The high level of interest in the ATSC M/H mobile/handheld standard and the creation of the Open Mobile Video Coalition indicate TV broadcasters are realizing that the future of off-air broadcasting is more about reaching portable and handheld receivers than transmitting TV to the big screen in the living room, where in most markets, the vast majority of viewers receive programming via cable or satellite. I’m not saying HDTV isn’t important for broadcasters—it certainly is.
However, to compete in a world with thousands of programming options from cable-only channels, video on the Internet, satellite TV and video from cell phone providers, broadcasters will have to have to look beyond programming one channel to the living room TV set. Many stations already offer updated news on the Web and have additional DTV program streams devoted to 24-hour news and weather and other specialized programming.
This month I’ll look at the conditions that make indoor reception more difficult. Next month I’ll discuss ways to optimize transmission facilities for indoor/mobile reception.
BUILDING ATTENUATION AND HEIGHT
Households that subscribe to cable or satellite TV are unlikely to put up an outdoor TV antenna for second or third TV sets. If they can’t get reliable reception on an indoor antenna, they will probably add another cable or satellite outlet rather than install an outdoor antenna.
The ATSC planning factors are based on the use of an outdoor antenna with gain that’s installed at a height of 30 feet. How much more signal is required to reach that indoor antenna? Unfortunately, there is no simple way to answer that question.
Consider a signal passing through a wall that has line-of-sight to the transmitter. The attenuation will depend on the wall construction. One study done in Finland states that a 70 cm (27.6 inches) brick wall attenuated signals in the 400-600 MHz band by 10 to 15 dB.
The book “Electromagnetic Compatibility: Applications and Principles” by David Weston lists building attenuation for various structures. At 500 MHz the mean E-field attenuation was 0 dB for a single-family detached split-level ranch style house framed with wood and brick siding. For a single family detached wood-frame raised ranch type using concrete block walls on lower levels, brick veneer front on the main level and aluminum siding covering the upper levels, the mean H-field attenuation was 10 dB and the mean E-field attenuation was 12 dB.
Other types of construction, including buildings using concrete block and multistory office buildings using steel framing showed mean attenuation around 10 dB. Mobile homes provided an average of 28 dB attenuation through the entire 20 kHz to 500 MHz range tested.
HEIGHT AND CLUTTER
Hammett and Edison quoted a 1963 FCC study showing building penetration loss of 25 dB at VHF and 21 dB at UHF in less cluttered areas of New York City (outside Manhattan), with UHF attenuation increasing to 26 dB in the most cluttered areas. A United Kingdom study showed UHF losses ranging from 16.4 dB at ground level to only 2.5 to 4.2 dB at the sixth floor.
These results show that building materials are not the only factor affecting indoor reception—height and clutter have a significant effect. One reason is the path from the transmitter site to a lower height indoor antenna is likely to involve loss from additional obstructions and reflections.
A paper from iBlast dated April 23, 2001, “iBlast Data Broadcasting Field Tests,” by Andrew Miller, Steve Lacey, Jerry Glaser, Mike Stauffer and Pete Lude, available at www.iblast.com/new/downloads/iBlastFieldTestWhitePaper.pdf, shows the impact of antenna height and buildings on DTV signal strength. Tests were conducted in Los Angeles, San Diego, Portland, Ore., and San Jose, Calif.
The study found reducing antenna height from the 30 feet used in the FCC Planning Factors to 8 feet caused, on average, a 6.7 dB reduction in signal strength. The average loss due to building attenuation for an indoor antenna compared with an outdoor antenna, both 8 feet above ground, was 18.5 dB in Portland and 21.5 dB in San Jose. Data was insufficient to calculate loss in Los Angeles or San Diego.
INDOOR ANTENNAS
The Planning Factors for DTV Reception in FCC OET Bulletin 69 are based on an antenna gain of 6 dB at high VHF channels and 10 dB at UHF channels. However, the downlead line loss of 2 dB at high VHF and 4 dB at UHF won’t apply with indoor antennas, which are likely to be located on or near the DTV receiver.
A PBS study referenced in Hammett and Edison’s exhibit for EchoStar in the SHERVA signal testing proceeding showed indoor antennas had an average gain of –1.1 dB.
On the other hand, Kerry Cozad’s study showed available indoor antennas had an average gain of 2.4 dB. As iBlast notes in its paper, adding a preamplifier, even one with modest gain, significantly improved the performance of indoor antennas. Many indoor antennas being marketed for HDTV reception include a preamplifier, so in practice the loss of antenna gain isn’t going to be as great a factor as clutter, height and building attenuation.
CONCLUSIONS
With this information, we can make some conclusions about the outside field strength (as measured at 30 feet) required for indoor reception. Using a worst case analysis of 28 dB for building attenuation (the mobile home) combined with 7 dB loss due to height reduction (iBlast study) and a loss of 7 dB in indoor antenna gain (based on the PBS study and eliminating line loss), a field strength of 83 dBµV/m will be required, based on the planning factors’ 41 dBµV/m threshold. An indoor environment will have more multipath, which will impair the receiver’s threshold SNR to perhaps 20 dB, 5 dB above the planning factor SNR, resulting in a required field strength of 88 dBµV/m.
A more optimistic analysis would use a building attenuation of 10 dB (from David Weston’s book), a loss due to reduced height of 7 dB (iBlast study) and a compromise gain of 2.4 dB for the indoor antenna based on Kerry Cozad’s average.
This scenario would increase the required field strength by approximately 21 dB, plus an extra 5 dB for impaired receiver SNR due to multipath, to 67 dBµV/m at UHF Channel 36.
Other factors to consider are noise generated by other electronic equipment, which will be a significant factor at VHF frequencies, and the difficulty consumers will face orienting an indoor antenna for best reception.
Next month I’ll discuss steps broadcasters can take in designing their post-transition stations to provide the best signal to portable and mobile device.
