Move to VHF proves challenging for many lower power stations
Jul 13, 2009 12:06 PM, By Michael Grotticelli
KSNG engineering staff reconfigured two Harris Platinum PA cabinets to operate as one to increase power output= from 7.4kW to 56.8kW ERP.
Now that more than 950 stations have shut off their analog signal, the DTV transition is revealing a number of real-world limitations that the industry didn't foresee and broadcasters are working frantically to fix.
For stations operating at lower power levels (7kW and below) that had to relocate from their UHF channels to VHF after June 12, several misguided calculations made by FCC planning engineers regarding adequate digital power levels for replicating analog service in the higher VHF band (175MHz-216MHz, including channels 7–13), resulted in lost viewers (or viewers lost them, depending upon your perspective).
As Warren Kunkle, chief engineer at KSNG-DT the NBC affiliate in Garden City, KS, and the Kansas State Network, explains it, over-the-air viewers had good VHF reception in analog but were unpleasantly surprised to see their favorite stations disappear when the station switched from the temporary digital UHF channels during the simulcast transition period back to the former (analog) VHF channel. Soon after the June 12 transition date, viewers began calling in with complaints about the loss of service. Among other things, VHF digital signals were not penetrating structures as well as had been predicted so indoor reception was problematic.
Before the transition KSNG broadcast an analog signal on channel 11 with an effective radiated power (ERP) of 200kW. It also transmitted a low-power (3.63kW ERP) digital signal on channel 16. The station dutifully followed the recommendations and calculated contour maps presented by the FCC and other industry consultants, and then saw many of its over-the-air viewers fall off the digital cliff. He called those early predictions “woefully wrong.”
“On June 12 we terminated the analog CH 11 and DTV 16, and fired up a digital system with an ERP of 7.4kW on CH 11,” Kunkle said. “The FCC and our consulting engineers concluded that this power level would permit replication of the former analog signal. However, the real world clashed with that assumption the first day of operation. Many unhappy viewers could not see us, even after correctly rescanning their DTV receivers.”
Fortunately, the FCC is taking DTV reception issues seriously. Once Kunkle surmised that he needed to output more power, he petitioned the commissioned on June 17 and within two days got a granted modification of his channel 11 construction permit and increased his power level from 7.4kW to 56.8kW ERP.
All new transmission plumbing was installed in a manner of one week to reach viewers left behind after the DTV transition.
“I knew that our [Harris Platinum] transmitter and antenna system at KSNG was capable of delivering much more power than was originally specified,” he said.
On June 23, with a new CP in hand, Kunkle and Rob Trull — a Harris Broadcast contract engineer who was helpful in devising a migration strategy — stripped out all of the former analog hardware and reconfigured two Harris Platinum PA cabinets to operate as one. The combined cabinets enabled the station to output a full 6kW, yielding an ERP of 56.8kW. Thus far, this proved to be adequate to reach the viewers in the 69th DMA.
“We had converted the Harris solid state analog transmitter into two digital transmitters, for redundancy,” Kunkle said, “but soon realized that if we scrapped that plan, and combined both PA cabinets, we could deliver 6000W to the antenna system instead of the 780W specified in the original construction permit.”
After the transmitter retrofit work was complete, Kunkle conducted signal tests at several locations using a consumer DTV tuner attached to his laptop gear. Many areas that yielded no reception on June 12 now showed strong signal levels “and flawless pictures,” even 40 to 50 miles from the station's 837ft tower in Finney County, KS.
By increasing the power level, Kunkle thinks he has the VHF reception problem solved, but isn't celebrating too loudly yet. “It's too early to tell,” he said, adding that he's optimistic for the future, but remains concerned about interference from harmonic radiation from FM radio broadcasters and the robustness of the VHF signal and its ability to penetrate buildings — a key element of any mobile video service. The size requirements of a VHF antenna also impacts mobile application design. (At present, mobile service appears to be completely out of the question for low-band 59MHz-88MHz VHF channels 2-6, due to power limitations.)
“More testing and sharing information with other stations will hopefully help,” he said.
Although nearly 80 percent of viewers in Kansas get TV service via cable and satellite, Kunkle said it's still important not to forget terrestrial consumers. “We got hundreds of calls after June 12, so it's clear to us that many people in our area rely on over the air service,” Kunkle said. “We will not give up on them.”
Part of the solution lies with consumers obtaining and correctly installing the proper VHF DTV antennas at their house. Many are recommending that if you live more than 20 miles away from the station's tower, an outdoor antenna 30ft off the ground is key. Also, consumers that used set-top UHF antennas before the transition might need a new VHF-compatible model. Some UHF antennas will receive VHF signals.
“I know that a lot of the set-top antennas in the market are not suitable for reliable VHF reception, and people need to be aware of that.”
On the bright side, KSNG has received a lot of positive feedback from grateful viewers who had flawed or no reception soon after June 12. Kunkle also jokingly wonders whether the Kansas State Network has set a world record for applying for a CP modification and getting it granted (48 hours), and building out the required system (one week).
“We're grateful that the FCC was so proactive and understood our problem,” Kunkle said. “We're getting some 'thank you' calls from outlying areas that are on the fringe (roughly 40 miles away), so I know that the signal is propagating well. The community spoke and said 'DTV isn't working' and we made adjustments accordingly.”
The Kansas State Network is made up of four commercial NBC affiliates (KSNW, KSNC, KSNG and KSNK) and one low-power (KSNL) DTV station. It maintains four full-power transmitter sites, with KSNC and KSNL broadcasting in UHF on channel 22 and 47, respectively.
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