Behind the power
Now that we've reviewed the common methodologies of HD Radio transmission used over the last five (or so) years, let's examine the implications of increasing the power. We'll assume the increase is 10dB, since this is the highest increase proposed. In fact it may be something lower (thus making the job of increasing the digital power somewhat easier).
If the station uses high-power combining (with a -10dB injector) it is clear that you will not be able to increase the digital power by simply developing more power for the digital input port on the injector. Neglecting the injector specs momentarily, you would need a much larger transmitter (electrically and physically), a much larger waste load, more ac power capability and much more air conditioning.
On the other hand, if you could eliminate the need for the -10dB injector, then the current HD Radio transmitter is probably sized correctly for the amount of TPO needed, assuming you could get its RF into an antenna with the right amount of power gain. One potential solution then may be to replace the main antenna. The popular antenna manufacturers, including Shively, ERI, Dielectric and Jampro all make antennas with dual inputs (one for the analog carrier, and another for the IBOC carriers), providing the end-user with a convenient means by which the right amount of IBOC ERP can be achieved.
One performance parameter critical to the success of antennas such as these has been the isolation from the analog antenna input port to the digital antenna input port. If there is too little isolation, circulators are needed on the output of the digital transmitter to prevent too much of the analog power from getting back in to the digital transmitter, thus creating intermodulation artifacts that show up as spectral regrowth. It was pointed out to me that a 10dB increase in the digital transmitter power is now going to potentially cause problems with intermod generation in the analog transmitter output as well; and for this reason it will be very important to have good isolation from the analog to digital inputs (as it has been) but also from the digital to analog inputs. This will be one critical specification to address with the manufacturer you consider.
ERI Lynx Series II
ERI offers the Lynx antenna, which provides separate inputs for analog carrier and the IBOC carriers. The elements of the Lynx are all excited by both the analog and IBOC carriers; the antenna presents the same power gain for both. According to Tom Silliman of ERI, the Lynx is already capable of handling a 10dB increase in power on its digital input. Published specs for the Lynx show that the two inputs are isolated from one another in excess of 30dB — but it should also be noted that the circulators ERI can provide for use with this antenna are rated up to 2,500W. (Higher power ones are available, according to Silliman.)
Dielectric HDR series
Dielectric offers its HDR series of interleaved antennas for IBOC and analog radio transmission. The two sets of antenna elements (with separate inputs of course) are configured for opposite circularity, which provides for a high amount of isolation from digital to analog and likewise from analog to digital. Matt Leland of Dielectric tells me that the digital input for the HDR is completely able to accommodate the proposed 10dB power increase for IBOC.
Shively Labs model 6813
Shively Labs offers interleaved antennas as well, for example, its model 6813. The IBOC input of this particular antenna will be able to accommodate the higher power IBOC levels.
Jampro offers its JSHD dual-input antenna for HD Radio. Like the other antennas previously mentioned, it has a separate input feed for IBOC, and thus has nice built-in redundancy. Greg Montano of Jampro told me that this antenna is also scalable for -10dBc IBOC carrier levels.
There are multiple instances around the country of the HD Radio signals being back-fed through an existing wideband combiner (such as the Shively 2540 used on top of Tiger Mountain in Seattle). Here in New York, we make use of a Shively combiner at our back-up transmitter site on top of 4 Times Square (although we don't transmit HD Radio from there). That system has been configured by John Lyons, the site's manager, as a back-fed system for the purposes of adding multiple HD Radio signals together for subsequent transmission by the Shively panel that is located there. I have learned from Bob Surette of Shively that this particular configuration can accommodate a 10dB increase in digital TPO; we would only need to size up the transmission line on the digital output side of the combiner. I point this out as a possible solution for some stations looking to achieve the 10dB proposed increase in digital power.
I also asked both Tom Silliman and Bob Surette if the hybrids used in their large panel antennas are able to accommodate the 10dB increase in digital power. In the case of the Shively antenna at 4 Times Square (at the very least), the answer was yes. Silliman also indicated that the answer was also yes in general for ERI panels but recommended a review and peak voltage calculation in each case.