Receive Antennas Revisited
May 1, 2002
My March column on DTV receive antennas generated more e-mail than any column I've written in a long time. I'll share the responses and look at what a report from the ATSC had to say about DTV receive antennas.
E-mail on DTV receive antennas fell into two categories - observations based on antenna experience and comments and observations on antenna technology.
I'll tackle the antenna technology portion first. I received a few messages that lamented the need to reposition directional antennas to receive multiple stations. One even wondered if a terrestrial TV antenna was worth the trouble if the signals become available via satellite. DTV reception difficulty varied widely and it didn't appear to be directly related to the type of antenna used. John Terrill reported a bow-tie dipole on the floor in the back of the TV set-top box provided excellent reception of Salt Lake City DTV, although it did not work for analog. In another case, Mark Schubin reported that he had yet to find a combination of 8-VSB chipset and antenna that would receive both NYC DTV stations from the same location, let alone the same orientation, in his famous apartment. Mark, as many readers know, has probably done as much testing of TV antennas and DTV chipsets as humanly possible! Two readers questioned whether the antenna VSWR really caused a problem with DTV reception.
Jeremy Lansman quoted the official definition of VSWR as "a measure of impedance mismatch between the transmission line and its load." Thus, he explained, "Since the antenna is almost no distance from the source, there is little room for VSWR."
He took issue with the AFCCE (Association of Federal Communications Consulting Engineers) comment that "the effective noise figure subject to a typical mismatch between the receive antenna and the receiver's input is higher by at least 3 dB for a VSWR of 2:1." Lansman commented, "That would be so if the mismatch is between the transmission line and receiver, but your article is about antennas. A 2:1 mismatch at the antenna side reduces gain by only 0.51 dB." He continued, "The AFCCE quote is meaningless, as antennas are normally not directly connected to TV sets, so 'mismatch' between the antenna and the receiver input is a confusing statement at best."
Looking at the AFCCE statement and Jeremy Lansman's comments, you will see they aren't talking about the same thing. The AFCCE statement refers to the "effective noise figure," not "gain reduction." In Bob Plonka's IEEE Broadcast Technical Symposium paper, Symmetry and Asymmetry Aspects of VSWR and DTV Transmitter Systems as Related to EVM Digital Measurements, a mismatch adds group delay and amplitude variations across the DTV channel's bandwidth. (See TV Technology, Jan. 6, 2002, for a summary of Plonka's paper.) The impact of these distortions on the 8-VSB signal's error vector magnitude (and thus its effective signal to noise ratio) is much greater than the impact of line loss. Although VSWR is frequently used to describe a mismatch in an antenna system, you can argue that the impact on system performance is from the effects caused by the mismatch, rather than VSWR. I should have clarified that point in my March column.
REFLECTIONS ON AN ATSC REPORT
The ATSC Task Force on RF System Performance published a Performance Assessment of the ATSC Transmission System, Equipment and Future Directions in April 2001. That report included a section on "Consumer DTV Antennas and 'Ease of Reception.'" In the section on set-top antenna performance, the report stated:
"These antennas display wide variation in the impedance match to the tuner versus frequency as shown in Table 1 and Table 2 of Appendix E. Impedance mismatch affects the RF power transfer from the antenna to the tuner, the noise figure of the receiver and, depending on the length of the antenna-to receiver cable, creates reflections between the antenna and tuner. These reflections affect the receiver in much the same manner as short delay multipath propagation. They create echoes (positive and negative) as well as linear (amplitude and phase) variations that may require compensation by the receiver."
The report includes outdoor antenna performance observations that specifically mention VSWR:
"Many outdoor antennas already contain amplifiers. Such devices redress the downlead and splitter losses and provide impedance stabilization (VSWR). When compared to the FCC UHF planning factors, a system improvement of 3 to 6.9 dB is achieved. Similar works are in progress for indoor antennas; and many models contain amplifiers that mitigate splitter losses, such as from VCRs, and stabilize VSWR." The complete report is available on the ATSC web site at www.atsc.org.
David Lawry modeled the UHF rhombic in EZNEC (an antenna modeling software) and found the VSWR was less than 2:1. Like Jeremy, he questioned whether that was important, saying "The real problem as I see it is that of receiver match. If that is off, it will send a reflection back from the set to the antenna, which could set up a longer delay reflection." David, by the way, not only modeled the rhombic but also built one! He found that elevation angle of the antenna changed as he adjusted the load resistance to improve the front-to-back ratio, requiring the antenna to be tilted up or down to compensate. More complex rhombic array designs use stacked rhombic elements with the spacing varied across the length to reduce this effect. To obtain David's EZNEC analysis, drop me an e-mail and I'll forward it to you.
So, what were the favorite antennas? Various members of the Winegard Chromstar family of corner reflector/director antennas were mentioned as favorites for UHF reception. The Channel Master Quantum 1100 was a favorite VHF antenna, but it is no longer available. David Lawry wasn't pleased with the performance of a high-gain Radio Shack antenna, but found a conventional Yagi design (he thought it may have been a Cushcraft) did a much better job eliminating ghosts. Henry Ruh liked the gridded parabolic reflectors with a bow-tie or bow-tie and reflector feed for UHF. For VHF, he preferred a Kay Townes antenna that used a large collection of folded dipole-driven elements.
The focus is moving away from outdoor antennas to reliable indoor antennas. The ATSC report I quoted earlier has comments on new "Smart" antenna technology. There have been some significant developments in this area that were reported on at NAB. I'll have a full report in next month's column.
I planned to wrap up my tutorial on spread spectrum technology this month, but I've run out of space! I will return to the tutorial after covering new RF developments at NAB. To learn more about spread spectrum, the Web site Spread Spectrum Scene at www.sss-mag.com is the best site I've found for spread spectrum information.
Comments are always welcome. Drop me a note at [email protected]