The Newsletter of Crawford Broadcasting Company Corporate Engineering
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The Newsletter of Crawford Broadcasting Company Corporate Engineering OCTOBER 2011 • VOLUME 21 • ISSUE 10 • W.C. ALEXANDER, CPBE, AMD, DRB EDITOR MDCL according to the stations’ engineers). The results For many years, decades even, shortwave were encouraging, showing an average 30% power and medium-wave broadcasters in Europe and other savings while employing around 4 dB of carrier parts of the world have been employing various reduction. Those results eventually led to a Public methods of carrier power reduction that are Notice from the FCC outlining a new procedure that dependent on modulation. The driving force behind will permit any U.S. AM station to employ MDCL this is the cost of power in that part of the world. If control technologies. power is available at all, it’s often $0.50 per kWH (as When the Public Notice came out in mid- opposed to $0.10 - $0.15 per kWH here in the States). September, I immediately began investigating our A lot of stations have to generate their own power at options. As it turns out, Nautel employs the MDCL an even greater cost. So anything that can save power feature in all its current-generation NX-series is way beyond “green” – it’s an economic must-do. transmitters, meaning that we already have it in the One of the first things I learned as I was NX-50 at KCBC. All we have to do is get the FCC studying for my ham license way back when was that authorization and then turn it on. I learned a great the carrier on an amplitude-modulated signal conveys deal more about MDCL at a technical session at the no information. The carrier is, however, necessary to NAB Radio Show in Chicago last month. As soon as the proper demodulation of an AM signal, just as the I got back to Denver from the show, I filed the 19 kHz pilot is necessary to the proper demodulation required paperwork with the FCC and we are now of the 38 kHz stereo subcarrier on a multiplex FM awaiting grant. The FCC told me after the filing that signal, so it has to be there. But does it have to be our application was the first such request they had there at full power all the time? Research by the BBC received. and others says no. Once we get the FCC go-ahead, KCBC will There are two methods of AM carrier turn on the MDCL feature in the transmitter. We will reduction in use in Europe. One method reduces the initially employ the ACC method with 4 dB of carrier carrier during periods of low modulation. The other reduction and see what happens. This is an almost does just the opposite, reducing carrier power during ideal test bed for the technology, since our target periods of high modulation. Both methods have coverage area is some distance from our site (but still advantages and disadvantages. As I understand it, the well within the 5 mV/m contour). We will method that reduces the carrier during high experiment with the different options and amounts of modulation reduces the peak envelope power of the power reduction and see what it does to the signal, transmitted signal, which may have some impact on both analog and digital, in both the local and distant coverage. The other method, if I have it right, parts of the service area. maintains the peak envelope power but may result in Hopefully we will quickly find a set of a lower signal-to-noise ratio at the receiver in fringe parameters that we can live with that will save us a areas (because the lower carrier power during “quiet” lot of money on our electric bill. If we do, I would periods will allow the receiver’s AGC to pump up the plan to purchase the outboard device that provides RF gain, and the noise along with it). the MDCL features for the KLTT transmitter (1995 The FCC recently allowed testing of the vintage Nautel ND-50). various MDCL methods on AM stations operating in A 30% savings on the electric bills of these Alaska (where power is mostly generated on site, 50 kW AMs amounts to a lot of “green.” That’s the 1 The Local Oscillator October 2011 kind of “green” operation I want to have. If we save a It didn’t take me long to figure out what was little CO2 at the same time, all the better. happening. The frequency of one downtown signal was doubling and subtracting the frequency of RITOIE another downtown signal and spitting out a new Back in the mid-1990s, I was introduced to a signal right on 92.3 MHz, the WPWX frequency. Of new acronym, “RITOIE” (pronounced ri-tü-ē). It course this new “receiver-induced” signal was stands for “Receiver-Induced Third-Order extremely strong and WPWX could not overpower it. Intermodulation Effects,” and the acronym was I did a little research and found several coined by the late Robert D. Greenberg who was at combinations of downtown stations that would the time a supervisory engineer with the FCC in produce third-order products on 92.3 MHz, all 2A-B Washington. products (which is what it usually is in the FM band). On that particular occasion, we were moving That should come as little surprise considering all the an FM station to the WCMF-FM tower on the east signals that are down there. side of Rochester. WCMF-FM had been on the So what does this mean for WPWX? It receiving end of some very serious RITOIE means that like WCMF-FM, we live with it. There interference created when another station in the are, in my estimation, very few “fixed” receivers in market moved its transmitter site to a nearby tower. the downtown area that would qualify for FCC- That station’s very strong signal would combine with required interference protection. Portables and another station’s very strong signal in the receiver mobile receivers are not protected devices, and let’s front-end, producing a third “phantom” signal right face it, that’s what people are listening on these days. on the WCMF-FM frequency. In effect, this punched It does, however, give us a good idea of a “hole” in the WCMF-FM coverage for several what we’re dealing with, and however frustrating it miles around the other stations’ tower, in an area that may be, we know not to waste further time, money or was normally very well served by WCMF-FM. The resources on signal improvement where there can be story of this incident is long and takes a number of none. twists and turns, but when all was said and done, the The good news, though, is that WPWX has a outcome was, “too bad, so sad” for WCMF-FM; they killer signal everywhere else. I have never heard it had to live with the interference. Our station’s sound so good, both in terms of signal strength and relocation to the WCMF-FM tower produced no such sound. The new antenna and transmission line made a ill effects and the move went fine (the station, which big improvement in coverage, and the new we no longer own, still operates from that site). transmitter and audio processor make for a In the years since, I have always been formidable combination. People listening anywhere diligent about running a RITOIE study anytime I outside the immediate downtown area can’t help but have considered a new site for one of our FMs. So far notice the difference. it has not been an issue. But last month, I observed a One more thing… while I was at the WPWX clear case of RITOIE interference involving one of transmitter site, I looked at the ends of the 20-foot our stations that has likely existed for many years. sticks of 3-1/8” rigid transmission line that we took The situation is in downtown Chicago, down to install the new Andrew 3-inch Heliax. What where 26-some-odd stations transmit primarily from I saw was unsettling. On what had been the top two “antenna farm” rooftops, namely the Sears and Teflon disk on just about every stick of line was an Hancock towers. Our station which is affected is accumulation of carbon, and several of these had WPWX with a transmitter site some 17 miles to the arced. All those years of chafing at each bullet had south-southeast. I was driving the signal of WPWX resulted in a deposit of tiny metal shavings or dust on with its new transmission line and directional the top Teflons. From the looks of some of those, we antenna, and I started downtown where I was were about one ice storm away from a catastrophic attending the NAB Radio Show. I noted that WPWX failure of that transmission line. Further, had a strong signal downtown, but in some locations discoloration at virtually every end Teflon revealed the receiver would be captured by another signal – how much power was being lost in the line due to signals (plural), actually. What I heard was audio heating in those no longer solid connections. It was a from two or more other stations, one of which was sobering observation and provided a lesson to be really overdeviated. I’ve heard that before. It’s the learned. telltale sound signature of RITOIE. 2 The Local Oscillator October 2011 The New York Minutes By Brian Cunningham, CBRE Chief Engineer, CBC – Western New York Hello to all from Western New York! Last Unless it’s a freebie commercial (or month, out of the clear blue, I received a visit from infomercial, however you want to look at it) that our Roy Sampson, the Broadcast listener heard on WDCX- Marketing Manager of FM, I seriously doubt that iBiquity Digital Corporation.