By: O. Bendov

Introduction

Most stations that operate as NTSC channels 2-6 may face a staggering cost in trying to replicate their service when assigned a UHF-HDTV channel in accordance with planning factors as described in the FCC’s 6th NPRM¹ .

Without a revision of the planning factors, the authorized Average Effective Radiated Power (AERP) for these UHF-HDTV stations would reach 5,000 KW. For 5000 KW AERP, a transmitter’s peak power for omnidirectional service will be around 1 MW, almost four times that of the largest NTSC transmitter in the US. Whether it is possible to build a practical transmission plant that can safely and economically accommodate even half that AERP is an open question.

Almost 90% of this extreme power will be used to provide HDTV service to a very small portion of the population in the outlying areas whose present reception of over-the-air NTSC service is correspondingly poor. In fact, merely one-tenth of this extreme power, 500 KW, will provide reliable HDTV service in all cases at least to the Effective Radio Horizon ². By replacing the receive antenna in the planning factors with a "smart antenna," service equivalent to 5000 KW AERP can be attained given a practical and economic transmission facility. What’s more, a "smart antenna" will permit connection of multiple receivers without loss of coverage contour -- a feature not possible given the planning factors in the FCC’s 6th NPRM.

LIMITS TO NTSC REPLICATION

HDTV will be related to NTSC much as FM is to AM. In both cases there is a fundamental trade of range for quality. Replicating NTSC service with HDTV is a laudable goal but for most stations it will be proven impractical for the following reasons:

• Through equalization, would-be Tx-to-Rx distortions are traded for perfect picture and sound with a reduced service contour.
• HDTV service to receivers with indoor antennas will be far more restricted than NTSC service is.
• The FCC/MSTV suggested service area replication assumes one receiver. At the fringe contour, loading a second receiver on the same downlead cable will typically reduce coverage by about 3 miles.
• Viewers will react more negatively to losing picture and sound a certain percentage of time than they do to similarly frequent fading effects in NTSC.
• For many VHF-NTSC stations moving to UHF-HDTV the implementation of replication will prove impractical and costly.

The interference that extreme power will bring to adjacent channels and mobile radio has not been fully researched. For example, the protection ratios published by the FCC should be applied at the receiver, not at the transmitter. Nor has the issue of average and peak RF hazard levels been adequately researched. The actual RF hazard levels may yet prove to be in conflict with the FCC’s own guidelines. Once understood, the interference and RF levels may well impose additional constraints on service replication.

REPLICATION BY BRUTE POWER

The most challenging case for replication is for NTSC channels 2-6 when assigned UHF channels for HDTV. The grade-B contours of the VHF channels extend well beyond the Radio Horizon and are therefore impractical to replicate at UHF frequencies without resorting to extreme power. Regardless of power, acceptable calculation of beyond - the - horizon coverage, supported by measured data, may not be possible. Section 73.683(b) of the FCC Code states that "…the F(50,50) curves when used for Channels 14-69 are not based on measured data at distances beyond 48.3 kilometers (30 miles)."

Calculation of NTSC replication has so far been based on the flawed mathematical model known as LR (Longley-Rice). The LR model is flawed in that it addresses a single carrier transmission, not a broadband transmission such as HDTV. Nevertheless, the LR model can serve as a good planning tool in the absence of a better propagation model.

To examine the limits of "brute power" on coverage replication and the associated implementation cost, two actual transmitter sites were chosen. One site, in Florida, is surrounded by a flat terrain with an Effective Radio Horizon of 94.3 km. The second site, in Oregon, is in a mountainous terrain with an Effective Radio Horizon of 58.5 km. An NTSC channel 2 moving to HDTV channel 40 was assumed for each. The HDTV coverage at both sites for AERP of 500-5000 KW is shown in Figure 1 for Florida and Figure 2 for Oregon. Conditions deemed practical for reliable service were used. For example, the receive antenna height was set at 5 meters and the percentage of time availability was set at 99%. The black shaded areas in Figures 1 and 2 show the increase in coverage for a tenfold increase in power.

Click Here for Figure 1
Click Here for Figure 2

The limits of "brute power" are demonstrated in a demographic analysis based on the 1990 Census:

FLORIDA

OREGON

The estimated minimum costs³ of ownership of the transmitter for the two levels of AERP assuming omnidirectional coverage are:

* Peak/Average power = 7 dB.
** Including tubes’ combiner, AC supply, air conditioning, loads and backup generator. It is not clear if a practical combiner for 14 tubes can be built.
*** Including tube replacement and electricity @ .08$/KWhr.

This analysis demonstrates that 85% of the population in the Florida case and 94% of the population in the Oregon case can be provided with reliable and economical HDTV service.

If 90% of the viewers can get reliable service with a 120 KW transmitter, can the remaining 10% of the viewers be provided with a service equivalent to a 1 MW transmitter without resorting to an impractical transmission plant? The answer is yes.

REPLICATION WITH AN OPTIONAL SMART RECEIVE ANTENNA

By replacing the receive antenna in the planning factors with a smart antenna, not only will the coverage be extended to the 5000 KW contour with only 500 KW of AERP but, the coverage contour will not shrink even when multiple receivers are loaded on the same downlead cable.

The smart antenna⁴ contains an LNA which is controlled by the receiver. Depending on the level of the intercepted signals for each of the tuned channels, the receiver connected to the downlead cable either controls the bias and the front end filter of the LNA, or completely bypasses the LNA. The connecting cables⁵ among the receivers and to the antenna pass both RF and control data.

By maintaining a constant system noise figure at the input of the LNA, multiple HDTV receivers can be loaded on one downlead without loss of coverage. The system’s noise figure, assuming 50 feet of downlead cable and 10 dB for the receiver’s noise figure, is shown in Figure 3. It demonstrates that for an LNA gain of 20 dB, the system’s noise figure converges to a fixed value of 4-6 dB for several receivers.

Multiple benefits would be derived from including the smart antenna in the planning factors:

• Maximum replication.
• Growth of HDTV service by lowering the cost of ownership and capital investment to practical and economic levels.
• Reduction of adjacent and cochannel interference.
• Increase of spectrum availability.
• Allowing for multiple receiver connection to a single antenna without loss of coverage.
• Reduction of RF hazards near short towers and towers with multiple antennas.

CONCLUSION

This paper has shown that the incremental increase in coverage for the last 10% of the viewers will require a tenfold increase of power from 500 KW to 5000 KW AERP. The last 10% of the viewers are typically in areas characterized by poor NTSC service and significant cable penetration. Even if transmitters can be built to deliver the high level power needed to reach the last 10% of the viewers, it will be uneconomical to operate them. The deployment of "Smart Antennas" will provide a rational and economical entry into the HDTV market without sacrificing long-term replication of service.

Click Here for Figure 3

¹ Sixth Further Notice of Proposed Rule Making, FCC Docket No. 87-268,August 14, 1996

² Defined as the average of the longest 50% of N equally spaced radials to the radio horizon,

³ O. Bendov, "Planning Your HDTV Coverage Area," 3rd Annual Conference & Workshop sponsored by Broadcast Engineering, November 1996, Chicago. Copies Available.

⁴ A more detailed description can be found in the reply comments of the Association of Federal Communications Consulting Engineers to the FCC's Sixth Further Notice of Rule Making, Nov 22, 1996.

⁵ Either triaxial or coaxial. For coaxial, DC blocking capacitors are added at the end terminals.