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HEAVY METAL, PART I |
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VHF-UHF Antennas Compared for Outdoor DTV Reception Over the past couple of years, I have run numerous tests of indoor and outdoor DTV reception. These tests have taken place in apartments, basements, attics, on roofs, on a nearby hilltop, and even on the rear deck of my house. I've used numerous first-generation and second-generation 8VSB tuners, preamps, and all kinds of test equipment to see just how difficult OTA reception really is. Regular readers will note that I had success in a surprising number of locations, particularly with newer tuners such as Samsung's SIR-T150, and even with PC card tuners made by accessDTV and Hauppauge Electronics. Along the way, I have compiled a stack of gain tables and hundreds of spectrum analyzer plots that occupy several CD-ROMs. Until recently, I had never done a simultaneous side-by-side-by-side test of ALL the antennas I had in my stockpile. The task seemed too daunting, but repeated requests for an all-inclusive table of antenna performance from readers and emails from members of various on-line forums finally did it. Over a period of two days, I dragged every antenna I could find out of my attic, basement, office, and studio; set them up on my backyard test range, and ran comparative gain tests for each and every one of them. Photo A and B show antennas piled up next to deck. Photo C shows test range and Photo D shows measuring equipment.
This month's column will go into the outdoor performance tests, and we'll check out the indoor antennas next month. Now, I should say that tuner performance has improved by several magnitudes. Samsung, Zenith, and Sony are all coming out with new third-generation set-top receivers this summer and fall, with Samsung bringing three new models to market - the SIR-T151, an "economy" OTA-only tuner; the SIR-T160 combo OTA/DirecTV box (enabled with HDCP-DVI), and the SIR-T165, another OTA-only design with IEEE-1394 output for interfacing with JVC's HM-DH30000U D-VHS recorder/player. There's more good news. Linx Electronics, a company made of up ex-Zenith employees, has developed an even better 8VSB receiver system that practically eliminates multipath problems. Dubbed "Casper" (because it makes ghosts 'friendly'), this receiver was demonstrated at NAB 2002 and also underwent rigorous tests at the Communications Research Center Canada to prove its worthiness. (The full report is available at www.linxelectronics.com). I hope to have more information about this new receiver as well as samples of the Samsung tuners to show at my INFOCOMM seminar on DTV Reception and Display at INFOCOMM 2002. Despite the fact that 8VSB receivers are getting progressively better, you still need an antenna of some kind to pick up the signals. And it still makes sense to use an antenna with enough gain at the desired frequencies, as well as an appropriate pattern to handle multipath and put the gain in the direction you need it. For my outdoor tests, I set up a battery of test equipment. A fifteen foot mast was used with a tripod support anchored to the rear of the deck. This particular location has a combination of high to medium signal levels and moderate to high multipath, which is pretty similar to the average suburban or city location. A 50' section of Radio Shack RG-6 was used for each antenna, and fed the Samsung SIR-T150 tuner as well as a spectrum analyzer through an RF loop. A Sadelco Mini-Max 800 signal strength meter was also used to peak the antenna for maximum signal on each channel. All of the outdoor antennas were tested on every available VHF and UHF analog and digital channel, even if they were VHF-only or UHF-only designs. Even though a given antenna may not have gain at specific frequencies, it might still work if the signal levels are strong enough. That could be to your advantage when trying to keep antenna profiles small, or disguised. I also shot selected JPEG photos of some of the waveforms, particularly when I had better-than-expected results with reception. This was true with some of the combo VHF-UHF antennas and high-band UHF DTV signals on channels 62 and 64. All outdoor antennas were tested without external preamps, while powered antennas were tested "powered up". Antennas that had a selectable gain switch (internal amp on or off) were tested in both modes. Here's how to read the tables. All measurements were taken in dBmV (subtract 47 to get the dBm reading) from the Sadleco Mini-Max. This meter has a lower sensitivity limit of -20dBmV (about -13.2 dBmV in digital mode), so any signal that fell below that threshold was simply logged as <-20 (analog) or <-13.2 (digital). Analog signal readings for each band are either in blue (above 0 dBmV) or red (below 0 dBmV). Digital signal readings are also color-coded depending on signal threshold. Analog and digital signals that fell below the sensitivity of the meter are in gray. Note that I cannot provide any data on VHF DTV reception, simply because there aren't any VHF DTV allocations in my area. In addition to tables for analog and digital signal strength, you'll also find a fourth table that shows carrier-to-noise measurements. Now, an accurate C/N measurement is not easy to do without more sophisticated test equipment than I had on hand. Plus, there's a correction factor to be added when measuring digital TV signals on a conventional spectrum analyzer (I covered this in an earlier column on indoor reception). The numbers you see in the C/N table are not absolutely
perfect measurements, but they are pretty close. A blue "L"
indicates the signal locked up the Samsung tuner and held it for at
least 3 minutes without dropout. A brown "MP" means moderately
severe multipath was seen on the waveform, but the signal still locked
up the SIR-T150 for at least three minutes. Entries in red indicate
that reception of that DTV channel was intermittent, and a gray entry
means no signal was received at all.
Among the amplified models, the CM 3010 STEALTHtenna
was the clear winner on channels 3 and 10. The TV55 worked best on channel
6, and the CM 3000A SMARtenna had a bit more gain on channel 12. None
of the UHF antennas amounted to much on any VHF channel with the exception
of channel 10 on the Channel Master 3028 8-bay bow tie. (That might
not be a good thing from an VHF interference standpoint.)
Among the amplified antennas, the CM 3010 was a better
choice up to channel 39, with the amplified TV55 doing well on channel
57. The UHF-only antennas held up (yes, I did stick a Silver Sensor
atop the 15' mast for this test!) across the board, with signal strength
measurements pretty much linked to antenna size. Channel Master's Model
3023 long UHF yagi took first place on each channel.
Keep in mind these are measurements of signal level, not whether a given antenna was able to receive the signal. (We'll cover those notes in Table 4). For reference purposes, here are the effective radiated power (EIRP) levels for each digital TV station received: KYW-26 = 787 kW WTXF-42 = 319 kW WLVT-62 = 48 kW WPVI-64 = 500 kW Most of the combo VHF/UHF antennas worked about as well as a dummy load in terms of gain. CM's 3020 and the RS VU-75XR were good choices for channel 26, and both managed to make the signal strength meter 'tick up' on channels 62 and 64. Among the amplified antennas, the CM3010 and Terk TV55 turned in slightly better numbers than the CM 3000A on channels 26 and 64, with the 3000A winning out on channel 42. The UHF antennas held their own. Once again, the CM 3023
was a clear winner on all DTV channels except 64, where the CM 3021
4-bay bow tie edged it out. Considering the UHF band covers about 500
MHz of spectrum, it's hard to find one antenna that works well at all
these frequencies, but the CM 3022 suburban UHF yagi also did well.
Competition was fierce among the amplified antennas. Both the CM3010 and Terk TV55 held channel 26 nicely, despite beaucoup multipath. The TV55 was a bit better on channel 42, and the CM 3010 was again on top with channel 62. Both antennas kept WPVI-DT's 720p signal cooking on channel 64, although the waveform was noticeably cleaner on the CM 3010. None of the antennas could pull in local DTV channel 67 (WCAU-NBC), though. The UHF tests were very, very close in all respects. Waveforms were clean on all signals - no significant multipath was seen in any case. Radio Shack's U120R took top honors on channel 26, edging out the CM 3023 which did best on channel 42. The CM 3023 pretty much handled the rest of the signals, except on channel 64 were it was bested by the CM 3021 and 3022. CONCLUSIONS My tests are by no means as accurate as those taken on a test range under optimal conditions. Rather, the results show you how given antenna models compared to each other under identical conditions with a baseline tuner and measurement equipment. (As Hugh Downs used to say, your actual results may vary.) Given the improvements in sensitivity and multipath equalization I've seen in successive generations of DTV tuners - and also the ghost-moderating improvements to the Linx DTV receiver system - it appears that antenna gain will become the most important factor for outdoor DTV reception (along with low noise figure and signal distortion in amplified antennas). Copyright ©2002 Primedia Business Media / Peter
H. Putman |
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