By: E. Mayberry

Broadcasters facing the addition of HDTV transmission facilities must choose an antenna system configuration that provides good signal coverage, minimizes tower wind loading, and maximizes the available tower space. Tower top, stacked antenna systems provide an approach for achieving these desirable characteristics. Issues examined include: existing antenna replacement by stacked HDTV/NTSC antenna systems, stack height impact on gain and effective radiated power (ERP), tower loading considerations, and implications of the proposed DTV allotments. Several examples of stacked antenna combinations are presented. Alternative antenna configurations are reviewed comparing relative advantages and disadvantages.

STACKED ANTENNA ADVANTAGES

The stacked antenna approach offers several advantages for the broadcaster. To consider these advantages, refer to Figure 1, which illustrates the stacked antenna arngement for a UHF HDTV antenna over a channels 2-6 VHF NTSC Superturnstile.

Stacking the HDTV antenna on top of the NTSC antenna offers significant performance advantages. Superior omnidirectional pattern performance is achieved for the HDTV transmission. Circularity of the azimuth pattern is primarily a function of the electrical radius of the radiating elements. The top antenna has the minimum structural demands and is made with the smallest possible pipe diameter. Circularities are well under ±1 dB, more typically ±0.5 dB. Since every dB of signal reduction results in approximately one mile loss of coverage at the fringe, circularity performance is an important consideration.

Figure 1

Another obvious advantage is that the HDTV antenna has the highest possible center of radiation. As the majority of HDTV service will be in the UHF band, the height of the radiation center can significantly impact the coverage achieved. Solid UHF coverage occurs where direct line-of-sight is attained. Terrain, buildings, and even vegetation can severely reduce the signal coverage. Increasing the center of radiation may yield a bigger return in improved coverage than increasing the radiated power.

By using the stacked antenna approach, the lowest possible wind force coefficients are applied to the antenna projected areas. The current structural standards for steel antenna towers and antenna supporting structures, TIA/EIA-222-F, specify force coefficients for cantilevered tubular pole structures which are much lower than non-structural, side-mounted cylinders. This usually results in lower overall wind forces for the stacked combination of antennas compared to other two antenna configurations on the same tower.

Keeping both the HDTV and NTSC antennas above the tower top affords the broadcaster efficient and flexible use of his tower space. Auxiliary services and leases can maintain their positions below the tower top.

EXISTING ANTENNA REPLACEMENT

The potential for directly replacing existing NTSC antennas with a stacked HDTV/NTSC antenna combo exists for some broadcasters. By making the appropriate tradeoffs in gain/height, it is possible to offer designs in both low band (chs. 2-6) and high band (chs. 7-13) that can replace some common NTSC antennas.

For channels 2-6, a standard design is available to replace a 6 bay Superturnstile antenna. Limiting the Superturnstile to 3 bays yields enough aperture for an HDTV antenna. This design, shown in Figure 1, offers equivalent height with no increase in windload.

Another standard design is available for channels 7-13. This configuration, Figure 2, replaces existing TW-15A or TW-18A antennas. In this case, a special TW-9B-R antenna is used to support the HDTV top mount. The combination provides equal height to the TW-15A with only a 15% increase in wind load. It is shorter and lower wind load than the TW-18A antenna.

All the stacks with top mounted HDTV antennas use electrically center-fed, coaxial designs for the UHF slotted cylinder antenna. The center-fed design offers the best possible antenna output response in terms of amplitude and phase variation across the 6 MHz channel. For HDTV, these designs ensure that the transmission system introduces minimum distortion reserving equalizer capacity to cope with the defects of real world propagation.

Figure 2

GAIN & ERP

As discussed above, in order to fit these standard stacked antenna designs within existing antenna apertures, it is necessary to limit the gains of the supporting VHF antennas. There is also an impact on the available gain for the UHF HDTV antenna depending on the channel.

The low band replacement reduces the VHF gain by ¸ which doubles the required transmitter power if the original 2 x3-1/8" transmission lines are maintained. However, the transmitter power requirement can be reduced by increasing the line size as shown in Figure 3. With 4-1/16" or 6-1/8" line, the maximum 100 kW ERP is achieved with 44-45 kW transmitters

Figure 3

One approach to cost savings is to replace the two 3-1/8" lines with a single 6-1/8" line. By using a broadband transmission line, e.g., Dielectric’s DigiTLine, the NTSC VHF and HDTV UHF transmitters can share the line. This allows the low band broadcaster to make the conversion to HDTV without increasing tower loading.

The aperture limitations for the HDTV antenna result in an available gain variation versus UHF channel. With the channel 2-3 Superturnstile stack, the HDTV antenna gain varies from 20.5 x power at channel 14 to 30.0 x power at channel 48. Although gains greater than 30.0 are possible for channels above 48, the maximum gain is limited to 30.0 in order to control amplitude variations with frequency and wind deflection. The channel 4-6 Superturnstile stack limits the HDTV antenna gain to 15.0 at channel 14 and 26.0 at channel 69.

Maximum available ERP for the HDTV antennas for the low band stacks is given in Figure 4. The HDTV antennas are designed with 4-1/16" inputs which provides a 32 kW average power rating at channel 14 reducing to 24 kW average at channel 69. Multiplying the power rating times the gain produces the available ERP curves. As indicated, 500 kW to 800 kW ERP is available. The proposed DTV Table of Allotments1 indicates 563 stations with ERP’s greater than 500 kW. Although the available ERP appears low relative to the proposed allotments, these designs offer practical, economical configurations with excellent electrical performance. Consider that the transmitters to match the input power capacity of these antenna designs ( assuming 75% line efficiency) would have output powers of approximately 40 kW average. However, the HDTV peak/average ratio is 5:1 (7 dB) which translates to a transmitter peak power requirement of 200 kW. This power level compares to the largest transmitters currently in operation which offer 240-280 kW output power.

Figure 4

Like the low band stack design, the high band design using the TW-9B VHF antenna trades off gain to configure a direct replacement stack for either TW-15A or TW-18A antennas. The resulting gain reduction requires an increase in transmitter power to maintain the maximum 316 kW ERP. Figure 5 shows that a 44-45 kW transmitter with 6-1/8" line for most tower heights is sufficient. As with the low band stack, it is possible to share the 6-1/8" line with the UHF HDTV transmitter to maintain tower loading.

Figure 5

Again, as with the low band stack design, limiting the stack height to match existing high band NTSC antennas reduces the available HDTV aperture. Depending on the VHF channel and the UHF channel, the HDTV UHF antenna gain varies from a low of 16.5 x power to the maximum 30.0 x power. Available HDTV ERP is given in Figure 6.

Figure 6

STACK HEIGHT ISSUES

The low and high band stack designs shown previously were configured as replacements for existing NTSC VHF horizontally polarized antennas with their overall heights limited accordingly. Custom designs with increased gain/height for HDTV and/or NTSC are possible. These custom designs and other stack antenna configurations that follow will not fit into existing tower top antenna apertures. In those cases, the FAA height limitations must be addressed.

The first option to consider is applying for a height increase. Unless the tower is below allowable heights established by taller structures in the vicinity, this approach has a low probability of success.

The second possibility is to simply remove tower sections and lower the height of the structure. Although this appears to be an significant project, it is feasible. The costs to do this are dependent upon specific site and tower conditions, but, if done coincident with the antenna change, budgetary costs are in the 50-100k$ range.

ADDITIONAL STACK DESIGNS

Three additional stack designs will find wide application. The first two are configured with circularly polarized VHF antenna systems. These will appeal to broadcasters with existing circularly polarized systems. Also, many broadcasters given the opportunity to upgrade their existing horizontally polarized systems will want to consider the advantages of circular polarization, especially since NTSC will be the primary revenue producer for at least the next decade.

Figure 7 shows the channel 2-6 stack configuration using a 5 bay TDM for the NTSC portion. As a circularly polarized antenna, the power gain of the TDM-5 is only 2.2 x power. For most sites, this antenna will require a 60 kW transmitter with 4-1/16" line or larger. Allowing for a 50 ft HDTV antenna aperture, the stack height for a channel 2 design is 125 ft reducing to 104 ft when combined with a channel 6 TDM.

Figure 7

A circularly polarized alternative is also available for channels 7-13 as illustrated in Figure 8. In this case, the NTSC antenna is a TCL-12. The gain of the TCL-12 varies from 5.0 to 6.0 from channel 7 to 13. Depending on line length, a 60 kW or larger transmitter is required to achieve maximum 316 kW ERP in each plane. Allowing for a 60 ft HDTV antenna, the stack height is 140 ft. The TCL height is constant across channel 7-13, therefore, the stack height reduces on a one-for-one basis with shorter HDTV antennas.

Figure 8

The next stack configuration is for existing UHF broadcasters. The base NTSC UHF antenna is a slotted cylinder design as shown in Figure 9. Directional or omnidirectional designs, as well as horizontally, elliptically, or circularly polarized designs are available. Due to the extreme variations of potential channel and gain combinations, it is not feasible to offer standard designs. These UHF/UHF stacks must be designed on a case-by-case basis.

Figure 9

ALTERNATIVES TO STACKED ANTENNAS

Several approaches to stacking the HDTV antenna system with a NTSC antenna system were presented. What are the alternatives? Two general approaches are: 1) panel antennas, and 2) side mount antennas.

Panel antennas may be considered for replacing the existing NTSC antenna to clear the top most position for the HDTV antenna. UHF panel antennas are possible for the HDTV service or multiplexing the HDTV and NTSC signals. The major drawbacks of panel antennas are higher wind loads, reduced circularity performance, and complex feed systems. Higher wind loads translate into increased tower modification costs. Feed system complexity can impact long term reliability.

Side mounted UHF antennas are possible for the HDTV service, but there are significant disadvantages. Although a side mounted UHF may be acceptable for directional service, omnidirectional performance is unacceptable due to the radiation scattering from tower members, conduits, and transmission line. Another negative is that a side mount antenna produces greater wind load than a top mounted antenna of equivalent size.

Figure 10

Figure 10 shows a channel 2-6 alternative. In this case, the low band NTSC antenna is replaced with a wrap around panel antenna. This approach has some advantages: the HDTV antenna is a top mount, a low band panel antenna offers acceptable circularity on large face towers, and it is possible to maintain the gain of the NTSC antenna. The principal disadvantages are the high wind load and feed system complexity of the panel antenna.

The circularity of a typical top mount, Superturnstile antenna is compared with a low band, 4 around panel in Figure 11. Although the panel circularity is worse than the Superturnstile, it is an acceptable ±2 dB.

Figure 11

For channels 7-13, a similar alternative exits. The main difference compared to the low band case is the need to reduce the tower face width for the high band panel antenna to achieve acceptable circularity. Implementing this tower face reduction further adds to the tower modification expense.

Figure 12 makes the circularity comparison for the channels 7-13 case. The typical traveling wave antenna circularity is compared to the 4 around panel on a 4 ft tower and a 10 ft tower. Circularity of the panel on the 4 ft face tower is an acceptable ± 2 dB, but it degrades to ± 3.5 dB on the 10 ft face. Note that the TW antenna circularity includes the effect of the 4-1/16" line feeding the HDTV antenna stacked on top.

Figure 12

For existing UHF broadcasters, an alternative to consider is the top mounted, wide band, UHF panel antenna. Shown in Figure 13, this approach provides the broadcaster with one antenna for both HDTV and NTSC. As a result, the radiation centers are identical and the performance of the HDTV and NTSC are similar. Negatives include reduced circularity, complex feed system, and high wind load.

Figure 13

The general alternative available to any broadcaster is to side mount the HDTV antenna as shown in Figure 14. This is by far the easiest to implement as no changes to the tower top installation are required. While this approach may be suitable for directional coverage requirements, the omnidirectional performance is unacceptable due to tower scattering. Also, the side mount antenna results in higher wind load compared to an equivalently sized top mount antenna.

Figure 14

A circularity performance comparison of the UHF HDTV alternatives is presented in Figure 15. The top mount HDTV slotted cylinder provides excellent circularity. A top mounted, UHF panel supported by a 27 inch face width square spine offers circularities that range from ±2 to ±3 dB across the UHF band. The side mount azimuth radiation plot is an example of what can happen when side mounting an omni antenna. Every tower site and operating frequency will have different results depending on tower and appurtenances configuration and antenna placement.

WIND LOAD COMPARISONS

Previous reference to the wind load advantages of the stacked antenna approach have been made. This advantage is due to the force coefficients applied to projected area as specified in the TIA/EIA-222-F specification for steel antenna towers.

For cantilevered tubular pole structures mounted on the tower top, the force coefficient applied to the projected area is Ca = 0.59 (assuming step bolts and other projections on the surface of typical antennas). Compared to a non-structural cylinder, e.g., side mounted antenna, with a Ca = 1.2 (aspect ratio > 25), the top mounted HDTV cylinder antenna can have a 2:1 load advantage.

Flat surfaces typical of UHF panel antennas have a force coefficient that typically ranges from 1.5 to 1.8 depending on the length/width aspect ratio. This can result in overall loads nearly 3:1 greater for UHF panel antennas compared to cantilevered stacks.

Table 1 presents an aerodynamic area comparison of the typical stack designs compared to the presented alternatives. The wind load advantages of the stacked antenna approaches are apparent.

Figure15

AERODYNAMIC AREA

Table 1

* Areas > 400 ftÓ occur with low band VHF Superturnstile antennas on the tower top combined with side mounted waveguide antennas for high power HDTV.

IMPLICATIONS OF PROPOSED DTV ALLOTMENTS¹

In the FCC’s proposed DTV allotments, 278 existing VHF stations would have greater than 2500 kW ERP for their HDTV assignment. Although this appears comparable to current NTSC standards, the HDTV power is average with peaks 5 times greater. For example, a 5 MW system design (83% line efficiency; 30 gain antenna) will require 200 kW average power or 1000 kW peak. To transmit these power levels, waveguide antennas and transmission line are necessary.

The need for waveguide feeds will require a rearrangement of the stack to place the VHF NTSC antenna on top of a waveguide antenna as shown in Figure 16.

Figure 16

This configuration is feasible mechanically and the circularity performance is good. It is necessary to feed the VHF antenna with coax running up the side of HDTV waveguide antenna. These lines have a minor degradation of the circularity as indicated in Figure 17. Circularity still exceeds a specified ±1.5 dB.

Figure 17

However, high power HDTV waveguide antenna systems have significant disadvantages. On a system basis, the tower wind loads are much higher. Rectangular waveguide can have up to 5 times greater wind load than 6-1/8" coax while circular guide is up to 3 times more for the larger waveguide sizes. The waveguide antenna has a larger diameter than coax designs which can increase antenna loads up to 35%.

Figure 18

The other significant disadvantage is the performance of the antenna output amplitude and phase response. The end-fed waveguide antenna experiences large beamtilt variation versus frequency. This results in large amplitude variations across the 6 MHz channel.
Figure 18 compares the calculated amplitude output response of a high gain, end-fed waveguide antenna to a center-fed coax design typical of the stacks with the HDTV antenna on top. While the center-fed design varies about 1 dB, the waveguide antenna has frequency response deviations approaching 7 dB. This amount of deviation greatly exceeds the goals established (±1 dB) for the ATV field testing.

SUMMARY

Several stack configurations for HDTV/NTSC antenna systems were reviewed. Advantages of these stack systems were shown as:

1) superior circularity

2) highest center of radiation

3) low wind loads

4) efficient use of tower space

Alternative approaches to configuring HDTV/NTSC antenna systems on towers were also presented. Circularity performance and wind load comparisons were made demonstrating the advantages of the stacked antenna arrangement.

Finally, the implications of the proposed DTV Table of Allotments1 on stack designs was considered. The high powers proposed for many current VHF broadcasters would require waveguide antennas and transmission lines for HDTV. A typical stack design incorporating a waveguide antenna was presented. However, waveguide antenna systems for HDTV were shown to have the significant disadvantages of higher wind loads and large output response variations.

¹ Sixth Further Notice Of Proposed Rule Making, FCC 96-317, released August 14, 1996.

Copyright © 1996 Dielectric Communications. All rights reserved.