RF Column 39 - January 1995 Copyright (c) 1995 H. Douglas Lung ALL RIGHTS RESERVED TOPICS: SBE "World Media Expo" Los Angeles 1994 - session reports RF radiation effects -- are they in your head? Broadcasters' auxiliary spectrum squeezed I.T.S. ITS-830 1 KW UHF transmitter review Transmitter tube life survey - tetrodes & klystons I.O.T. problems and solutions ----------------------------------------------------------- Last month, if you remember, I said that the effect of RF radiation may be in your head. This month I'll explain that statement using information gathered at the 1994 World Media Expo and SBE Conference in Los Angeles. Broadcasters' auxiliary spectrum space is coming under increasing pressure as the government finds it can auction off spectrum space for big dollars. This was one of the topics discussed during the FCC regulatory sessions at SBE. On the hardware side, I'll report on the results of my tests on the I.T.S. Model ITS-830 1 KW low power TV transmitter. Response to my high power TV transmitting tube survey started off strong but slacked off in the last month. Even though I didn't get a wide enough response to be statistically reliable, conversations with manufacturers and customers alike gave me a good idea how TV transmitting tubes compare today and what problems are cropping up. One of the panels during the SBE Saturday regulatory session was titled "ANSI C95.1- 1992 - Heads Up for Broadcasters." The panelists included Robert Cleveland Jr. and Robert Greenberg from the F.C.C., SBE General Counsel Christopher Imlay and professional engineers Dane Ericksen (from the firm Hammett & Edison) and Robert Denny (from the firm Jules Cohen and Associates). The F.C.C., as detailed in Docket 93-62, is in the process of revising the RF radiation hazard compliance standards to match the ANSI C95.1-1992 standard. You can order a copy of the standard from the Institute of Electrical and Electronic Engineers (IEEE) by calling 800-678-IEEE. I've discussed ANSI C95.1-1992 in detail in past columns. As a quick summary, it significantly tightens allowable RF exposure in "uncontrolled" environments where the public may be exposed and institutes new exposure limits for transmitters operating below 100 MHz. based on RF currents induced in the human body. Many broadcasters are questioning how they will show compliance with these rules, which may be in place shortly before the next radio renewal cycle starting in June 1995. Robert Cleveland Jr. said there is a basis for using tables or calculations rather than measurements for demonstrating compliance with the standards. Robert Greenberg indicated the F.C.C. was working on a check sheet to make it easier for stations to help determine compliance. Last year there was a lot of publicity about people claiming some brain tumours or cancers were caused or exacerbated by RF energy from handheld cellular phones and two way radios. Obtaining zoning approval for new cellular or communications towers has become difficult because of the RF radiation issue. While TV broadcasters' towers tend to have less problems with this because they usually have more isolated tower site outside of major population areas it has become a problem in cities like Seattle, Honolulu and even on mountains in Colorado. Until specific standards are adopted TV broadcasters may have a difficult time obtaining liability insurance for RF exposure. One participant noted that the insurance agent for Suttro Tower in San Francisco attempted to exclude RF exposure from their liability insurance. Robert Cleveland Jr. pointed out there have been substantial out of court settlements to people claiming harm from RF exposure. This is likely to lead to more restrictive local ordinances regarding RF exposure and more difficultly in obtaining insurance coverage. While government preemption may help the situation, there was considerable disagreement over whether or not it could solve the liability problem. As the session was winding down, there was some discussion about thermal versus non-thermal radiation effects. As a ham radio operator, I've read the warnings about looking into microwave dishes and yagi antennas radiating at UHF and microwave frequencies. The warnings mentioned that the eyes could be damaged by internal heating caused by the RF radiation. Like many people who work with RF, I believed that RF heating was the main danger and that the current ANSI radiation limits were based on thermal effects. After all, the limit is based on energy absorbed -- specified in millwatts per square centimeter -- and energy absorbion, whether at infrared or microwave, will cause an increase in temperature. Jules Cohen was one of a four member committee that drafted the standard. He said the standard was not based on thermal effects but on biological effects caused by exposure to radiation at specific absorption rates (SAR) which might be damaging over a long period of time. The first harmful biological effect noticed in the test animals as the SAR was increased was related to behavior, specificially failure to perform a trained task. This effect was noticed at a rate of 4 watts per kilogram, then dropped to 0.4 watts per kilogram for safety. This is not to say there weren't other effects of RF exposure. When I asked Jules Cohen about eye damage from RF exposure, he mentioned that years ago RF diathermy was used to heat the eyes to such an extent that the skin surrounding the eyes was darkened, without obvious harmful effect on vision. What does this mean? First, thermal effects from RF exposure may not be significant, compared with other effects. Second, the other effects are likely to affect behavior, which, as we know, can be difficult to measure in humans. Finally, the amount of RF exposure required to cause a harmful effect in animals was high - 4 watts per kilogram. Of course, what subtle changes lower amounts of RF energy might cause is difficult to determine from animal studies. Where does this leave us? It's difficult, if not impossible to prove a negative. We can't prove that RF at a low level doesn't cause some harmful effect, just as we can't show that peanut butter or city tap water at a low level doesn't cause some harmful effect. The best advice is to keep the danger in perspective and don't take unnecessary risks. TV broadcasters' have another concern -- competition for spectrum space. Chris Imlay, SBE General Counsel noted that a TRW proposed rulemaking wants to reallocate channels A1 and A2 in the 2 GHz. band. He said the F.C.C. doesn't want to change the band, but one solution might be to shift it down 40 MHz. to accommodate TRW. Richard Rudman from the SBE National Frequency Coordination Committee commented that government agencies are after the 2.5 GHz. band for helicopter survellance uses. Someone commented that one agency started to cooperate with broadcasters after the broadcasters threatened to broadcast the video the agency was transmitting on 2.5 GHz. The sources of interference I've mentioned so far are fairly easy to track down. That isn't the case with new digital modulation methods and spread spectrum transmissions. These can be extremely difficult to identify and locate. As spectrum capacity becomes more saturated this type of interference is going to increase. Howard Fine recommended that all S.T.A.'s (Special Temporary Authority grants) be put on Public Notice. Transmitters are often authorized for temporary use on broadcast auxiliary frequencies. Broadcast temporary use shouldn't be a problem, since the F.C.C. rules require the operation be coordinated through the local frequency coordination committee. Unfortunately, this coordination is not always required for non-broadcast operations by manufacturers and communications contractors. Putting all S.T.A. applications using broadcast auxiliary frequencies on Public Notice broadcasters would make it easier for broadcasters to prevent or stop interference before it causes significant damage. Recently Howard told me about some research he and Dane Ericksen have been doing looking into broadcasters' use of the 2 GHz. band. He found that the way the F.C.C. database tracked broadcast remote pickups, a system consisting of a half a dozen receive sites and twenty ENG trucks would appear the same as a system with one receive site and one ENG truck. Howard said this makes the 2 GHz. band look a lot less congested than it is because the F.C.C. doesn't recognize the extra receive sites. Howard suggested that broadcasters file new or modification applications that list each individual receive site with coordinates instead of only one site. Without this information in the F.C.C. database, the F.C.C. may not understand why non-broadcast fixed links can't share the broadcast auxiliary bands. As the auxiliary session at the SBE was winding down, both representatives from the F.C.C. implored broadcasters to make their case known. "The squeaky wheel gets the grease" was one comment. Lots of actions are pending which will affect broadcasters -- one proposal wants to use channels B7 and B8 in the 7 GHz. band for digital audio radio (DAR) uplinks. Here are the names and phone numbers of two F.C.C. members that want your comments. Riley Hollingsworth is with the Private Radio Branch in Gettysburg, PA. That branch is now handling all broadcast auxiliary applications. His phone number is 717-337-1311. Gordon Godfrey, head of the Policy and Rules division of the F.C.C., is well known to broadcasters. His number in Washington D.C. is 202-632-9660. I finally had a chance to take a look at a new I.T.S. Model ITS-830 1 KW solid state LPTV transmitter when North Carolina Public Broadcasting was testing it prior to installation. David Neff, I.T.S.'s Broadcast Systems Product Manager and one of the engineers that designed the transmitter, flew in from Pittsburgh for the checkout. Adjusting the transmitter for differential gain, ICPM and intermod performance wasn't difficult, but like most transmitters using common amplification adjusting linearity meant watching intermod, differential gain and low frequency linearity all at the same time. I was impressed with the construction of the transmitter after seeing it at NAB. The features vs. price ratio was also among the best in the business. We were able to get the transmitter within four percent linearity with the in-band 3.58 MHz. vs aural carrier peak intermod at 54 dB below peak carrier. We did notice some intermod riding on video peak white. I.T.S. was still tweaking the design of the intermod corrector and David felt that could be eliminated. One thing David pointed out was that the usual method of adjusting chroma / aural intermod using a high chroma red field wasn't always the best. Because the luminance level of the red field didn't change, adjusting with that signal often allowed intermod levels to go much higher at other video levels. He recommended using a modulated ramp to adjust for best intermod performance. I noticed some ringing on the 2T pulse and 1T bar. This appeared to be coming from the SAW filter, which I.T.S. offered to replace. I understand that the both the intermod and SAW filter problems were corrected after my visit. I.T.S. includes a bandpass filter as part of the transmitter. Many LPTV stations are located at sites with two way radio users. These users are concerned about new transmitters increasing the noise floor at the site. I've found that broad-band solid state amplifiers, unlike cavity tuned tube amplifiers, put out a measurable amount of out of channel noise. This can be a problem for translators working with weak signals. This noise is usually 80 to 90 dB or more below the carrier so reducing it by another 30 dB or so with a bandpass filter usually puts it below most sites' existing noise levels. Unfortunately, most bandpass filters do not do a good job rejecting harmonics. I.T.S. includes notch filters for harmonics, but I'd recommend additional filtering if second or third harmonic would fall near one of the receive frequencies at the site. If your channel is in the upper 40's or low 50's I strongly recommend an additional filter to protect broadcast 2 GHz. ENG receivers. I've had good luck with filters from Micro Communications, Inc., in New Hampshire. I.T.S. has their own filter division, so they may also be able to provide the additional filtering. It doesn't take much time with the transmitter to see that it was designed by engineers who not only know how to design TV transmitters, but also understand what happens in the field after the transmitter is sold. Most of my experience with solid state LPTV transmitters has been with Acrodyne units, so that is my point of reference. I was concerned that the two final amplifier modules where too heavy for one person to lift. Several times I've had to remove and install Acrodyne's final amplifier modules by myself for repairs or return. A close look at the amplifier showed me it was possible to remove individual amplifier subassemblies from the amplifier chassis with a little unsoldering. Each subassembly with heat transfer plate was small enough to ship in a FedEx overnight boxes. Enough metering was provided on the front of each amplifier chassis to verify it was working, but I'd like to have seen a way to monitor individual amplifier currents without sliding out the amplifier assembly. The way the transmitter is designed the amplifier must first be disconnected from the combiner before it can be pulled out, then it has to be reconnected with a jumper cable, which, of course, upsets the phase balance between amplifiers. This isn't a problem, since each final amplifier has an isolator and load its output, but it would be nice to take readings from the front for logging without shutting down the amplifier. Acrodyne now includes that feature on their new 600 watt amplifier drawer. I.T.S.'s exciter layout is easily accessible. Instead of one huge board, like Acrodyne's economy exciter, the I.T.S. unit has functions logically divided between individual boards. Each board has test points for checking the RF or video signal (as appropriate) and each board can be easily removed from the chassis. I.T.S. supplies the cables needed to get their pin type connectors to a standard BNC connector. Most adjustments are clearly labeled. The local oscillator is a non-synthesized and mounted in an oven. This results in a very phase stable oscillator. Acrodyne shifted to a synthesized local oscillator in their economy exciter because of the long lead time (three months or more) involved in obtaining aged crystals. Unfortunately, with the synthesizer comes increased phase noise and sensitivity to vibration. I.T.S. gets around the lead time issue by keeping a crystal in stock for each channel and offset. I've found the Acrodyne synthesized L.O.'s to be more stable in frequency than most broadcast part 73 exciters, inspite of the poor phase stability. The non-synthesized oscillators may not be as frequency stable because the crystal is operating at significantly higher frequency, is thinner and hence more susceptible to aging. So how does the ITS-830 stand up? I like the construction, the remote control interface and the attention to detail. I like the exciter. The only major disadvantage I see to this unit is that the final amplifier is a bit under powered. According to I.T.S. it does not have the head-room to operate with more than 5% aural carrier. I.T.S. tells me that in spite of operating the transistors near the maximum power, the transistors have been very reliable -- they knew of no field failures. While the transistors they are currently using are made by Motorola, they are testing transistors from other sources as well. Acrodyne's new transmitter, using Acrodyne private labelled transistors, has lots of head-room, even with 10% aural power. My ideal transmitter would have Acrodyne's finals and I.T.S.'s exciter. As of now, I can't find a strong reason to recommend one transmitter over the other. I plan to order one of each for my next two LPTV sites. That will give me a chance to give both a real world test. Many thanks to Wayne Estabrooks, Ben Beech, Ron Evans and Don Lakey at N.C.P.B. for inviting me to their shop and letting me participate in the transmitter testing. Two months ago I asked transmitter engineers to send me life data on their high power UHF transmitter tubes. I was trying to get some handle on relative performance of klystron versus MSDC klystron versus klystrode. I received no detailed responses from klystrode (Varian/Eimac or EEV IOT) users nor did I receive any detailed responses from tetrode users. I did receive several reports on external four cavity klystrons like the EEV 3762. Several stations reported getting from 30,000 hours to over 40,000 hours from these tubes. One third hand report said an EEV 3762 had accumulated 100,000 hours, but I couldn't substantiate that. External cavity MSDC klystrons seemed to be getting similar life, with the exception of one station that couldn't make the tubes last over 8,000 hours. I suspect a magnet assembly might be at fault here. The big red integral cavity Varian klystrons hold the records for the longest life - I received several reports of tubes lasting over 70,000 hours in aural service. Some of these tubes have even approached that life in visual service, though the norm seems to be 30,000 to 40,000, same as the external cavity tubes. While I didn't receive any first hand life reports on klystrodes, I did hear some news about what was happening with them. Several new transmitters are being installed with the new Varian klystrode, which is designed to correct some of the problems I've heard users of the old Varian/Eimac klystrodes complain about. I'll let you know how they do. Meanwhile, I've heard several reports of EEV IOT input cavities arcing over. The problem turns out to be in the material used to insulate the input cavity from the high voltage on the tube. The material worked fine as long as the temperature stayed low enough. As temperature increased, defects in the material caused it to break down. This problem wasn't noticed earlier because most well designed transmitter sites have enough air conditioning to keep the temperature from reaching the critical point. EEV should have a fix for the problem by the time you read this. I received one second hand report of EEV IOT life from San Francisco. The tube was removed from service while still functioning after 20,000 hours because its grid current was starting to increase. Joe Wozniak and Dr. Tim Hulick of Acrodyne gave me some information on the performance of the Thomson tetrodes in their 25 KW and 30 KW transmitters. According to Acrodyne, most of these tubes are lasting 20,000 hours, even at 30 KW output. I'm out of space for this month. Next month I'll return to the discussion I started two months ago on RF path analysis. I'll add path loss and fresnel zone clearance formulas to the spreadsheet and explain how to calculate them. I'll also have some new sites for you to check out on the Internet for RF and broadcast related material. Your comments and contributions are always welcome. E-mail them to me via the Internet to dlung@gate.net or via Compuserve to 70255,460. Fax them to me at 305- 884-9661 or call me at 305-884-9664 -- after 6:00 PM please -- since I'm usually too busy during the day to get into long discussions. Copyright (c) 1995 H. Douglas Lung ALL RIGHTS RESERVED