In my November column, I described some of the papers covering DTV reception presented at the 2001 IEEE broadcast symposium. This month I'll look at two papers dealing with DTV transmission. One covers the effect of VSWR on DTV error vector magnitude, and the other describes a new combination VHF/UHF antenna that attracted a lot of attention at NAB2001.
VSWR (voltage standing wave ratio) – as well as DTV error vector magnitude monitoring VSWR – is the easiest way to see if the RF system is performing correctly. A high VSWR indicates a mismatch somewhere in the transmission line or antenna. Transmitter and high-power tube manufacturers set limits on the amount of VSWR a transmitter will tolerate and still meet specifications.
Robert Plonka gave transmitter engineers additional reasons to strive for a low system VSWR in his paper, Symmetry and Asymmetry Aspects of VSWR on DTV Transmitter Systems as Related to EVM Digital Measurements, presented at the 2001 IEEE broadcast symposium.
He pointed out that, although there has been much discussion of VSWR and DTV performance, there has been very little discussion of the effect of group delay associated with VSWR. This group-delay component can have a significant effect on the error vector magnitude (EVM) of the transmitted signal.
As EVM increases, coverage area decreases, and more correction capability is required in DTV receivers' adaptive equalizers. In one example, a VSWR of 1.3:1 resulted in an amplitude variation of a little more than 2 dB and a group delay variation of 80 ns – resulting in an EVM of more than 11 percent.
The amplitude ripple can easily be seen with a spectrum analyzer on the forward port of a directional coupler on the output of the transmitter. This effect, however, may not be as noticeable when the line and antenna are measured using a network analyzer alone.
This is because the network analyzer provides true 50-ohm impedance to the reflected signal, while the impedance at the output of the transmitter is usually more complex.
As a result, some of the reflected signal bounces off the output of the transmitter and passes back through the forward port of the directional coupler, increasing the amount of amplitude ripple observed.
Robert Plonka created a "poor man's" transversal filter to analyze the effect of amplitude and group-delay variations. He found that if the amplitude and group-delay variations are not symmetrical – a high-frequency roll-off in the amplitude and no corresponding change in group delay –EVM increases significantly when compared to a case where the amplitude response dips in the center of the channel and group delay drops symmetrically at the same frequency.
In the example above, with a 2 dB variation in amplitude response, the EVM dropped from 7.2 percent for the asymmetric case down to 4.3 percent for the symmetric case. Plonka found that only 10 ns of asymmetrical group delay produced an EVM of 4 percent, corresponding to the ATSC-recommended SNR of 27 dB. With no group delay, an amplitude variation of more than 2 dB is needed to raise the EVM to 4 percent.
It is clear that real-world antenna and transmission lines can have sufficient levels of mismatch and raise the EVM above acceptable levels. Fortunately, Robert Plonka offered some solutions to this problem.
EQUAL AND OPPOSITE
The simplest solution is to let the adaptive precorrection in the DTV transmitter's exciter cancel the ripple by creating an equal and opposite distortion. If the RF sample applied to the exciter correction circuitry is taken from the output of the transmitter's filter system – as is usually the case – this happens automatically!
In one system with a transmission line 1,736 feet long, Plonka found applying adaptive precorrection reduced the EVM from 11 percent (SNR of 18.275 dB, which is unacceptable) to 2.8 percent (SNR of 29.856 dB – quite acceptable).
It is important to understand that precorrection cannot compensate for very high mismatches. The reason is that the precorrection requires additional power from the DTV transmitter's high-power amplifier. Correcting for a 2 dB dip in the amplitude response will require an additional 2 dB of power from the amplifier. This is a good reason to operate DTV amplifiers conservatively, even if it means making slightly less power.
Adaptive precorrection can be considered an active way to improve the transmitter reverse impedance. Passive methods include using a phasing-hybrid combiner with two high-power amplifiers or using a high-power circulator on the output of the transmitter.
In one analysis, Plonka found that reducing the transmitter-reverse impedance (S22) from -1.0 dB to -17 dB reduced the group delay – on a system with an antenna return loss of 26 dB and 1,000 feet of transmission line – from 182 ns to 29 ns. Amplitude response improved from 0.76 dB to 0.12 dB.
The paper demonstrated the importance of analyzing the DTV transmitter plant as a system. The transmitter may be operating as a perfect load with a very low EVM – and the antenna and transmission line may show a VSWR of 1.1:1 when driven with a perfect source load – but the EVM performance of the system when the transmitter is connected to the antenna may be much worse than either measurements separately would indicate!
This paper was not on the Harris Web site (www.broadcast.harris.com), but copies should be available from your Harris representative.
COMBINED HIGH VHF/UHF SLOT ANTENNA
Broadcasters who have a high VHF NTSC channel and a UHF DTV allocation (or vice versa) were very interested in the antenna Dielectric debuted at NAB2001.
This antenna – Dielectric type "TUV" – allows transmission of both the high VHF and the UHF signals in the same aperture without the compromises usually present in multiband antenna installations.
Kerry Cozad, from Dielectric Communications, described the TUV antenna in detail in the paper, Common Aperture Antenna Systems for Multichannel NTSC/DTV Broadcasting.
Separate antennas are typically used when transmitting a VHF signal and a UHF signal. Using two antennas requires some compromises. When stacked, if the tower height cannot be changed, the length of the antennas and thus the gain of the antennas have to be sacrificed, requiring increased transmitter power or lower effective radiated power.
If the antennas are installed side-by-side in the same aperture, careful engineering can reduce the effect on coverage in key areas.
However, if the tower was not originally designed with a candelabra or T-bar structure, it could be very expensive to modify.
THE ART OF COMPROMISE
The UHF antenna could also be side-mounted on the tower below the VHF antenna or even at the same elevation interleaved with the VHF panels. Both approaches involve compromises in the UHF antenna pattern that may not be desirable.
The TUV antenna allows UHF and high-band VHF radiating elements to share the same aperture. The VHF portion is a traveling wave slot antenna. The UHF portion is pylon slot antenna.
Slot antennas have been used by TV stations for decades. Slots are cut into a pipe and fed with a center conductor inside the pipe. It is possible to obtain a wide range of patterns by changing the position and number of slots.
Building a slot antenna that works on both VHF and UHF isn't as simple as cutting big slots for VHF and little slots for UHF in the same pipe. A pipe of the proper diameter for UHF will be too small for use at VHF. Make the pipe bigger to accommodate VHF slots and it won't work at UHF frequencies.
Cozad said that Dielectric's invention (patent pending) uses smaller diameter pipes containing the UHF slots inside a larger pipe with the VHF slots. (See Fig. 1. VHF slots are located in the space between the UHF slots.)
Six columns of UHF slots (each with its own pipe and transmission line) can be arranged between three columns of VHF slots to create a dual-band, omnidirectional antenna. By eliminating individual columns of slots, the VHF and UHF patterns can be modified independently to create skull/cardioid or peanut-shaped patterns.
One antenna could use four columns of VHF slots for an omnidirectional VHF pattern and one column of UHF slots for a skull-shaped UHF pattern. This can be useful when one of the channels has to protect another station or to reduce transmitter power requirements when omnidirectional coverage isn't needed.
Although the VHF and UHF slots have separate inputs, the difference in frequencies makes it easy to combine both signals on one transmission line using a "shared-line tee" at the bottom of the tower. Another tee is used at the top of the tower to split the channels.
Mechanically, the antenna presents the same windloading as a VHF slot antenna of similar gain. Windloading is comparable to that for a VHF batwing on the same channel – weight is greater, however, due to the addition of the pipes and transmission line for the UHF slots inside the larger diameter pipe used for the VHF slots.
Performance is similar to that of single-band slot antennas. The TUV antenna is capable of VHF elevation gains up to 13 and UHF elevation gains up to 33. Electrical beam-tilt up to 1.25 degrees is possible.
As with the azimuth patterns, elevation patterns for VHF and UHF can be changed independently. VHF power levels are limited by the size of the transmission line. With UHF, the number of slots limits the power. An omnidirectional UHF antenna with six slots can handle up to 85 kW peak NTSC power, allowing a maximum ERP (effective radiated power) at an elevation gain of 33 at 2,805 kW – about 2.5 dB less than the 5,000 kW FCC maximum analog (NTSC) ERP.
The same configuration will take 60 kW average DTV power, resulting in a DTV ERP of 1,980 kW at the maximum elevation gain, well above the FCC maximum of 1,000 kW.
This paper has not been posted with the other papers on Dielectric Communication's Web site (www.dielectric.com/broadcast/dtv.asp), but copies should be available from Dielectric.
Next time I'll look at some of the problems that can develop when trying to install DTV antennas in less than ideal locations, and I'll also offer some tips on choosing antenna patterns for these spots. Ignore the impact of the real world on the catalog antenna patterns and you may find your real-world coverage doesn't come close to what the FCC coverage maps show! Comments and suggestions are always welcome by e-mail to [email protected].
Doug Lung is vice president and director of engineering for the Telemundo Group of stations.