IEEE II: Receive Antennas & Unlicensed Devices
This month, I continue my coverage of papers at the IEEE 2004 Broadcast Technology Symposium with a look at a paper by Kerry Cozad outlining receive antenna measurements, and William Meintel's evaluation of the FCC Notice of Proposed Rulemaking (NPRM) to allow unlicensed devices in vacant TV channels. ANTENNA MEASUREMENTS
Cozad's paper on DTV receive antenna measurements was not included in the proceedings, so this discussion is based on notes taken during his presentation. Any errors are likely to be mine. He hopes to have a report available soon.
Cozad, senior vice president of broadcast products for Dielectric, found that the gain figures for the antennas tended to be lower than those calculated using NEC modeling and lower than the values specified in manufacturer brochures, so he is checking his measurements. Since some stations will be using high-VHF DTV channels, he checked the performance of the antennas on Channel 11, even if the antenna wasn't designed to receive high-VHF signals.
The Zenith Silver Sensor, which performs well at UHF, did a relatively good job receiving Channel 11, apparently using the log-periodic fed system as the peak was off the side, 90 degrees from the UHF peak. UHF variable standing wave ratio (VSWR) was 4:1 at band edges and good at the center.
The Radio Shack RS 15-1864 "Budget UHF/VHF TV Antenna" using rabbit ears and a loop had a more omnidirectional pattern. The worst case VSWR was 3:1, with an average 2:1 VSWR across the band. The average vertical-to-horizontal polarization ratio of this antenna was only -5 dB, indicating a reason to include some vertically polarized energy in the transmitted signal.
Cozad reported on the Winegard Square Shooter, an inconspicuous outdoor antenna for high-VHF and UHF DTV reception. While this antenna has been widely touted for improved DTV reception, he found the UHF gain was only 4.5 dB, significantly lower than the 10 dB specified in the planning factors and with an average VSWR of 3:1.
Years ago, I found that stacked bowtie (fan-dipole) antennas or fan-dipole corner reflector antennas worked well over the full UHF band if maximum gain was not important. I was pleased to see Kerry had included the Channel Master 4228 array of eight fan dipoles and a reflector in his tests. He found the antenna had a good front-to-back ratio, narrow azimuth pattern and a tight elevation pattern. Even at VHF, the antenna patterns were not bad, although relative gain was down 3 to 4 dB compared to UHF. The measured pattern was close to the calculated pattern. VSWR was about 3:1 on average.UNLICENSED DEVICES
William Meintel presented the paper, "Unlicensed Operation in the TV Broadcast Bands--Evaluation of FCC NPRM ET Docket Nos. 04-186 and 02-380." As with Cozad's paper, a copy was not available, so this discussion is based on my notes, the FCC NPRM and an MSTV/NAB FCC filing.
Earlier this year, the FCC released an NPRM to allow the operation of unlicensed devices on vacant TV channels. The devices were divided into two classes--low-power portable devices for use by consumers and somewhat higher-power devices for fixed use. The maximum conducted output power for portable devices was set at 100 mW, with fixed devices allowed up to 1 W output.
Antenna gain up to 6 dBi was allowed before the peak output power had to be reduced to offset antenna gain, allowing effective radiated powers up to 400 mW for portable devices and 4 W for fixed devices. The proposed rules require the power outside the channel in which the device is operated to be at least 20 dB below the highest power inside the channel. Both measurements must be at 100 KHz bandwidth and can be based on either RF-conducted or radiated power. This high level of out-of-band emissions is one of the major concerns broadcasters have with the proposed rules.
Fixed unlicensed transmitters have two options to avoid transmitting on a channel that would cause interference.
One is to use a GPS receiver to determine the location of the device and check it against a database to see if the TV channel at that location is vacant. The other option is to have the device professionally installed and configured to operate only on unused TV channels.
In a joint filing, MSTV and NAB said these methods would be ineffective due to problems receiving GPS signals at some locations, and difficulties in maintaining an accurate database, particularly during the DTV transition. Under the proposed rules, a portable device would have to be able to receive a control signal telling it what channels are vacant. If the control signal indicates no channels are vacant, the device would not transmit.
Meintel explained that a 400 mW ERP portable device was capable of producing a strong signal in areas that are not likely to be under the control of the user. Due the requirement that out-of-channel emissions can be as high as 20 dB below the desired signal, adjacent-channel interference is likely.
Even if the device meets the FCC requirement for a D/U ratio of -26 dB for the upper adjacent digital channel and -28 dB for the lower adjacent digital channel, with out-of-channel emissions only 20 dB below the maximum in-band signal, these adjacent channels, and perhaps others, will experience interference.
Meintel used the interference ratios and a hypothetical unlicensed device at a height of 30 meters operating at the maximum ERP of 4 W to see how many channels might be available. There were none in Miami and Orlando when existing TV stations, land-mobile designations, international border distances and required D/U ratios were considered.
He concluded the proposed rules would likely result in interference, and the viability of unlicensed devices may be severely limited in populated areas. Even in areas where portable devices would be permitted on adjacent channels, the high field strengths from nearby TV stations would likely overload the device's receiver.
After Meintel's presentation, it was pointed out that the IEEE 802.18 advisory group is working on an FCC filing and seeking participation by those affected. Tom Gurley of MSTV has been attending meetings at the Consumer Electronics Association to present broadcasters' views, but broadcast organizations and license holders are under-represented in 802.18.
Meintel's company, TechWare, was commissioned by MSTV to conduct a study to determine the availability of vacant TV channels for unlicensed devices. Fig. 1 shows few locations along the northeast coast where even a single channel is available. Fig. 2 indicates there are vacant channels in California, so long as the device is outside major population centers.
In addition, the NAB/MSTV filing included a study by Canada's CRC demonstrating the potential for unlicensed devices on unused TV channels to cause interference to TVs tuned into other channels. This filing is at www.mstv.org/fccyear.php
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