RF Column 40 - February 95 Copyright (c) 1995 H. Douglas Lung ALL RIGHTS RESERVED TOPICS: RF Path Analysis - Calculating Fresnel Zone Clearance UHF Tetrode tube life survey results from Bill Barrow FCC & Intelsat World Wide Web Site note... This document was written using standard ASCII text characters only. Therefore, some of the formulas may look a little strange, especially if viewed with a proportional font from a word processor. I recommend using a plain old ASCII text editor for viewing. Notepad works for Windows. Email me if you have any questions. Thanks, Doug dlung@gate.net -------------------------------------------------------------------------------------------- This month I'll get back to the discussion of RF path analysis I started a couple months ago. That column generated a huge amount of reader response. Bill Barrow sent me a very detailed report on his experiences with tetrode UHF transmitting tubes. I'll cover some of the highlights. If you use UHF tetrodes, you'll find it most interesting. Finally, I'll give you the address for another interesting home page on the Internet. The spreadsheet I presented in my November column provides an easy way to check path clearances at various K-factors. Providing path clearance along, however, isn't enough to guarantee a solid microwave path, if there ever can be such as thing as a guaranteed microwave path. Fresnel zone clearance is also important. Here's why. It's easy to think of a microwave signal as a pencil thin beam shooting through space like a bullet heading to the receive dish. It's also dangerous to think of it that way, not only when plotting a path but when aiming a dish. A couple years ago I discussed my "donut" method of visualize microwave dish patterns to make alignment easier, so I won't repeat it here. If the microwave signal traveled like a bullet clearance of a meter would be a good as clearance of a kilometer. We know that isn't the case. Why? The microwave beam isn't like a bullet. Even with a very narrow beamwidth on the antenna, the beam will have spread quite wide after a distance. You can do the trigonometry yourself to see what I mean. Furthermore, the microwave beam isn't a single object, like a bullet. It is a wave, or a wave front, which moves though space. (Same quantum scientists might argue with this, but would still have to agree it has wave like properties.) Like me, at some time or another you probably had a case where a stereo audio recording had the left and right channels recorded 180 degrees out of phase. For mono material, like center channel voices, the output disappeared. The same thing happens at the microwave dish when two waves arrive 180 degrees out of phase. I've discussed the affect refraction and reflection can have on microwave paths here before. The first thing the comes to mind when discussing Fresnel zone "clearance" is that if an object protrudes into this zone it is the blockage of part of the signal that causes the signal loss. Its also easy to see why some people dismiss Fresnel zone clearance as not really critical because, well, "hey, its only blocking part of the signal". Fresnel zone clearance is more complicated than blockage. The Fresnel zone clearance is determined by calculating the points where an obstruction would cause a portion of the beam to be defracted back to the receiving dish out of phase with the main beam. If the defracted beam was equal in amplitude and opposite in phase the signal would be totally canceled! If we know the wavelength of the microwave signal, we can calculate the Fresnel zone clearance by looking for defracted path lengths a half wavelength longer than the direct path. That is, the size of the clearance required is inversely proportional to the frequency. The lower the frequency, the larger the clearance needed. That's why, for equal tower heights, stations near the edge of a mountain are going to have better coverage than stations further back from the edge. You might want to try calculating the Fresnel zone clearance requirements for your station's TV transmitter as well as the microwave. Here's the formula you'll need to calculate the first Fresnel zone clearance. Like our K factor analysis, it takes advantage of the right angle triangle formed between the path to the potential obstruction, the direct path to the receive dish and the line from the top of the obstruction to the direct path (that is the clearance required). The most clearance will be needed at the middle of the path. (DOMESTIC EDITION - FEET & INCHES) _________________ / Fresnel zone clearance (feet) = 2280 x / (d1 x d2 / F x d) \/ All path distances are in miles. "d" is the direct path distance to the receiver, "d1" is the distance from the transmit antenna to the obstruction, "d2" is the distance from the potential obstruction to the receive antenna and "F" is the frequency in MHz. (INTERNATIONAL EDITION - METRIC) _________________ / Fresnel zone clearance (meters) = 17.31 x / (d1 x d2 / F x d) \/ All distances are in meters. "d" is the direct path distance to the receiver, "d1" is the distance from the transmit antenna to the obstruction, "d2" is the distance from the potential obstruction to the receive antenna and "F" is the frequency in MHz. You may hear reference to second and third Fresnel zones. Those correspond to the distances to potential obstructions that would create defracted paths one wavelength, for the second Fresnel zone or one and a half wavelengths, for the third Fresnel zone, longer than the direct path. The affect Fresnel zone obstructions have on the signal is more complex than that of a single out of phase reflected wave. Indeed, given the proper aperture the obstruction may actually increase the signal strength if the defracted waves add rather than subtract from the direct beam. If the goal is to have a reliable microwave path with different atmospheric conditions, it's best to design the path to avoid any obstructions in the first Fresnel zone. The formulas above can be plugged directly into my K-factor spreadsheet. If you want to a quick path analysis without considering K-factor, the Fresnel zone clearance normally used is six to seven tenths the Fresnel clearance calculated from the formula, based on a normal K-factor of 4/3. That's because the atmosphere will tend to refract a microwave beam back towards the earth, slightly reducing the effect of obstructions near the middle of the path. Refer to my November column for more information on K-factor. Next month I'll build this formula into the spreadsheet along with a standard formula for free space path loss calculation. The editors are promising me some extra space, so I'm planning on including figures showing the spreadsheet layout, formulas and resulting graphs. Getting back to more practical matters. In December I commented that even though I'd received lots of response to my tube survey from klystron users I'd gotten very little response from users of UHF tetrodes or klystrodes, either those from Varian or the EEV I.O.T.'s. Bill Barrow, former engineer at Acrodyne and currently Chief Engineer for Video Jukebox Network sent me a very detailed listing of his experience with various UHF tetrodes. At these two companies Bill has probably worked on more UHF tetrode transmitters than any other engineer in the U.S., so I was happy to see his fax. I can't reprint the whole thing, but here are the highlights. I'll start with the 100 watt Thomson TH328. Bill reports he's heard of "life figures up to 50,000 hours" and says "Average life has been 18,000 to 20,000 hours." In the popular 1 KW range, Bill has at least 15 years experience with Thomson TH347 tubes. "Most of the [Thomson] tubes usually fail for other reasons than burn out.", Bill says. "I say this because the TH347 has routinely lasted over 20,000 hours IF the transmitting rig does not experience a damaging problem. The problem with the TH347 is that they are so good that the engineer never has cause to do maintenance until something fails big time. The average life of these tubes from my experience can be pegged at 15,000 hours." Bill mentions heat as a potential problem in some transmitters and noted that narrow tuning "increases plate impedance and the result is more heat." Bill reported less life with the 9017 Burle 1 KW tube, averaging around 9,500 hours. He found that the best tube life comes from ITS transmitters, which use a lower filament voltage than the others. This results in less gain, but they make up for it with a bigger driver amplifier and more correction. Bill warned bias supply failures in Emcee rigs can cause pre-mature failures. He said "Another factor is the pick-up truck/multimeter crowd. They have tried to modify the Y1400 amplifier to make tube replacement easier. Unfortunately, it will cause a tube to burn up in 3,000 hours or less..." What about the BIG tubes? Bill found the average life of the Thomson TH382 5 KW tube was around 12,000 hours. but that the "tube's life is directly related to the type of rig and the knowledge of the site engineer." I know this from my own past experience, having worked on two of Acrodyne's first 5 KW transmitters (serial numbers 1 and 2 at Channel 18 in L.A. and Channel 26 in Honolulu, respectively). I found if the tube was set up as specified in the data sheet it would fail in under 1,000 hours. Carefully reducing the filament voltage to the point where the plate current started to drop off and carefully tuning the tube extended the life considerably. I've been hearing more lately about the higher power tetrodes - over 10 KW. Bill has had first hand experience with these during his years at Acrodyne. Writing about the Thomson TH382 15 KW tube, he said "my first experience with this tube was a disaster. A visit from the French and a year later it turned into gold. The best tube life was 28,000 hours and the least was 12,000." Like the other tubes, he said the tubes usually failed from other external problems before they wore out. The larger Thomson TH563 30 KW tube is likely gather the most interest from high power UHF broadcasters. Bill personally developed the techniques for using this tube. The tube life has been excellent, according to Bill and also based on the reports I've received from Acrodyne. Bill said the first application of the tube lasted 24,000 hours. The second ran for 27,000 hours and he's only heard of one tube lasting less than 16,000 hours. That tube was able to be repaired by Thomson and returned to service. Most of the tubes are used outside the U.S. in France, Turkey and China according to Bill. You may remember I reported on Thomson's new Diacrode 60 KW tetrode in my SBE report last month. Bill Barrow had the opportunity to visit the Thomson factory in France last year and see the tube. I was concerned about the heat such a small tube (the shape and size of a lawn-mower tire) could generate. Bill says not to worry, that the Hypervaptron cooling can easily dissipate the heat in the tube and parts of the cavity. He did mention the low 15 dB of gain means a big driver will be needed to get 50 KW output in combined service. I'd estimate Bill's tetrode transmitter experience has come close, if not exceeded, 100 sockets. As I reported earlier, I received a good response on klystron tube life and even reports from a few sites with MSDC klystrons. So far, I have very few reports of klystrode / I.O.T. tube life. Undocumented numbers I've heard seem to place klystrode / I.O.T. life between 8,000 and 20,000 hours. Considering that the vast majority of transmitters over 30 KW sold today are based on this technology, I'm surprised I've gotten such a light response from their users. I'd welcome a fax or e- mail from transmitter or tube manufacturer's willing to document actual tube life, like Varian did with the MSDC tube at NAB a couple years ago. That's about it for this month. Here's this month's hot Internet World Wide Web home-page: "http://www.intelsat.int:8080/ ". The URL is what's between the quotes. Here you will find information on the entire Intelsat satellite system, coverage maps, satellite configuration, phone numbers to order service and lots of graphics. If you have a slow computer link, you might want to turn off the graphics to speed things up. I recommend you leave them on for the first page, which includes a map of the earth with the various satellites and global coverage contours overlaid. Click on a satellite or region to find more about it. The first week of the new year the Federal Communications Commission started playing around with their World Wide Web server. I hope this will lead to some of the enhancements I've been looking for, like being able to click on a document listed in their "Daily Digest" to see a copy of it. Of course, I'd much rather see the broadcast actions such as CP's granted, applications filed and such available on the Web. As I'm writing this they are not. Anyway, the old URL for accessing the FCC files directly no longer works. Use "http://fcc.gov/AAA_HOMEPAGE.html" to get the text based listing of files, the same as you would see using the gopher or ftp interface. You can continue to use "http://fcc.gov:70/0h/AAA_HOMEPAGE.html" to get a more graphic menu which includes the FCC logo and links several other communications related servers around the world. Don't include the ""'s when typing in any of these URL's! The full text of the Notice of Proposed Rulemaking on permitting unattended operation of broadcast station is available in the Mass_Media directory, under Public_Notices, file name pnmm4005.txt. If you still aren't connected to the Internet and are wondering how to get started, I'll give you the basics next month, including ways to get some of the data using America On Line and Compuserve. Don't be surprised if direct Internet access isn't cheaper, however. A few final notes. I mentioned earlier I'd received a large number of requests for the path calculation spreadsheet. If you e-mailed your request to me, you should already have a copy of the spreadsheet. If not, please try again. If you send me a disk to copy the spreadsheet on, I'll return it to you after adding some of the new features you'll be reading about here in the next month or so. If you sent me a fax and no disk, I will gather up some of the free sample disks I get in the mail (America On Line sends me one every week or so, now that I've joined that will probably stop) and mail the spreadsheet to you after I've finished the updates and after I've returned the disks that others have sent me with pre-addressed mailers and postage. E-mail is the best and fastest way to reach me and to get copies of the various programs I've written for this column. I can also e-mail text copies of back columns to you, minus graphics. I can now send binary files to accounts on Compuserve, America On Line and the Internet (provided your Internet software supports either attached files or UUENCODED files). If you are on Compuserve, please use my Compuserve number -- 70255,460. If you are on America On Line, the Internet, MCI Mail or any other service, address messages to me at dlung@gate.net. I have an AOL address but please don't send mail there. I check Compuserve at least once a day and my Internet mail a couple times each day. I'm on AOL once a week or so. If you must use more archaic ways of communication, you can fax me at 305-884- 9661, though faxes will sit while I'm traveling. You can also call me at 305-884-9664, but please avoid peak business hours. After 6 PM eastern time is best. Mail should go to Doug Lung, 2265 Westwood Blvd., Suite 553, Los Angeles, CA 90064. It is forwarded to me on a regular basis, about once a month if I'm not in L.A. Expect a six to eight week delay for replies, more if I'm busy. Thanks for all your comments and special thanks to people like Bill Barrow who are willing to contribute something to the column! Copyright (c) 1995 H. Douglas Lung ALL RIGHTS RESERVED