RF Technology: Doug Lung

DTV On Low-Band VHF Channels

Last month I heard from a reader asking my opinion on whether they should keep their UHF DTV channel or go back to their low-VHF analog channel for digital at the end of the DTV transition when analog TV is shut off. I responded that I thought low-VHF DTV could have advantages in rural areas where viewers are likely to have outdoor antennas and be located far from the transmitter site, but the need for a larger receive antenna and a much higher level of man-made noise could make the low-VHF channel less desirable in an urban area. I told him I hadn't actually compared coverage of a maximum power low-VHF DTV facility with a maximum power UHF DTV facility and would take a closer look at it in my next column.

Given the number of major market network affiliates with analog channels in the low-VHF band, there has been increased interest in the viability of channels 2 through 6 for DTV. The FCC CDBS showed 39 DTV stations currently authorized on these channels:

So far, few comprehensive field test results have been published on the real world performance of low-band VHF DTV facilities. The original Grand Alliance testing in Charlotte, N.C. included a Channel 6 transmitter. In that test reception, Channel 6 was hurt by man-made noise and interference from nearby FM stations. A limited number of tests conducted at higher power showed that increasing power significantly improved coverage.

More recent field tests were conducted in Cleveland using WKYC DTV channel 3 as part of the MSTV 8-VSB/COFDM Comparison Project. Measurements showed successful indoor 8-VSB reception was achieved only 26 percent of the time. However, for sites (indoor plus outdoor at 6 feet and 30 feet antenna heights) that received an NTSC Channel 2 signal from the same tower with a CCIR rating of 3 or better (acceptable picture quality), the 8-VSB channel 3 DTV signal was successfully received 92 percent of the time, although the report noted higher power would have helped the overall WKYC-DT coverage.

The FCC rules on the maximum power for low-band VHF stations state:

"A DTV station that operates on a Channel 2-6 allotment created subsequent to the initial DTV Table will be allowed a maximum ERP of 10 kW if its antenna HAAT is at or below 305 meters and it is located in Zone I or a maximum ERP of 45 kW if its antenna HAAT is at or below 305 meters and it is located in Zone II or Zone III. A DTV station that operates on a Channel 2-6 allotment included in the initial DTV Table of Allotments may request an increase in power and/or antenna HAAT up to these maximum levels, provided the increase also complies with the provisions of paragraph (f)(5) of this section."

Refer to the FCC rules for the charts showing the areas included in these zones. As a rough guide, Zone I covers the northeastern states south to Virgina and states around the Great Lakes. Zone III covers parts of the states around the Gulf coast; Zone II is everything else. Therefore, if a station has a height above average terrain (HAAT) of 305 meters or less, the maximum power allowed is 45 kW unless they are located in Zone I. At 610 meters HAAT, this drops to 10 kW.

FCC COVERAGE VS. REAL WORLD

Before we can look at the real-world coverage of low-band VHF DTV, we need to determine what field strength levels will be used to plot the coverage. The planning factors in the July 1997 edition of FCC OET Bulletin 69 consider thermal noise only, using -106.2 dBm for the noise power at the low VHF geometric mean frequency of 69 MHz. Considering all the OET-69 planning factors, the minimum field strength required for DTV reception was determined to be 28 dB_V/m. For determining community grade coverage, the FCC adds an additional 7 dB margin, increasing the field strength to 35 dB_V/m.

The Canadian version of OET Bulletin 69 is JTCAB-97-03, Digital Television-Service Considerations and Allotment Principles. The JTCAB Ad Hoc Group on DTV Planning Parameters include a 8.2 dB man-made noise parameter (Ta equivalent), along with other factors that differ from the U.S. Planning factors, to come up with a 35 dB_V/m minimum field strength for reception. The paper "Planning Factors for Fixed and Portable DTTV Reception" authored by Oded Bendov, Yiyan Wu, Charles W. Rhodes, and John F.X. Browne and presented at the IEEE 2003 Broadcast Technical Symposium has a detailed analysis of impact of noise on DTV reception. The paper says expected noise levels could be up to 20 dB higher than those used in the original planning factors at low-band VHF channels. In Table III of their report, they show a shortfall of 9.5 dB between the FCC planning factors and the link budget developed in their paper, excluding the impact of interference, multipath and fading.

Before proceeding with my plots of Longley-Rice predicted DTV field strength, I should warn that in "Planning Factors for Fixed and Portable DTTV Reception," the authors show that the use of "field strength" is not valid in the case of DTV, as there is no well-defined center frequency and this can lead to errors. They recommend the use of received power in watts, based on total received power within the channel. Oded pointed out in his 1998 paper, "On the Validity of the Longley-Rice(50,90/10) Propagation Model For HDTV Coverage and Interference Analysis," that "the LR model should be modified for wide-band signals, its statistical margins adjusted and multipath margins for urban and suburban areas added." The errors resulting from treating a wideband DTV signal as a single carrier will be more pronounced at 60 MHz, where the channel bandwidth is 10 percent of the center frequency than it will be at 600 MHz, where the channel bandwidth is only 1 percent of the center frequency.

To see how low-band VHF DTV coverage compares with UHF DTV coverage, I used an antenna with a radiation center 600 meters above ground at a tower in Walnut Grove, the main antenna farm in Sacramento, Calif. For both cases, I used an omnidirectional antenna with the appropriate OET-69 elevation pattern. For the UHF DTV case, the maximum ERP is 1,000 kW. For the low-band VHF at this height, I used the maximum power allowed by the FCC rules, 10.3 kW. Channel 3 was used for the VHF DTV channel and channel 35 for the UHF DTV channel.

The signal level considered adequate for DTV reception varies widely. The information I decided to use is in Tables 1 and 2.

The map in Fig. 1 shows the Longley-Rice predicted signal level for the Channel 3 DTV facility. Comparing it with the map in Fig. 2, which shows the predicted coverage from the Channel 35 DTV facility, you can see that even at the maximum FCC allowed power, Channel 3 coverage does not equal that from the full power UHF facility at the same location. Look at the edges of the coverage, around Merced to the south, to see the difference in the FCC threshold coverage. The yellow area shows the coverage after making all the adjustments described above. This, however, is still "best case." The authors of "Planning Factors for Fixed and Portable DTTV Reception" warn, "If we characterize the FCC factors as 'idealistic,' our results could be characterized as 'optimistic.' Certain margins due to interference, propagation fading, uncorrectable multipath and added equalizer noise are not included."

The low-VHF situation improves significantly if the effective radiated power is increased to 45 kW, the maximum allowed by the FCC at heights above average terrain less than 305 meters in Zones II and III. The map in Fig. 3 shows the predicted field strength at the same signal levels as Fig. 1 but with a Channel 3 ERP of 45 kW.

As these plots demonstrate, whether or not low-VHF channels are better than UHF channels for DTV depends a lot on the ERP available, the terrain and the local noise environment. A 1,000 kW UHF DTV station's predicted coverage exceeds that of the 10.3 kW channel 3 VHF DTV station over flat terrain, but at 45 kW ERP the Channel 3 DTV becomes quite competitive. Looking at the impact of terrain, compare the coverage just west of Vallejo. Even at 10.3 kW ERP, the Channel 3 DTV put a signal that meets the L,T (99,75) margin requirements in this area while the Channel 35 DTV at maximum power doesn't quite make it.

There are a lot of trade-offs to consider. Look at the audience you are trying to reach with your DTV signal and where they are located. If low-band VHF still seems attractive, consider participating in MSTV's efforts to study low-band VHF DTV and obtain FCC approval for power increases.

All these graphs were created using RadioSoft's ComStudy Version 2.2.12.50 with the default OET-69 parameters, Longley-Rice Version 1.2.2, a cell size of approximately 1.0 km, terrain extraction at 0.2 km intervals and a receive antenna height of 10 meters. The contours shown in each figure are the FCC noise-limited F (50,90) contour (largest circle) and the FCC community grade contour (inside). Land use was not considered.

Drop me a note if you have any questions or comments or if there is any other RF related topic you would like me to discuss. I can be reached at dlung@transmitter.com.