Are you ready for an HD Radio Power Increase?

July 1, 2009


The digital sideband power for HD Radio may get a boost. Are you prepared for it?

Are you ready for an HD Radio power increase?

In April of this year, the Corporation for Public Broadcasting engaged NPR Labs to undertake a further study of the potential interference effects of higher-power HD Radio transmissions on the analog FM radio service. The results of this study are expected by the end of this summer, prior to the NAB Radio Show that will be held in Philadelphia this year. Although I did not attend the 2009 NAB Show myself, several of my colleagues who did all formed the impression that some amount of increase in digital power is inevitable.

I'll look at the practical means by which a station can increase its digital power level. If your station is waiting until the dust settles prior to committing to the purchase and installation of an HD Radio transmission system, hopefully you can gain some insight in to what will be necessary when you do make the decision to go forward. Let's first review the common means used up until this point to get HD Radio transmissions on the air.

Probably the most common method of creating the hybrid HD Radio signal is by way of a high-power, -10dB, four-port injector installed between the output of the analog transmitter and the main antenna. The analog transmitter's power output needed to be increased by 10 percent to make up for losses through the injector; the HD Radio transmitter had an output that was 10 times the TPO needed to get the correct HD ERP, because the coupling of the high-power injector is -10dB (in other words, only 1/10 of the power inserted on that port of the injector, or -10dB, actually makes its way out of the injector and in to the main transmission line). The other 90 percent of the digital transmitter power was terminated in a waste load and dissipated as heat.

This method proved relatively easy, as long as the analog transmitter was sized adequately, and the facility could provide the extra ac power and air conditioning capacity. Because the analog and digital made use of the same antenna, the -20dB ratio between the analog signal and the digital signal was consistent for coverage throughout the listening area.

Another way to use the same antenna for digital and analog was to procure a transmitter that was a common amplifier — one that had the analog and digital signals common in its output. This of course required buying a new transmitter, and was somewhat limited in the amount of power it could supply — at least early on. The transmitter technology for HD Radio has moved along well in the last four years.

If a facility had a licensed auxiliary antenna that had a HAAT of at least 70 percent (and no more than 100 percent) of the main antenna, while being within three arc-seconds in both longitude and latitude, a station could use it to transmit the HD Radio signal. This ended up being a convenient method for some stations. I have used this method, and early on we were concerned that the -20dB ratio of digital to analog would not be consistent in the field due to the use of different antennas. In practice it did not turn out to be an issue though.

Finally, if the station had a high-power analog transmitter that lacked 10 percent headroom in its output capability, but you were determined to use the same antenna to transmit both signals, then Split-level combining was sometimes a way to go. A combined amplifier analog and digital transmitter was combined with an analog transmitter to develop the correct amount of digital RF and analog RF for a common antenna feed.



The digital sideband power for HD Radio may get a boost. Are you prepared for it?

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 Lynx Series II

Antenna applications

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 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 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.

Jampro JSHD

Jampro JSHD

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.



The digital sideband power for HD Radio may get a boost. Are you prepared for it?

Harris HT/HD+

Harris HT/HD+

Perhaps a new transmitter

Of course another possibility (especially if you have not invested in HD Radio at all) is to go with a new combined-amplifier transmitter. Several of the manufacturers of combined amplifiers have already published specs on their various transmitters' analog power limits with respect to the 10dB power increase. For example, the Harris Z12HD+ has an analog power limit of 2,876W in this mode. Its HT/HD+ (which is the vacuum-tube combined amplifier) tops out at just under 8,200W of analog power in this mode. Harris recently introduced the HPX40, which was designed with increased IBOC levels in mind. This transmitter will produce 16kW of analog power in the -10dBc mode, making use of the standard PAPR algorithm.

Nautel has developed a different version of the PAPR (peak to average power ratio) reduction algorithm for use in its IBOC transmitters and thus has different set of published specs for the amount of analog power its transmitters can generate in the -10dBc digital mode. For example, the NV20 will deliver up to 8.8kW of analog power in this mode; the NV40 will go up to 17.5kW.

Nautel NV Series

Nautel NV Series

I have learned from Broadcast Electronics that its current set of solid-state combined amplifiers can be operated in the -10dBc mode, with the obvious reduction in analog output power capability. Like BE, Continental Electronics is taking more of a conservative, wait-and-see approach as to where higher digital levels really end up before publishing elevated level performance specs on their vacuum-tube combined amplifiers.

If you currently use a separate antenna for HD Radio (often referred to as space combining) then your ability to move up in power by 10dB is probably going to depend mainly upon how much TPO you currently generate. If it was difficult to find the right amount of floor space, electricity or air conditioning for the current HD Radio transmitter, how easy is it going to be to increase it by 10dB? Is there the necessary amount of isolation between the main and auxiliary antennas? These are obviously very site-specific questions. One possible solution may be to change your auxiliary antenna to one with higher gain. This will not only lower your TPO requirement, but could also increase the isolation from your main antenna as well.

Broadcast Electronics FMi Series

Broadcast Electronics FMi Series

The split-level combining method will likely be problematic when trying to increase digital power because there generally isn't that much headroom available in either the IBOC amplifier or the analog transmitter. Assuming there is some amount of headroom available in the current IBOC amplifier, one possible solution would be to add a second (identical) IBOC amplifier with a 3dB hybrid combiner to increase the overall IBOC power. This would obviously necessitate extra space, ac power and air conditioning at the site.

I've used the -10dBc levels because it provides the most stringent requirements. It's certainly possible that the amount stations are allowed to increase is going to be less than that, making the solutions to the problems associated with increasing HD Radio power considerably easier. I'm a believer in HD Radio — not only in the efficacy of the technology, but in its future importance to our industry. If we continue to make it available to the radio audience, I believe it will experience a continually increasing level of acceptance. It took FM radio 20 years to catch on after all.


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


Resource Guide

HD Radio transmitter and antenna manufacturers

Armstrong Transmitter
315-673-1269
www.armstrongtx.com

Bext
619-239-8462
www.bext.com

Broadcast Electronics
217-224-9600
www.bdcast.com

Broadcast Technology
719-336-3902
www.broadcasttech.com

Continental Electronics
800-733-5011
www.contelec.com

Dielectric
800-341-9678
www.dielectric.com

ERI-Electronics Research
812-925-6000
www.ERIinc.com

Harris
800-622-0022
www.broadcast.harris.com

Jampro Antennas
916-383-1177
www.jampro.com
Larcan
303-665-8000
www.larcan.com

Nautel
207-947-8200
www.nautel.com

OMB America
800-662-4872
www.omb.com

Propagation Systems Inc
814-472-5540
www.psibroadcast.com

PTEK
888-889-2958
www.ptekpower.com

QEI
800-334-9154
www.qei-broadcast.com

Shively Labs
888-SHIVELY
www.shively.com

SWR
800-762-7743
www.swr-rf.com

Transcom
800-441-8454
www.trcorp.com



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