RF Column 46 - September 1995 Copyright (c) 1995 H. Douglas Lung ALL RIGHTS RESERVED TOPICS: Review of the Broadcasters' ATV proposed channel allocation plan Transmitter implications for TV broadcasters under the plan UHF TV transmitter harmonics may pose a problem for GPS and PCS! Acrodyne Diacrode screen overload solved - it wasn't the tube! News from MicroCommunications and Varian Associates, Inc. --------------------------------------------------------- This month I'll be taking a look at the Association for Maximum Service Television's preliminary channel allocation plan for ATV. This comprehensive plan not only assigns a channel to each existing TV station but also shows the calculated power required to come close to matching the ATV and NTSC service areas. The implications for VHF stations are especially interesting. Dane Ericksen from Hammett and Edison and the Society of Broadcaster Engineers alerted me to a potential new RFI hassle for UHF TV stations. It will be discussed in detail at the World Media Expo (WME) in New Orleans. I'll give you a brief overview here and follow it up with a detailed report after WME. Finally, Mitch Montgomery at Acrodyne updated me on progress with the Diacrode, including a simple fix found for the overload problem discussed last month. In January the Association for Maximum Service Television, Inc., along with most broadcast TV group owners and networks ("Broadcasters") filed comments on the FCC Mass Media Docket 87-268, "Advanced Television Systems and Their Impact Upon the Existing Television Broadcast Service". The Broadcasters had four premises in mind when drafting the proposed allocation table. First, all existing NTSC stations, permittees and applicants should have an ATV channel paired with their NTSC channel. Second, the coverage from the paired ATV channel should closely match that of the NTSC channel it is paired with and ATV interference to existing NTSC channels "should be avoided wherever possible." Third, licensees should be allowed to increase ATV coverage beyond their current NTSC coverage., provided it does not create new interference to other stations' NTSC or ATV service. Fourth, vacant non-commercial NTSC allotments should be sacrificed for ATV "only as a last resort." I'm surprised this proposal hasn't generated more discussion. I have to commend the engineers that created this model. Considering the number of variables and the task at hand, I'm surprised the results turned out as well as they did. Here's a quote from the Broadcasters' filing: "The Broadcasters' approach achieves remarkable results. The attached table provides that (i) _all_ existing stations, permittees and pending applications will receive ATV assignments; (ii) 97.5% of existing stations will have ATV coverage areas at least 95% as great as their existing coverage areas; (iii) 88% of stations will achieve at least 98% replication or better of their existing viewers; (iv) 91.7% of stations will achieve ATV coverage that exceeds their NTSC coverage; (v) 89% of the viewership of NTSC service will experience _no_ new interference and the remaining 1% will be affected only slightly; and (vi) approximately two-thirds of the ATV stations will have the possibility of greatly expanding their service areas according to the maximization principle and many of these will be able to maximize up to the largest service area in the market." The filing goes on to note that after NTSC is shut down, many ATV stations will be able to increase service areas without increasing ATV interference. What's the downside? For existing UHF broadcasters, there is very little downside. I'll use the Mount Wilson transmitter site north of Los Angeles as an example. Most UHF stations on Mount Wilson operate near the maximum ERP the FCC allows at the site's approximately 900 meter height above average terrain (HAAT). That power is around 2,600 KW. Reviewing the Broadcasters' table, the maximum ATV power for a UHF station on Mount Wilson is 165.8 KW. Assuming a peak to average ratio of 10 dB, worst case, this means the transmitter facilities required to provide ATV coverage comparable to NTSC coverage will be less than those currently used for NTSC. The worst case for high VHF channels isn't bad -- Channels 7 and 9 on Mount Wilson still achieve 95% or better matching coverage. Channels 11 and 13 do better, achieving over 99% matching coverage. The problems start to appear when low channel VHF NTSC stations are paired with UHF ATV channels. Channels 2 and 4 achieve about 87 percent matching coverage. To achieve this, the ATV power required for channel 4's matching UHF channel is 1,025 KW. Even if the peak to average ratio is reduced to 7 dB, which many consider acceptable, the peak ERP required will be 5,137 KW, similar to the maximum NTSC ERP's used by UHF broadcasters at lower elevations. Is this the worst case? Definitely not. The worst case appears to be in Billings Montana, where NTSC channel 2 would have to go to 5,188 KW (average) on ATV channel 68! I hate to even think about the peak power requirement! Channel 3 in Lakin, Kansas would also need over 5,000 KW on its ATV channel 25. Channel 3 in Rapid City, South Dakota, channel 2 in Vermillion, South Dakota, channel 4 in Colby, Kansas, channel 6 in Ensign, Kansas, channel 5 in Lubbock, Texas and channel 2 in Anchorage, Alaska are all shown with ATV powers over 4,000 KW. Are low band VHF TV stations doomed to a choice between building UHF transmission plants even bigger than those 240 KW behemoths popular in North Carolina or living with greatly reduced ATV coverage? The situation may not be as bad as it appears. First, you'll notice that most of the stations I listed above are in fairly isolated areas. Many already use translators to supplement coverage and this should be an alternative for ATV. Second, the current low band NTSC coverage may not be as great as the numbers would indicate. Low band TV stations experience much more interference from automobiles, power lines and CB radios than higher channels stations. Indoor antennas, as commonly used, are not nearly as efficient at low VHF channels as at higher VHF channels. Beyond line of sight interference from co-channel stations is much greater at low band VHF due to skip (both E and F layer) than at higher channels. Even when outdoor antennas are used, the longer elements (for channels 2 and 3) tend to get broken first. There are other options for low band VHF stations, particularly after NTSC is shut down. The Broadcasters' filing points out that without using VHF channels for ATV in congested markets the loss of ATV service compared with NTSC service would be exaggerated. The F.C.C. has acknowledged that the VHF band is needed for ATV in the transition phase. The Broadcasters' filing notes that "...a planned phase-out of VHF channels would penalize those licensees initially assigned the few VHF channels because they would have to make the conversion to ATV twice, first on VHF and then on UHF." I'd take it a step further. Allow those small market, big area low-band VHF stations the option of going back to VHF once the ATV conversion is complete. While it is unlikely these small market stations would ever construct a 4 megawatt plus UHF ATV transmission facility, allowing them to build a more reasonable VHF ATV station would allow their distant viewers to continue to receive the station after the VHF NTSC channel is shut down. Forcing these stations to move to UHF for ATV and stay there even after their NTSC channel is turned off would not serve the public. Furthermore, the demands for (and auction value of) their VHF spectrum space should be less in these markets. As time allows I've been scanning the Broadcasters' proposed channel allocation plan into my computer and converting the text into a spreadsheet format. Using the spreadsheet, I've generated text tables showing the proposed ATV allocation for each channel, sorted by city and state. The spreadsheet and text tables in either wide column (all details) or narrow column (basic information) are available on my World Wide Web site - . Don't type the < or > symbols when entering the URL in your browser. The spreadsheet is also available by FTP at . As mentioned in previous columns, there is a 20 user limit on the FTP site. Several tries may be necessary to connect. You shouldn't, however, have any problems getting the text listings from the Web page. By the time you read this I hope to have listings for all states available. on-line Dane Ericksen traditionally has led the broadcast engineering community in ferreting out changes in regulation and technology that could have a significant effect on broadcast stations. Since I'm a member of AOPA (Aircraft Owners and Pilots Association), the latest area of concern shouldn't have caught me by surprise, but it did. Dane told me the FCC and FAA were looking at the problem of UHF TV transmitter interference with GPS (Global Positioning System) receivers. The FAA is starting to certify these receivers for aircraft navigation. This makes sense, because it has the potential to eliminate the need for the expensive and maintenance intensive ground based navigation systems currently used. A few years ago LORAN receivers were the hot new navigation item in the aviation magazines. Now, its hard to find LORAN mentioned and GPS receiver ads file the pages. Why would UHF TV transmitters interfere with GPS? By now you probably know GPS consists of a constellation of satellites orbiting the earth, each transmitting location and time information to airborne or ground based receivers. When the receiver locates at least three satellites, it can calculate its position. The satellites transmit near 1.5 GHz. using spread spectrum modulation. A quick mental calculation shows that the second or third harmonics of several UHF TV channels fall near that range. Dane Ericksen told me the F.A.A. has received reports of interference to GPS receivers from pilots flying near UHF transmission towers. It isn't clear if the problem is due to harmonics or fundamental overload. Broadcast engineers familiar with the problems FM stations had with overload in non-selective VHF aircraft receivers know it can't be ruled out. Manufacturers are currently required to reduce spurious and harmonic emissions from TV transmitters to a level 60 dB below peak visual carrier. The F.C.C. rules Part 73.687(e) state: (e) Operation. (1) Spurious emissions, including radio frequency harmonics, shall be maintained at as low a level as the state of the art permits. As measured at the output terminals of the transmitter (including harmonic filters, if required) all emissions removed in frequency in excess of 3 MHz above or below the respective channel edge shall be attenuated no less than 60 dB. below the visual transmitted power. (The 60 dB. value for television transmitters specified in this rule should be considered as a temporary requirement which may be increased at a later date, especially when more higher-powered equipment is utilized. Stations should, therefore, give consideration to the installation of equipment with greater attenuation than 60 dB.) In the event of interference caused to any service greater attenuation will be required. Note that although this requirement has been on the books for over 20 years, the 60 dB value is still considered temporary! If harmonic related interference to GPS receivers becomes a problem, the F.C.C. has laid the basis for requiring greater attenuation. I'd recommend designing new UHF transmission facilities so that the harmonics are attenuated at least 80 dB below the visual power. This matches the requirement for AM and FM broadcast stations. While harmonic related GPS interference is the current concern, I'm also worried about the potential for interference to the new PCS service operating near 2 GHz. Most broadcasters in a major market with a UHF channel in the upper forties or lower fifties know that third harmonic radiation can make use of the harmonically related 2 GHz. microwave frequency difficult. A 5,000 KW peak ERP UHF station could transmit 5 watts at a third harmonic in the 2 GHz. and still be legal. Transmitting antennas often will not significantly attenuate third harmonic energy. (Remember how a 7 MHz. forty meter dipole would also work well at 21 MHz. / fifteen meters?) Most broadcasters have been able to work around this problem. In one case, locating the 2 GHz. ENG receive dish on the same tower as the UHF transmitting antenna solved the problem. Unfortunately, the PCS bands will not be used by broadcasters. Instead, they will likely be populated by large, well funded telecommunication companies that bid millions of dollars at auctions for the frequencies. What will happen when one of their million dollar frequencies won't work because of UHF TV harmonic interference? Something to think about! Last month I gave you a report on the first high powered tests of Acrodyne's new Au- 60D Diacrode based transmitter. I also mentioned there was a problem with the tube that prevented operation at high average powers (low APL's) for more than a short period of time. First, I want to thank Acrodyne for inviting me to witness the testing of a product so early in its development stage. Many companies would not have invited outsiders in to see a product at such an early stage of development. Second, I'm pleased to report that, according to Acrodyne's Mitch Montgomery, the problem with the screen current overload under low APL was found and fixed. It turns out it wasn't a tube problem. A simple output tuning adjustment on the cavity was all that was needed to cure the problem. As this column went to press, Acrodyne had successfully operated the Diacrode at 60 KW peak visual power, with sync and blanking (worst case average power) without experiencing any instability or screen current problems. VHF transmitter engineers are probably chuckling at this revelation, since they know that tuning can cause screen current overloads in VHF tetrode transmitters. The explanation is simple. When I was at Acrodyne I noticed the plate voltage on the Diacrode was 8,500 volts and the screen was at 695 volts. If the tube was tuned so that the RF voltage swing on the plate caused it to drop below 695 volts, the screen would start to pick up the charge and current would increase. Therefore, to get full power from a tetrode without damaging the screen it is important that the impedance at the plate of the tube be low enough that the plate RF voltage doesn't drop near the screen potential. I may be oversimplifying the problem, but from my conversations with Mitch I know only a minor adjustment was needed to restore stability to the Diacrode. Mitch offered to answer any questions readers might have about the Diacrode. He can be reached at Acrodyne 1-800-523-2596 in the U.S. or +215-542-7000 elsewhere. It's encouraging to see progress with the Diacrode, particularly in light of some of the high powers it looks like ATV stations will require. A while back, after the World Trade Center explosion, I asked manufacturers for ideas on backup transmission facilities. MicroCommunications, Inc. sent me a short news release on their new emergency UHF antenna. I've posted a copy of the press release in the manufacturers' section of the RF Page. Another news release received recently was from Varian Associates, Inc. which announced they are selling their Electron Devices business to Leonard Green & Partners, L.P. The new company will be called Communication & Power Industries. Both the news release and a letter from Al Wilunowski, Varian executive vice president and head of the Electron Devices business, indicated that it would be "business as usual" after the sale. Full text of the news release is available in the manufacturers' section of the RF Page. That's it for this month. Be sure to check my RF Page on the Web for updates and more information. Next month, more news and, space permitting, a report of a low voltage pulser retrofit I did recently on an old RCA 110 KW transmitter. I found taking a few preliminary steps before the installation made the actual installation go a lot easier. If you have one of these old beasts you might find it encouraging! I welcome your comments. My email address is dlung@gate.net or dlung@ix.netcom.com. Compuserve users can use my Compuserve PPN - 70255,460. You can also fax me at 305-884-9661 or phone me after 6 PM eastern time (when things quiet down a bit) at 305-884-9664. Both numbers are at the Miami Telemundo office, so expect a delay in a response if I'm traveling. My mail service address is 2265 Westwood Blvd., Suite 553, Los Angeles, CA 90064. Because I'm often traveling, if time is critical please contact me for a local address before mailing me material. Copyright (c) 1995 H. Douglas Lung ALL RIGHTS RESERVED