By: O. Bendov

INTRODUCTION

The availability of suitable locations for new TV towers is diminishing even in the secondary markets. It is practically non-existent in major markets. Even after the hurdles of zoning variance and suitable tower location are overcome, FAA restrictions and environmental concerns may delay the construction of a new tower for years.

Not surprisingly, many broadcasters are looking at the pros and cons of using existing towers to support their new HDTV antennas even though the prime tower-tops are occupied.

For any given antenna, directional or omnidirectional, the tower will modify the as-designed antenna patterns. For optimum coverage, the as-installed patterns must be known not just at the carrier frequency but throughout the entire channel before the relative position of the antenna and its azimuthal pattern orientation can be fixed. There is one, and only one, position which would provide the optimum coverage without exceeding the structural limitations of the tower. The optimum position can be calculated.

This paper will describe the application of the optimization process to HDTV and NTSC antennas on a typical triangular tower with a 10’ wide face.

TRANSMISSION TRANSFER FUNCTION

Figure 1 shows a side-mounted HDTV antenna positioned 8’ from the center of a typical triangular tower with a 10’ face. The tower contains two runs of 6-1/8" transmission line and one run of 18" circular waveguide. The tower lattice, the tower legs and the components inside the tower intercept a portion of the energy radiated by the HDTV antenna. The intercepted energy is then scattered in all directions. The scattered energy is combined, constructively in some directions and destructively in other directions, with the primary energy of the HDTV antenna.

Click here for Figure 1

The total field of the antenna with the tower, relative to the primary field of the omnidirectional antenna without the tower is shown in Figure 2. The patterns shown in Figure 2 were calculated at the mid-frequency of channel 38. Even if the transmitter and the antenna system are assumed to exhibit a constant amplitude and linear phase transfer function (output/input) across the channel and in all directions, the scatter produced by the tower structure will distort that transfer function, at least in some directions.

Click here for Figure 2

As an example consider the transfer functions of the distorted pattern shown in Figure 2 in the azimuthal directions of -71⁰ and +113⁰. The two transfer functions are plotted in Figure 3. In the direction of -71⁰ , the system’s transfer function remains undistorted with an overall power gain of 1.3 dB. In the direction of +113⁰ , the system’s transfer function is distorted, with power loss depending on frequency. For simplicity, the phase distortion is not shown. The antenna system is analogous to a network with multiple output ports, one in each direction of interest. Moreover, the transfer function at each output port may be different than that of other ports.

Click here for Figure 3

Knowledge of the transfer function of NTSC antenna systems is not necessary for contour optimization because NTSC is a narrow-band transmission. Most of the NTSC picture information is contained within 1.5 Mhz of bandwidth. For this reason, the patterns are calculated at a single frequency, normally the carrier frequency, and are assumed constant within the critical bandwidth of 1.5 Mhz.

Because knowledge of the transfer function of NTSC antenna systems is not necessary, the optimization of NTSC contours is based on the calculation of the distorted field pattern such as that shown in Figure 2, at the carrier frequency only. The pattern may be optimized by changing the location of the antenna relative to the interfering structure and by recalculating the distortion at the carrier frequency until an acceptable pattern is achieved. Increasing the distance between the NTSC antenna and the interfering structure is generally advantageous as long as the distance remains £ 100’. At larger separations the reflections resolve into visible ghosts. The end point of NTSC optimization is the starting point of HDTV optimization.

Knowledge of the transfer function for HDTV antenna systems optimization is necessary because HDTV is a broad-band transmission. All of the HDTV picture and sound information is spread equally within most of the 6 Mhz of bandwidth. A carrier is not visible in the channel’s spectrum, and good response at all frequencies within the channel is necessary for optimum transmission. For this reason, the patterns must be calculated throughout the channel. An HDTV pattern calculated at a single frequency is not sufficient to determine the coverage unless the transfer function in each direction is known.

The number of cycles in the transfer function, and the possibility of creating a notch in it, depend on the distance (in wavelengths) between the antenna and the interfering structure. Because the antenna in Figure 1 is only 8’ away, less than 1/4 of a cycle is evident in the direction of +113⁰, as seen in Figure 2. A notch in the frequency response would cause a loss of HDTV picture and sound due to the high penalty, as explained in the next section.

COVERAGE PENALTY

Once known, the transfer function can be processed to determine the total penalty, in each direction, that must be assessed against the undistorted pattern of Figure 1.

The total penalty, expressed in ± dB, is composed of two factors. The first factor expresses the loss (or gain) of the total signal power within the 6 Mhz channel. For example, the penalty due to the loss of power in the direction of +113⁰ is 5.55 dB even though the power is down 8.4 dB at the low end of the channel and 3.2 dB at the high end of the channel. The second factor expresses the loss of carrier-to-noise ratio margin caused by the equalizer at the HDTV receiver. The equalizer, in attempting to flatten frequency response will increase the system’s gain and noise at selected frequencies thereby lowering the carrier-to-noise ratio margin. The equalizer penalty at +113⁰ is 2.30 dB. The total penalty in that direction is 7.85 dB. The total penalty (± dB), in any direction, against the undistorted pattern of Figure 2, is shown in Figure 4.

Click here for Figure 4

Coverage penalty is unique to HDTV because all undesired energies, such as reflections, translate into a loss of coverage, whereas in NTSC the undesired energies translate into a loss of picture quality.

COVERAGE OPTIMIZATION EXAMPLES

The objective is to optimize the coverage of a side-mounted, directional antenna, such as to provide service as similar as possible to that of the omnidirectional antenna mounted on the tower-top. The viewer population is not evenly distributed around the tower. The population density is heavy in the NW direction (city of license) and sparse in the SE direction. The tower and the antenna’s location in the NW quadrant are shown in Figure 1.

Two different antennas, a broad-band panel antenna and a single channel slotted-pipe ("pylon") antenna, with cardioid azimuthal patterns shown in Figures 5a and 6a were selected.

Click here for Figure 5a

Click here for Figure 6a

The first step is to minimize the distortion of the azimuthal pattern due to the tower effects at the mid-channel frequency for HDTV and at the carrier frequency for NTSC. This is done by changing the antenna’s position relative to the tower and by rotating the antenna on its axis to orient the antenna’s pattern within the service area.

The results of this first step are shown in Figures 5a and 6a. The position of the antenna is 8’ from the tower’s center. The direction from the antenna to the tower’s center is -80⁰. The direction of the center-of-symmetry of the antenna’s pattern is +122⁰.

At this point, if the objective were NTSC coverage optimization, the panel antenna of Figure 5a would be chosen because it is less directional than the pylon antenna, and as such, would provide coverage over a wider sector than the pylon antenna.

For HDTV coverage optimization, the same conclusion cannot be reached without assessing the appropriate penalty. The penalties on the two antennas are shown in Figures 5b and 6b. The sector of no service behind the support tower is twice as wide for the panel antenna as it is for the pylon antenna.

Click here for Figure 5b

Click here for Figure 6b

The advantage of the pylon antenna for this HDTV application becomes clearer with the calculation of the FCC contours (flat terrain) for the two antennas after the penalties were assessed against the as-designed antenna patterns. The use of the FCC contours here is not to depict actual service but to compare the relative merits of the two antennas. Actual service determination requires the knowledge of actual terrain and cochannel interference.

FCC HDTV Contours of Cardioid Panel Antenna

FCC HDTV Contours of C170 Cardioid Pylon Antenna

CONCLUSION

NTSC coverage optimization is the process of maximizing the radiated power at the carrier frequency within the area of critical service. The scattering from nearby structures will affect the picture quality but will otherwise play a secondary role in determining the coverage contours. A notch in the frequency response across the channel, if not at any of the three carriers (picture, color and sound), may not significantly affect program viewability. However, the availability of the NTSC carrier does not guarantee the availability of HDTV signal.

HDTV coverage optimization is the process of reducing the power penalty in the area of interest, possibly at the expense of areas where viewers are sparse. The penalty depends on the total signal power within the channel and the frequency response across the channel. In particular, a notch (or nearly one) in the frequency response, anywhere within the channel, will cause a loss of HDTV signal.