Radio World - July 1, 2010 - (Page 14)
RadioWoRld | radioworld.com
A Case of 1900 MHz Interference
by Mark Persons
July 1, 2010
As frequencies are reallocated and the RF spectrum becomes more crowded, we run into problems. Case in point, I was called to analyze what was going wrong at a radio station where a wireless provider was complaining of interference to their equipment. When I got there, a technician from NewCore
The best way to do this was to empty the transmitter cabinet of the sub-assemblies and install the screen, holding it down with existing hardware and some additional bolts through the cabinet. The goal was to cover 1 inch long or longer ventilation holes that are 1/8 wavelength or more at 1900 MHz. That took care of most of the cabinet radiation. The second part of the equation was to install low-pass filters in the STL antenna lines. I found that
Wireless was using a 1900 MHz Yagi antenna to search for the source of RF. It reminded me of fox hunting in ham radio. NewCore is a cell phone and wireless Internet provider in St. Cloud, Minn. The technician identified the problem as unwanted 1900 kHz signals coming from five 950 MHz band STL transmitters, which a local radio station uses to send audio to transmitter sites from the same tower. The relationship between 950 and 1900 MHz is that 1900 MHz is the second harmonic of 950 MHz. Yes, all transmitters emit second, third, fourth and so on harmonic radiation. It is just a matter of how much. I measured the second harmonic on those STL transmitters at about 65 dB below the 950 MHz carrier. From the 944.5 MHz to 951.5 MHz band, those second harmonics range from 1889 to 1903 MHz. That includes all of the 1895 to 1900 MHz receive band that the wireless Internet company was using in offering their service to local customers. Look at it this way, customers’ home and mobile modems are flea-powered transmitters and are as much as a few miles from the site. Those signals are trying to compete with 10 watt STL transmitters with, say, –65 dBc harmonic signals at 1900 MHz that might be many times stronger. Ouch! No wonder they were having problems. The wireless provider was expecting to use their equipment right down to the noise floor at –110 dBm too. What was the broadcaster to do? The answer was a two-pronged approach. I verified that the STL transmitters were in specification before adding RF shielding inside the STL transmitter cabinets. In this case, most of the transmitters were Marti STL-10, but two were of a newer vintage with internal rather than external heat sinks. A photo shows the inside of one. I chose aluminum window screen because the STL cabinet parts are aluminum and it could be purchased at a local hardware store inexpensively.
The technician identified the problem as unwanted 1900 kHz signals from five 950 MHz band stL transmitters.
Telewave (www.telewave.com) has some nice TLF-860 filters, which can be factory tuned to give a whopping 45 to 60 dB attenuation at 1900 MHz. Loss in the 950 MHz band is less than 0.25 dB. They are just 4 x1.5 x 1.25 inch boxes with N connectors at each end. At $212 each plus shipping, it was a bargain. So far the system is working. It will probably continue to work until some other factor like corrosion on the tower might become a place for RF mixing and harmonic generation to take place. See you further down the road. I’ll leave the soldering iron on for you. Mark Persons, WØMH, is certified by the Society of Broadcast Engineers as a Professional Broadcast Engineer and has more than 30 years experience. His website is www.mwpersons.com. extremely robust hum and buzz elimination, and at least $300 or $400 saved in the maintenance budget. Chuck encourages readers to Google the Western Electric 111C coil. You’ll be amazed at the information you’ll find. Chuck Bullett, CSRE, can be reached at chuck. email@example.com. om Norman is a senior engineer with Burst Video; he writes relative to the May 5 Workbench comment by Paul Sagi regarding grounding conductors. He says everything written was correct, but notes that parallel conductors have lower bulk resistance and lower bulk inductance than single conductors. Tom has used this principle to good effect in providing isolated grounding systems for rows of equipment racks, for example. Not every lesson learned in RF has applications restricted to RF! Two runs of #10 round wire would have greater surface area and less inductance than a single run of #10 square conductor, for example, and it would be available locally, and virtually anywhere. Tom Norman can be reached at firstname.lastname@example.org. John Bisset marked his 40th year in radio in broadcasting recently. He is international sales manager for Europe and Southern Africa for Nautel and a past recipient of the SBE’s Educator of the Year Award. Reach him at email@example.com. Faxed submissions can be sent to (603) 472-4944. Submissions for this column are encouraged and qualify for SBE recertification credit.
Living in harmony
that depending on how they’re jumpered, a 600/150 ohm, 600/600 ohm or 150/150 ohm match is available. Plus, they’ll ring out to well past 15 kHz. (continued from page 12) Bill Whitlock’s “Handbook for Sound Engineers, 3rd cations are tremendous considering what they really are Edition” extols the virtues of the Western Electric (iron and copper), and how old they are. The aged, trusty 111C Telephone Repeat Coil, and provides plenty of 111C coil provides for an honest and balanced 600/600 design data for anyone wishing to experiment in more ohm connection that is guaranteed to block any hum or modern applications. noise source. But rest assured, the venerable 111C coil will Engineers familiar with the 111C coils will recall exceed your expectations and preserve your maintenance and repair budget. You won’t have to invest in any of the numerous hum and buzz eliminators that are available on the market. Cumulus San Francisco Transmitter Supervisor and Chief Engineer John Buckham quickly soldered up appropriate XLR termination pigtails for each transformer, as seen in Fig. 1. Meantime, Chuck drilled out an old rack panel and mounted each repeat coil on it as shown in Fig. 2. The panel was put in an old, forgotten Anvil road case to give the assembly the mechanical and electrical stability required for its mission, an intensive season of broadcasts from music festivals throughout the Bay Area. Fig. 3 shows the finished product in the case. The result? About two hours of some Fig. 3: Mounted in an Anvil case and ready to knock out some buzz! shop time yielded five channels of
If you would like to try to load the digital publication without using Flash Player detection, please click here.