Trends in Technology: Higher-power HD Radio

February 1, 2011


Looking to add or upgrade IBOC? Here''s what you need to know.

Does your 2011 project list include either adding IBOC for the first time, or otherwise taking advantage of the new rules with respect to the allowable IBOC power? If so this article is for you. In reality the solutions to either of those two problems are nearly identical – as usual it depends upon your transmitter site particulars.

Of course the very first thing to do is to determine just how much IBOC power you can now run. As of May 10 2010, most stations were allowed to run up to -14dBC without giving prior notice to the Commission. (After higher-power operations commence, the station has 10 days to send the FCC an informal letter informing them of the change.)

Details on the power upgrade.

It is quite possible that your station may be able to run more than the “blanket” increase – in other words, some power level between -14dBc and -10dBc. Bear in mind that the results of those calculations may show that your station can run more power in one group of sidebands than the other. You will need to file a request with the Commission prior to increasing IBOC power to beyond the -14dBc blanket power level; see FCC Public Notice DA 10-866 for the fine details.

Details of variance in power level in FCC order 99-325.

Once you''ve determined the amount of IBOC ERP that will be allowed, you will need to file an informal request of the Commission before doing so. It is also possible that -20dBc is the IBOC limit for your super-power station.

What super-power stations need to know.

Interference

Before you plan out and build your higher-power IBOC transmission system it is important to know that there is at least the potential for interference between your new digital facility and an existing analog facility. The FCC Order 99-325 not only spells out the details of the IBOC power increase, but also the means by which interference from digital transmissions to analog transmissions will be mitigated. It is possible that you will need to throttle back to the original -20dBc level (but no lower) to mitigate the interference. That being said, paragraph 14 of the order reads as follows: "Since the commencement of 1 percent FM IBOC Power operations in 2004, the Bureau has not received any well documented complaints of interference to analog FM stations from digital signals. Since May 2006, the Media Bureau issued a total of 15 experimental authorizations to permit operations at up to 10 percent FM IBOC Power, including authorizations for 10 grandfathered short-spaced stations with as many as four first-adjacent channel short spacings. Some of these short spacings are severe. These stations operated their FM digital facilities with different levels of increased FM digital ERP throughout the experimental period, with the preponderance of the time spent operating with the maximum permissible FM digital ERP of -10dBc. The Bureau did not receive any complaints of interference to analog FM stations from licensees of analog FM stations or the listening public as a result of the experimental operations."

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Looking to add or upgrade IBOC? Here''s what you need to know.

Solutions

Shively digital injectors

Shively digital injectors


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 CD324-8DB

ERI CD324-8DB


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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Looking to add or upgrade IBOC? Here''s what you need to know.

ERI Lynx Series II

ERI Lynx Series II


Decisions

For many stations, making the jump to the -14dBc level (let alone the -10dBc level) is going to be like starting over again. Two major choices exist. Either you can decide to use your current main antenna for analog and IBOC, in which case you''ll choose a transmitter that is a combined amplifier, having the analog signal plus the IBOC signal in its output; or, you''ll opt to use your current analog transmitter, along with a smaller IBOC-only transmitter that feeds its own antenna. This antenna will have a power-gain that''s the same or close to your main antenna. This is a good option for those of us who have used a -10dB injector previously, because this size transmitter will likely have enough output capability to accomplish the -10dBc level (that is assuming the digital antenna''s power gain is the same as the analog antenna).

Shively 6813

Shively 6813


The major transmitter manufacturers that we know all have their own lines of IBOC-capable transmitters. Since the most basic requirement is being able to meet the TPO requirements, I''m including the latest information from these manufacturers – their published specs regarding their transmitter products (see below). As I wrote earlier, the other choice for increasing IBOC ERP is going with a different antenna. ERI has two different offerings. The first is called Lynx. This is a dual-input, dual-polarized antenna in which all elements are excited by both the analog and digital signals; power gain is the same for analog and digital. According to ERI it has the capability of achieving 40dB of isolation between the digital and analog inputs. The second is called Rototiller SA, which is a shared-aperture circularly-polarized antenna array. This type of antenna has the bays for the digital transmission interleaved within those of the analog antenna. The analog elements use opposite circularity than those of the digital antenna elements, providing the isolation needed. Shively Labs offers an interleaved antenna, their model 6813; Dielectric (SPX) also offers an interleaved antenna solution known as the HDR; and, Jampro offers the JSHD dual-input antenna.

As I wrote earlier, the other choice for increasing IBOC ERP is going with a different antenna. ERI has two different offerings. The first is called Lynx. This is a dual-input, dual-polarized antenna in which all elements are excited by both the analog and digital signals; power gain is the same for analog and digital. According to ERI it has the capability of achieving 40dB of isolation between the digital and analog inputs. The second is called Rototiller SA, which is a shared-aperture circularly-polarized antenna array. This type of antenna has the bays for the digital transmission interleaved within those of the analog antenna. The analog elements use opposite circularity than those of the digital antenna elements, providing the isolation needed. Shively Labs offers an interleaved antenna, their model 6813; Dielectric (SPX) also offers an interleaved antenna solution known as the HDR; and, Jampro offers the JSHD dual-input antenna.

I should note that each of the manufacturers listed here told me their dual-input antennas (or interleaved antennas) are ready to handle the -10dBc IBOC power level.

Now that the new power levels for IBOC transmission have become clear, it''s time for those of you who have been sitting on the fence with regard to IBOC to make the commitment to add it to your station.

Jampro JSHD

Jampro JSHD


It is much easier to put a digital transmission on-air now than it was five years ago; and even though many of our expectations haven''t been met (at least in terms of overall listenership), I continue to be a firm believer in the efficacy of this technology to provide a better experience for our listeners. We need to do everything we can to retain the ears that are giving us their attention.


Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at doug@dougirwin.net.



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