine whether or not the pattern stability can be maintained within the necessary tolerances in practice.

An example of this sort is given in Fig. 2.51. This shows a desired resultant field-strength vector R, which is the sum of vectors 1, 2, and 3 from the three radiators. A circle of radius M represents the maximum allowable field strength under operational tolerances. Vector 1 must beŽallowed some variation of phase and amplitude, as indicated by the dotted angular deviations and maximum and minimum amplitude limits.

FIG. 2.54. Base of a guyed broadcast vertical radiator, coupling house, protective fence, and the 10-inch concentric transmission line suitable for 500-kilowatt service. Radio Station WJZ at Bound Brook, New Jersey (location now at Lodi, New Jersey). (Photograph courtesy of National Broadcasting Company.)

The end of vector 1 can therefore lie in a small rectangle A between the maximum and minimum amplitude limits and the maximum and minimum angular limits. Vector 2 starts somewhere within this area, at the end of 1, and must, of itself, have certain possible limits of amplitude and phase, bringing its end somewhere in a larger rectangular area B. The same takes place in turn with 3, and the possible area of total variation due to 3 with the cumulative deviations of the other two must always fall within the circle M.

Mutual impedances make it impossible for any one vector to vary independently from normal, so that a deviation in one implies a deviation in the others. If more than three vectors are involved, the problem is that much more complicated. An exploratory examination of this sort is qualitative only (unless one undertakes all the labor required to recompute the entire impedance network of the system with each change of value), but with experience a great deal of information regarding stability tolerances and the degree of refinement that may be required to maintain operational limits can be gained by such a qualitative study.

Array instability is due to changes in the impedances of the system at the frequency for which it was designed, the impedance changes being caused by spurious influences. The same kind of effect occurs when the

impedances change, not because of spurious effects, but with a change of frequency, as during modulation.

FIG. 2.55. A top-loaded sectionalized guyed vertical radiator and its base detail. Radio Station WMAQ, Chicago, Illinois. (Photograph courtesy of National Broadcasting Company,)

The impedances are different for each side frequency, the difference increasing with increased side-frequency separation. The impedance changes cause deviations in the amplitude and phase of the various radiator currents with consequent deviations in the pattern shape. In systems with high degrees of suppression, the carrier frequency may be sufficiently suppressed, but the side frequencies may leak out of a null during modulation and cause some interference. This is another reason why the bandwidth of the system as a whole has to be engineered to avoid or to minimize this effect.

Last Update: 2011-03-19