One group of broadcasters has been ignored in the rulemaking so far on Advanced TV. This month I'll outline a proposal made by the Community Broadcasters' Association to preserve the Low Power TV (LPTV) service after the digital transition. In February's RF Technology column I discussed how slot antennas work. This month I'll cover panel antennas. They are getting a lot of attention due to their wideband characteristics and multiple use capability. Finally, the FCC has acted on the rulemaking to make tower owners, not the tenants, responsible for registering and maintaining the tower. Tenants who hold FCC licenses aren't totally off the hook. I'll detail what licensees must do if the owner fails to follow the new FCC tower regulations.
An LPTV broadcaster reading through the FCC's Fourth Further Notice of Proposed Rulemaking (NPRM) on Mass Media Docket Number 87-268, Advanced Television Systems and Their Impact Upon the Existing Television Broadcast Service would wonder what the future held for their station. The only mention was in an item on "impact on small entities" which stated:
"There are approximately 1,539 UHF and VHF, commercial and educational television stations, 2,509 UHF translator stations, 2,261 VHF translator stations, and 1,648 UHF and VHF low power television stations which would be affected by decisions reached in this proceeding. The impact of actions taken in this proceeding on small entities would ultimately depend on the final decisions taken by the Commission. However, the Commission, in taking future action will continue to balance the need to provide the public with affordable, flexible, accessible high definition television service with the economic and administrative interests of the affected industries."In previous NPRMs the FCC indicated that translator and LPTV stations were a secondary service and not entitled to any protection. I won't go into the legal arguments concerning this. Refer to the Community Broadcasters' Association (CBA) filing for their opinion. What I would like to discuss is a proposal mentioned by the CBA in their comments on this rulemaking.
One of the CBA's suggestions for providing a future for LPTV and translators deserves serious consideration. If you've been following the budget battle, you are aware the current thinking is that once the t transition to digital TV is complete, perhaps as early as 2003, digital TV stations would be moved to a contiguous group of channels and the remaining VHF and UHF TV spectrum auctioned off to the highest bidder. LPTV (which, for use here, I'll take to include TV translators) has been ignored in this discussion. Assuming that full power TV stations are allowed to move to a digital channel without bidding on it (no longer guaranteed), the channel packing envisioned probably won't allow much room for LPTV in large and medium sized markets. The CBA recommends allowing LPTV operation in a guard band between full power digital TV stations and other services in the newly auctioned space next to it.
Why should the FCC (and Congress) consider "giving" existing LPTV stations spectrum? Now we come to the engineering issues. First, full power digital TV stations are likely to require peak power levels not much lower than those used by NTSC full power UHF stations. Indeed, the Association for Maximum Service Television, Inc. proposed channel allocation plan requires substantially higher power levels than this for VHF stations desiring to duplicate their VHF coverage on UHF. How much power? Five million watts for some low VHF stations moving to UHF. While it's too early to know for sure what services will end up in the reclaimed UHF TV band, it's likely they will not be services operating with anywhere near 5 million watts of power. More likely the service will be something similar to PCS, with power levels in the one watt range or less for portable units. It doesn't take a degree in engineering to realize that 5,000,000 watt digital TV stations and 1 watt portable stations don't make good neighbors.>
Broadcasters and non-broadcasters alike should support the CBA's proposal for a guard band, occupied by LPTV stations, between full power digital TV stations and other services. It solves several problems. Broadcasters won't have to deal with interference complaints, which may actually be signal overload or blanketing, from non-broadcast neighbors who paid a high price for their frequency. Wireless operators in the newly auctioned bands won't have the extra expense of increasing the power of transmitters and/or the selectivity of receivers to work near megawatt TV transmitters. The public benefits because the diversified programming offered on LPTV will remain available in the digital age. Another FCC Rulemaking is planned in the near future that will address allocation issues for ATV. I'll be covering it here and in my RF Current newsletter at http://www.transmitter.com/current.html.
UHF panel antennas have long been popular in Europe but are just now starting to receive interest in the United States. VHF broadcasters, familiar with the arrays used at their frequencies find UHF panel antennas' characteristics similar enough that they are comfortable with them. UHF broadcasters, looking for a way to maximize tower space by sharing one antenna among several stations, see the broadband characteristics of panel antennas desirable, particularly for future ATV use.
What is a "panel antenna"? A quick search through the indices of several classic antenna text books reveals no listings for "panel" antennas. The closest reference is usually for a linear or planar array. The NAB Engineering Handbook offers a clue as to how this term arose. The "panel" refers to the reflector on which the radiating element of the antenna is mounted. Typically the radiator will be placed a quarter wavelength in front the reflector, resulting in the reflected signal adding with signal from the radiator itself. Before you say "hey - if the signals are half a wavelength apart they will cancel", remember that the mirrored image of the radiator appears to be a quarter wavelength behind the reflector, resulting in a reflected signal one wavelength apart and in phase with the signal from the radiator. The combined signal is unidirectional and has a cosine or cosine squared pattern in both elevation and azimuth planes. depending on the radiator.
The gain from a single dipole element in front of a reflector is small compared with what is needed to obtain UHF effective radiated powers commonly used today. For that reason, most manufacturers put four phased dipoles in each panel, providing gain per panel that is greater than 10 dB over a single dipole with no reflector. The resulting pattern is too narrow for most broadcast applications, so additional panels are added in the horizontal plane to broaden the coverage. With four panels it is possible to obtain a nearly omni-directional pattern. Two panels perpendicular to each other yield a lotus shaped pattern approximately 120 degrees wide. Three panels provide a wide cardiod shaped pattern in excess of 180 degrees in beamwidth. By varying the power fed to each panel almost any pattern desired can be created.
Antenna text books provide complicated formulas for calculating the patterns created by antenna arrays. Antenna manufacturers and many consulting engineering firms can do this analysis by computer. For the discussion here, I'll rely on an intuitive method to explain how patterns are created. First, we know that that the energy from two antennas feed from a common source and in phase will add. Second, if the signals are out of phase, whether due to location in space or due to differences in the phase of the signals applied to the radiators, we know there will be some cancellation. Given this, the patterns from two panel antennas perpendicular to each other will add where they overlap in phase. This is evident in Figure 1, taken from a data sheet provided by MicroCommunications, Inc. describing an antenna they sell with this configuration. With this understanding, you should be able to see how four antennas, each mounted perpendicular to its neighbor, create a nearly omnidirectional pattern.
In my antenna comparison in the January RF Column I mentioned how difficult it was to duplicate a desired panel antenna pattern in the field if the panels weren't installed exactly as designed. One of the problems, particularly on wide faced towers, is that the spacing of the panels ends up so far apart that the radiated signals are no longer in phase and don't combine as expected. The result is usually deep nulls in the pattern. The companies that responded to my request for information on their panel antennas -- Scala, Kathrein (now merged with Scala) and MicroCommunications, Inc. (using Sira antennas) -- all showed me their top mount panel antenna designs which can be factory tested and avoid the problems of proper location around an existing tower. Kathrein even has room for a medium sized rigger to craw up inside one of their panel arrays for repairs. See figure 2 for a photo from Kathrein's catalog showing how this feature works on one of their larger panel arrays.
As I pointed out in January's column, at many sites it is important to pay attention not only to the azimuth pattern but to the elevation pattern as well. I've seen panel antenna designs developed in the eighties that did not take this into account and had deep first and second nulls below the main beam. Both MicroCommunications/Sira and Kathrein/Scala showed null fill was available, at some loss of gain, in their panel arrays. While the catalog patterns shown did not match the smooth pattern I was able to obtain with the Andrew TRASAR slot antenna built for KVEA, a panel array with additional bays and carefully adjusted power splits should be able to approach this smoothness. I say approach because typically the power splits would apply to a group of four dipoles per panel where in a slot antenna such as the TRASAR each element (slot) can be adjusted individually. The "real world" difference shouldn't be significant.
One of the attractive features of the panel antenna is its broadband characteristic. Both the antennas I mentioned here and those from other manufacturers such as Dielectric are capable of covering the entire UHF TV band. Typically a panel will have a VSWR of 1.1 to 1 or less over the UHF TV band. If the lines driving each of the panels from the splitters is of equal length, performance should be maintained across the band. Because of the differences in wavelength, there will be some variation in the elevation and azimuth patterns due to physical spacing of the radiators. These will be minimal if the panels are closely spaced. The design of broadband radiators, whether dipoles or rings such as those used by Dielectric Communications, is an interesting topic and it deserves its own column. Look for it in the future.
I've used slot antennas with transmitter powers up to 180 KW visual and several sites with 240 KW visual transmitters exist around the country. Panel antennas can match these levels, however, a little more consideration is required. Unlike slot antennas, where energy is tapped off a center coaxial conductor or plucked out of field on a waveguide feed, panel antennas require a feeder for each array, usually consisting of four dipoles. A high power design may include four panels in each bay and sixteen or more bays. The power handling capability of the splitter and the feed lines becomes and issue. Another issue, not immediately evident, is that the feed lines must fit in the space behind the panels and if the space is too large, it will distort the pattern. Scala told me its Kathrein array will take up to 110 KW input power, "beyond which the size of the feed harness prevents us from getting a good omni pattern." MicroCommunications, Inc. send me a list of current UHF antenna installations. One, in Kuwait, is used at 240 KW input power. All other antennas listed were used at 60 KW or less. These power levels should satisfy most demands for single stations. If multiple high power stations are to be included on the same antenna, special designs, like the one in Kuwait, will be required.
More information on the panel antennas here is available from Scala at (503) 779-6500 or Fax (503) 779-3991 in Oregon, Kathrein at (0) 80 31.184-0 in Rosenheim Germany and MicroCommunications Inc. at (603) 624-4351 or Fax (603) 624-4822 in New Hampshire. Several other companies that manufacture panel antennas did not respond specifically to my request for information but are listed in the TV Technology Buyers' Guide mailed with your December TV Technology magazine.
Whether you are using a slotted cylinder antenna, a series of panels or a dish for transmitting a signal, it is probably on a tower. If you own or lease space on a communications tower in the U.S., you will be interested in the FCC's Report and Order on tower registration. If you subscribe to an FCC Rules service, you should be receiving updated versions of 47CFR Part 17 describing the changes. This information is also available on the Internet. A link to the text in the Federal Register is available in the February 12th issue of RF current at http://www.transmitter.com/curr1996/curr9602.html or you can search for it yourself using one of the GPO gateways such as the one at the University of California at San Diego, URL http://ssdc.ucsd.edu/gpo/.
Here are the details of the registration process. Notice that registration will be required for new facilities starting in July this year and for existing structures by July 1998.
One major change from previous regulations is that the structure owner is now responsible for maintaining the tower, performing daily tower light checks and quarterly tower inspections. If you lease space, are you off the hook? Not really. If the owner defaults on his or her responsibilities, the licensees using the tower must maintain it. Here's an excerpt from the rules regarding this responsibility:
I left in the section regarding Form 854. This is the form to be used in registering towers. The FCC will return a Form 854R, which the owner must distribute to all the tenants on the tower. You will also notice in the above excerpt that even though Federal law may prohibit an antenna owner from filing the Form 854, they still remain responsible for distributing the form to tenants and posting it. Of course, the owner also remains primarily responsible for the maintenance.
If you are planning a new installation this summer, be sure to obtain a copy of the new FCC Rules and follow them when applying for your construction permit or license. The FCC warns that failure to do so will stop application processing until the tower registration process is completed.
That's all for this month. I'd hoped to bring you the details on our installation of a Comark IOX transmitter at KVEA in Los Angeles, but that will have to wait until next month. The transmitter went on the air one week before the start of the February ratings period and experienced numerous "infant mortality" component failures, as often happens with any new, complex piece of equipment. What you'll find interesting about the KVEA design is that even though there were component failures, the station was able to continue operating at 100% power. Next month I'll tell you how we did it. The Au60d Diacrode based single tube 60 KW transmitter is turning out to be a popular, successful product for Acrodyne. In their literature Acrodyne has showed that their Diacrode based transmitter, watt for watt, is cheaper to operate that an IOT equipped UHF transmitter at similar power levels. Not so fast, says EEV, a major supplier of IOT's. They dispute this claim. I'll air both sides of the debate next month.
For current news on topics affecting TV and satellite broadcasters, refer to the RF Current pages at my web site at http://www.transmitter.com. RF Current offers a weekly summary of news affecting TV broadcasting and transmission with links to the full text of FCC actions, company news and press releases, technology news and Congressional speeches. Back issues of my TV Technology RF Columns through 1993 and previews of topics for future columns are available. While I'm no longer able to send copies of programs I've written in connection with the RF column on disk, the latest editions are available at http://www.transmitter.com. Select the link to my FTP site or go directly to ftp.transmitter.com/pub/. You may contact me at email@example.com. Your comments are always welcome!
Edited September 13, 1998 for conversion to HTML
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