To John Schadler, director of Advanced Antenna Systems Development at SPX Communication Technology (formerly Dielectric), the future success of mobile DTV depends largely on the service reliability consumers experience when using their new mobile receivers.
“Nobody will want to watch an intermittent video signal,” he says. “You probably are willing to accept a dropped call when talking on a cellphone, but if you are watching a baseball game and the video is cutting in and out, it will be unacceptable,” he says.
According to Schadler, service reliability is dependent upon the “spatial immunity of the receiver.” In other words, mobile DTV reception can’t be allowed to degrade because of the location or orientation of the receiver. Delivering a signal to account for moving receivers that shift in orientation as viewers move their hands holding the receivers is a challenge for broadcasters, but by no means an insurmountable obstacle, he says.
What’s needed is a high probability of signal. “SNR margin is reliability,” says Schadler, “and circular polarization improves margin by about 5dB over linear — that relates back to reliability.”
As Schadler recalls, a few years ago when mobile DTV emerged as an opportunity for TV broadcasters, clients began inquiring about how circular and elliptical antenna polarization might improve signal delivery. To meet their requests, the company launched several studies.
“We’ve done a lot of tests,” he says. “We started with basic tests three years ago and looked just at CP. The first question that popped up was, ‘What about elliptical? Is there an optimal amount for reliable service?’”
The company looked at different amounts of vertical component added in and at the resulting improvement in margin. It found that in a heavy scatter environment — one in which there are lots of signal obstructions, like trees and buildings — the most improvement accrued at 66 percent of power going into the horizontal component and 33 percent into the vertical component, he says. “If you look at that in terms of ERP (effective radiated power), it’s 50 percent — twice as much H as V,” says Schadler.
“As you remove scatter, that peak shifts to full CP,” he says. “I would still go with full CP, but that comes at a cost — a transmitter cost. If cost wasn’t a factor, full CP is the best.”
The good news for many broadcasters is the antennas for sale today are ready for elliptical polarization, and most television broadcasters deploying them are using them in the 66/33 split, he says.
However, others aren’t as lucky. “A lot of the tier one broadcasters purchased what they thought was going to be final RF systems — high power stacks and antennas — first,” explains Schadler. “At that time, ATSC M/H wasn’t even envisioned. All of the antennas shipped at that time were all horizontally polarized because with early 8-VSB receivers, any vertical component was detrimental.”
“Those tier one broadcasters will want to replace those systems,” he says. “Unfortunately, the one thing we haven’t come up with is a CP kit for them.”
Adding a separate vertically polarized antenna is an option, but it’s not optimal, says Schadler. While doing so improves margin 2dB, he says, it is less than an elegant solution.
“The reason it’s not as good (as elliptical or CP) is the spatial difference between the horizontal pole and vertical pole creates a poor axial ratio as you are moving on the ground,” he says. For mobile DTV viewers, a poor axial ratio means those holding a mobile DTV receiver oriented horizontally or vertically will have a signal, but those rotating receivers 45 degrees will have no signal at all, Schadler explains.
Given the need to deliver a reliable signal to mobile DTV receivers, broadcasters need to focus effort on reducing the variability in the signal they deliver to receivers on the move, says Schadler. Doing so is equivalent to improving the signal-to-noise ratio margin. CP and elliptical polarization are two powerful tools to do just that, he adds.
