Low-Band VHF DTV Revisited
My March 10 column "DTV on Low-Band VHF Channels" generated a large amount of e-mail. Many of the comments described experiences receiving low-VHF analog and DTV stations (Channels 2-6) and a few others focused on the reasons low-VHF DTV reception is difficult. This month I'll share some of those comments with you and offer some ideas on why low-VHF DTV reception is such a challenge.
I received the most comments about reception of WBBM-DT, which is on Channel 3 in Chicago. Henry Ruhwiedel, Chief Engineer of WYIN TV in Gary, Ind. reported that at his house 30 miles from Chicago, DTV Channel 3 was not receivable when his antenna was pointed at Chicago (340 degrees), but was marginally receivable (he estimated about 1 dB over threshold) at 140 degrees. Obviously at that low a margin, the signal drops out frequently. Henry uses a Winegard medium fringe antenna with 40 feet of coax, an amplified splitter and another 30 feet of coax to the receiver. He reported he receives all the Chicago UHF DTV signals "100 percent 24/7."
Another reader wrote that an outdoor antenna was needed to receive WBBM-DT if you were more than 20 miles from the John Hancock Center and warned, "However having a big honkin' antenna on your roof doesn't guarantee you won't have to endure frequent audio and video signal breakups due to power line interference, airplanes, and multipath distortion." He also referred me to www.avsforum.com
where there are numerous discussions about reception of WBBM channel 3. POWER LEVELS
WBBM-DT may not be the best example for low-band VHF DTV coverage. According to the FCC CDBS, the station is operating under special temporary authority (STA) with an effective radiated power (ERP) of only 2.5 kW. The maximum power on the construction permit for WBBM-DT is only 3.7 kW. If you recall from the example in my March column, I used an ERP of 10.3 kW and showed how much coverage improved if the power was increased to 45 kW. The Chicago Channel 3 signal, on power alone, will be over 6 dB weaker than the 10.3 kW example and almost 13 dB below the 45 kW signal. Based on Henry Ruhwiedel's observations, if WBBM-DT could increase power by 13 dB, the signal would be a lot more reliable.
It is unlikely, however, that WBBM or the FCC would want to increase power that much. When WBBM-DT went on the air, I heard that interference into the cable system was a problem even at the lower power level and that with the channel lineup, the interference was affecting reception of WBBM analog on some cable systems.
Gunther Meisse, President of WMFD-TV and WMFD-DT which serves the Cleveland market ("America's First Independent Digital Station"), had problems with WMFD's DTV signal on Channel 12 trashing the WMFD analog signal, which was carried on Channel 12 on the cable system. Gunther points out that cable ingress could be a problem for any VHF DTV station, whether high-band or low-band.
I did receive one positive comment about low-band VHF DTV. Birney Dayton, CEO of Nvision, watches analog and DTV signals from the Walnut Grove antenna farm I used as the site for my hypothetical Channel 3 DTV station. He notes that reception of Sacramento's KCRA's DTV on Channel 35 (operating under STA at 687 kW ERP) is "pretty reliable," but KXTV-DT, on Channel 61 (operating at 1,000 kW), is "unusable" most of the time. All power levels mentioned here were obtained from the FCC CDBS files from March 28, 2004. KCRA analog on Channel 3, at 100 kW circularly polarized, "is always there."
His feeling was a Channel 3 100 kW circularly polarized DTV signal would be "awesome." I tend to agree with this. Extra power is needed at low-VHF to overcome man-made noise (more on this later) and in the mountainous areas on either side of the Sacramento Valley, the needles on the tall pine trees can act like RF absorbers at UHF.
He notes that at another, more difficult site, KCRA's Channel 3 performance "improved markedly" when they switched to circular polarization (CP). Birney wondered whether CP would improve DTV reception and mentioned Charlie Rhodes' "Digital TV" columns discussing CP for DTV. However, he isn't optimistic we'll see a CP receive antenna, given the limited availability of outdoor TV antennas and the lack of new designs.THE VSWR FACTOR
Dr. Oded Bendov, one of the authors of the comprehensive study on DTV reception I used as a guide for determining appropriate signal DTV levels for the study in my March article, added another item to consider when deciding to move from UHF to low-VHF. If most of your viewers will be using an indoor antenna, VSWR could be a problem. Oded pointed out that, "Indoor antennas have very high VSWR at VHF thus raising the in-situ noise figure to a point where SNR is below or near threshold even without interference present." He also noted that while new "smart" antennas are being designed for indoor use, they are designed for high-band VHF and UHF, not the lower VHF channels.
I asked Dr. Bendov if ferrite-loaded antennas, like the ones that were used in the old Cue pagers to pick up FM radio subcarrier data, would work for low-VHF DTV reception. Oded thought this might be worth looking into. He responded that the SNR problem was due more to VSWR than physical aperture. Even with NTSC, when a portable TV with a monopole antenna was switched to Channel 3 and the noise was terrible, this was usually attributed to the size of the antenna. "Not so," he replies. "It's not just the 'dipole factor' that caused loss of efficiency. It was mostly the VSWR that raised the noise figure." He explained that it is possible today to build an active antenna (at least in high-VHF and UHF) that by itself is physically short and very reactive, but when integrated with the right amplifier gets a good impedance match [to the tuner] and better SNR. TOO MUCH NOISE
Solving the VSWR problem may not be enough if the electrical noise is too strong. Charlie Rhodes, who is also one of the authors of the paper on DTV reception I referenced in the March column, wrote to comment that the man-made noise problem is only going to worsen year-to-year as electrical power consumption rises. He said it increases by 3 percent per year, which when compounded becomes significant after a few years. Due to the large amount of man-made noise on these frequencies, adding a low noise pre-amplifier doesn't help. The pre-amplifier can overcome receiver noise and cable loss, but only serves to boost man-made noise picked up by the antenna.
Household appliances, computers and other electronic equipment contribute large amounts of noise inside the house, making low-VHF DTV reception with indoor antennas difficult, if not impossible. Charlie mentioned the large amount of RF interference generated by plasma TV displays. Another reader mentioned a widely distributed electric blanket that generated large amounts of interference even when switched off!
Charlie warned about the threat of interference from broadband over power line (BPL) systems. I've covered the debate about these systems in my weekly RF Report but haven't discussed it here. The system couples data modulated carriers at frequencies up to 80 MHz (or more) into the power line. While interference is supposed to be limited, the American Radio Relay League (ARRL) has found it can cause interference to amateur radio operations over a wide area. It and other organizations have filed comments with the FCC expressing concern about the impact of BPL on reception of HF and lower VHF frequencies.
Summarizing the e-mail messages I received concerning the use of low-VHF channels for DTV, a few points stand out. Antenna VSWR is an issue for indoor low-VHF antennas and for newer DTV- only outdoor antennas like the Winegard "Square Shooter," which does not cover Channels 2-6.
After solving the VSWR problem, the noise level problem has to be solved. As with the VSWR problem, it is now a greater issue for reception with indoor antennas, but BPL interference could create noise problems for low-VHF DTV antennas outdoors. The only way to overcome man-made noise is to either eliminate or quiet the source of the noise (unlikely to happen) or increase the power of the low-VHF DTV transmitter. Based on the reports from viewers around Chicago, low power at low-VHF frequencies doesn't work.
It is likely the maximum power level the FCC set for low-VHF DTV will have to be increased before low-VHF DTV reception is reliable in many areas. However, as studies have shown, increasing low-VHF DTV power creates new problems with interference to existing analog stations. Interference from distant stations due to skip has been a problem for analog TV stations on Channels 2-6. While DTV can handle a greater amount of co-channel interference than analog, theoretically to within 15 dB of the desired signal, if the undesired signal gets over this level even briefly, the signal is lost completely.
In addition to asking for increased power from the FCC, low-VHF DTV broadcasters will also have to convince manufacturers to include coverage of low-VHF channels (with low VSWR) in antennas designed for DTV, especially indoor antennas. If a large number of stations elect to go back to their low-VHF analog channels for DTV after the transition, they may get the extra power from the FCC and antenna manufacturer support. Unfortunately, if man-made noise continues to increase, even these wins may not be enough to provide reliable reception for a significant part of the audience, especially in urban areas.
As more data becomes available on low-VHF DTV performance, I'll report on it. Please continue to let me know your experiences with low-VHF DTV transmission and reception! Your comments and questions on any RF topic are always welcome. Drop me an e-mail at email@example.com