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Trends in Technology: Higher-power HD Radio
Looking to add or upgrade IBOC? Here’s what you need to know.
Once you've established the correct amount of IBOC ERP that you'll be able to run, it's time to figure out the most cost-effective way to do so. If you already have an IBOC transmission system going, it's obvious that the way to increase the ERP is by a change in the antenna system, an increase in the IBOC TPO, or some combination of both. (It isn't that likely that when you built the original system that you bought components big enough to increase the ERP by four-fold.) There again, the station’s transmitter site particulars may have worked out favorably. In New York, at least two stations that have increased their IBOC ERPs (WRKS and WBLS) and both were able to do so just by turning up the digital power. WRKS uses a Broadcast Electronics IBOC-only transmitter into an auxiliary antenna; WBLS just cranked up the IBOC power with its Nautel NV-15. Like WRKS and WBLS though, it’s crucial to consult your antenna manufacturer to make sure that the antenna you currently operate can withstand the peak voltages associated with the new analog plus digital power levels.
As I wrote earlier, the solution to getting a higher IBOC ERP is nearly the same whether or not you have a system already operating or you plan on building anew. Let’s examine first some possible solutions assuming you already have it on the air.
Say, for example, that your station uses a -10dB injector. Would it be possible to simply change that coupler to one with greater coupling? The answer to that question is yes, but with caveats. Shively Labs makes a series of digital injectors with coupling values of -9dB, -8dB, -7dB and -6dB (part numbers 5636, 5646, 5656 and 5666 respectively). The major factor here is the amount of throughput loss from the analog input port to combined output port. Whereas the -10dB injector that many of us have used accounts for a 10 percent loss in analog power, injectors with greater coupling also cause more analog throughput loss. The Shively 5656 injector has -7dB of coupling; let’s look at this as an example. According to Shively, this injector has an analog throughput loss of 20 percent and a digital loss of 80 percent. Let’s say our analog TPO (before IBOC was added) was 10kW. In order to make use of this -7dB injector, your analog transmitter would then need to have its TPO increased to 12.5kW. The digital transmitter's output (for -14dBc) would then need to be 2kW. Your "waste" load would then be dissipating 4.1kW. Compare this to the -10dB injector: Analog power would have been 11.1kW; digital power would have been 1kW; and the load dissipation would have been 2kW. So, if you built your original IBOC system with that much headroom and you have enough ac power to support the higher TPOs and you have enough air conditioning, then changing the injector may be an option for you.
ERI also makes a -8dB injector, model CD324-8DB. ERI's published specs for the performance of this coupler are different from that of Shively’s equivalent.
Another thing to keep in mind when using one of these injectors is that the amount of isolation between the analog port on the injector and the digital input on the injector varies considerably with the VSWR seen at the output of the injector. This isolation is critical and must be at least 30dB (preferably more). Too little isolation between the digital input port and the analog input port (whether we are referring to an injector or a dual-input antenna) will allow too much IBOC power to get back in to the analog transmitter, causing intermod products that will subsequently be radiated by the analog antenna. This isolation is a "figure of merit" for this type of injector and for the antenna systems that we’ll discuss shortly.
One more factor to consider before raising the power for IBOC is the power rating of your transmission line; answer this question by talking with the line's manufacturer and/or your normal consulting engineer.
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