The Newsletter of Crawford Broadcasting Company Corporate Engineering MARCH 2011 • VOLUME 21 • ISSUE 3 • W.C. ALEXANDER, CPBE, AMD, DRB EDITOR Interference for several months after that. I have, of late, been involved with an AM Then in August, the complaints started up interference resolution case that I think has again, so back to The Springs I went. I observed application throughout the industry. This particular essentially the same thing during that first August trip case involves some stations in Colorado Springs, two that I had noted the previous May, that 1040 and of which (1040 and 1530 kHz) are co-owned and 1530 were well within the limits specified in §73.44, diplexed into a single tower, and the third of which is that there were no observed third-order products and located a few miles southwest. The owner of the third that the 1580 station was very weak. station, which operates on 1580 kHz, is the Prior to making that trip, I did a quick complaining party. calculation of what the expected 1580 field should be This all started almost a year ago, in May of at the measurement points based on M3 (FCC) 2010. The 1580 station licensee began complaining ground conductivity. I was fairly comfortable with of interference to his station in a fairly small this calculation because the path between the 1580 geographic area near the other stations’ site. The site and the measurement location traverses the valley local engineers made some measurements and around Fountain Creek with no other geological observations but were unable to come to any irregularities along the path. Further, I had in years conclusion other than yes, in some locations near the past made measurements in this area and knew that 1040/1530 site you could hear 1530 station audio on 15 mS/m was a reasonable ground conductivity to top of the 1580 signal. assume in that area. The calculation showed that the At the request of the 1040/1530 station 1580 field intensity should be on the order of 44 owner, I made a trip down to “The Springs” and used mV/m; I measured 3.5 mV/m. That represents an 11 our Anritsu Spectrum Master to make some dB difference, indicative of something seriously measurements. These measurements clearly showed wrong with the 1580 transmitter or antenna system. that 1530 and 1040 were operating well within the I returned a couple of weeks later and took limits imposed by §73.44, and there were no another look. This time the 1580 field strength at that measurable third-order products. reference point was 22 mV/m, a big improvement but I took a FIM with me on that first trip and still 3 dB below what the signal should be in the area. observed that 1040/1530, which both operate with 15 I have no idea what the 1580 folks did, but that signal kW daytime, were very strong at the three reference level was sufficient to provide interference-free locations I chose. I also observed that the 1580 reception in the area. The 1530 field strength station was very weak, with nowhere near the field consistently measured 320 mV/m at that location. intensity that I would expect. It was so weak, in fact, I heard nothing else about the situation until that the reason for the interference reports became last month when the interference complaints started quite clear: the very strong 1530 signal was up again. As had been the case in May of 2010, the overloading receivers in the area that were tuned to new complaint was generated when a potential client 1580. It was a simple case of inadequate desired-to- of the 1580 station would not sign an airtime contract undesired signal ratio. because the 1580 signal was not clear in the area After making these measurements, I sent around his place of business. The 1580 station them on to all the parties along with a brief narrative engineer was making noise about a provision of of my observations and conclusion. I heard nothing §73.44 requiring 1530 to install additional filtering 1 The Local Oscillator March 2011 because of this interference. Based on my prior your own transmitter and make certain that the power experiences, I knew that additional filtering would be is correct and that it is being radiated. You’re going worthless because the “interference” was being to be embarrassed if you make an allegation and find generated in the front-end of listeners’ receivers and out that the problem was your own all along. was not the result of out-of-band emissions from Another lesson is that there is no substitute 1530. for empirical measurements. Oftentimes, interference Before I would agree to grab the FIM and complaints are backed up with only anecdotal SA and head back down to Colorado Springs, I asked evidence. Measurements tell the story. Spectrum the 1580 station engineer to provide me with some analysis in both AM and FM interference cases and measurements. I asked for seven close-in field intensity measurements in AM cases are measurements on the 1580 radial of interest, and I invaluable tools. Are emissions what they should be, asked for field intensity measurements at the three within the FCC-specified emissions mask and free of reference points I had been using (one of which was harmonic and third-order products? in a large mall parking lot across the street from the Finally, good engineering sense must be client’s place of business). So far I have heard used when evaluating an apparent interference nothing else. I suspect that the engineer found that situation. Several years ago, our KBRT began 1580 continues to have transmitter or antenna experiencing interference across a five mile wide problems and that the station’s field intensity is very swath in the Burbank area. The interference took the low in the area of interest. form of noise (“hash”) and it sounded for all the While the final chapter in this story has yet world like weak signal. A little sleuthing revealed to be written, there are some lessons that we can take that Disney’s KSPN (50 kW and 30 kHz below the from what we know so far. KBRT carrier frequency) was in the center of the The first (and perhaps most important) is to “weak signal” area. A little more sleuthing and be sure your own house is clean before complaining analysis of the facts and it wasn’t a great leap to about interference from someone else. This point was come to the preliminary conclusion that we were driven home in recent months in two separate listener actually hearing third-order IM products from the complaint episodes. In one, a woman who lives near KSPN HD Radio operation. These were 65 dB or one of our transmitter sites was complaining that she better below the KSPN carrier, but 50 kW is a lot of could not hear her favorite station, which is 90 kHz carrier! KBRT had about 5 mV in that area but the above our station. She was certain that we were the KSPN IM products were in the 1 mV range in the problem, but some field measurements and a phone affected area. call by our engineer confirmed that the other station I called Clay Steely over at ABC/Disney and was having some tower work done and was operating arranged to try a little experiment wherein KSPN at reduced power. Once full power was restored, would reduce the level of its lower digital carriers by normal reception was restored. 2 dB. That did the trick, reducing the IM products by In the other case, the issue was a noisy about 6 dB and reducing the “hash” to a level where power supply in a cable box. It was wiping out AM it was no longer objectionable. A little sleuthing and reception in a listener’s house. Our engineer the application of some engineering sense provided a (Amanda) advised this elderly listener how to good, working hypothesis that was the basis for a troubleshoot the problem (turn off circuit breakers quick resolution. one at a time while listening to a battery powered The first radio interference case in history radio) and this procedure quickly showed up the no doubt occurred when the second transmitter took problem – again, not ours. to the air, and we have been battling interference ever Down in Colorado Springs, the problem has since. It’s a fact of life in the broadcast business, historically been low signal strength from the something we have to deal with from time to time. If complaining station, and I strongly suspect that it is we start with a clean house, obtain good empirical again. My advice to any station engineer trying to data and apply good engineering sense, we can work sort out an interference problem would be to start at through most interference issues in a hurry. 2 The Local Oscillator March 2011 The New York Minutes By Brian Cunningham, CBRE Chief Engineer, CBC – Western New York Hello to all from Western New York! In last independently run and operated companies, and in months edition of The Local Oscillator, Art Reis some cases they even employ former engineers and pointed out the difficulties he has experienced in repair/rebuild personnel from the manufacturing obtaining replacement parts for Sony and Denon plants. They have found that the cost of warehousing broadcast equipment. From what I am told by those replacement parts for all manufacturing runs, the in the broadcast equipment sales end of the industry, labor costs for repairs, and maintaining the required parts availability closely coincides with the amount stable of test equipment and support staff, was just of sales of any given product.