In the intervening year since Radio magazine covered transmitters in the article titled Clear Transmissions in the July 2005 issue, much has happened with respect to RF transmission in our FM band, and in particular HD Radio. A year ago the coming killer app for HD Radio was going to be surround — but not much has happened on that front. However, HD Radio multicast has come into its own since then. This is a new technology that many believe to be the real killer app for HD Radio.
Living in Class C country and having become accustomed to using at least 35kW of RF (from one box), I find the rapid development of high-powered, vacuum tube, combined amplifiers to be of extreme interest. A year ago, Continental Electronics (CE) offered the only high-powered linear amp in its 816HD series. Evidently I'm not the only one interested in this technology, because Broadcast Electronics and Harris have stepped up development of their own vacuum tube, high-powered linear designs.
At NAB2005, the power limit of the Continental 816HD series was 20kW. At NAB2006, Continental's latest, most high-powered version on the convention floor was running into a test load to produce 25kW of analog and digital RF. One primary difference between this unit and last year's 20kW unit was the use of an external power supply, a simple way to limit the heat generated inside the main transmitter cabinet itself.
Mike Troje, sales manager of Continental Electronics, said that CE has its eye on even higher power. The key difference this time: water cooling. Don't let that scare you; UHF TV transmitters (among others) have used water cooling for years. This transmitter will use the Eimac 4CW30000A and makes use of a closed, purified water system. The tube anode will be attached to rubber hoses that make their way through the tube chimney. This tube still uses the same socket and cavity section that so many of us have become familiar with over the years. The power output of this transmitter, operating as a combined amplifier for analog and digital, is estimated to be between 28kW and 30kW.
Harris has introduced the HT/HD+ transmitter, which is a linearized, vacuum tube, combined amplifier based on the HT35 single tube FM transmitter. The published spec for this transmitter has an upper power limit of 22kW, but Harris said that there is ongoing development of this transmitter, and higher power levels may well be realized in the near future. This transmitter is also sold in a dual-transmitter-with-combiner package, thus allowing for 44kW of analog and digital RF. With all the talk about high-powered amplifiers, we should not forget that Harris has its line of solid-state combined amplifiers, such as the Z16HD+, that also can be doubled with a combiner, for a total of 16kW of analog and digital power.
Broadcast Electronics is working on its own version of the high-powered, vacuum tube, combined amplifier, but was not willing to go in to details when I made contact at the beginning of July. The company's highest-powered solid-state transmitter is the FMI-1405, which will generate 14kW of analog and digital RF and thus 28kW total for a combined pair.
Nautel's highest power (completely solid-state) transmitter to date is the V10d, which is rated for 8kW of analog and digital RF. Add a combiner and you're up to 16kW of RF; not bad for a 100V power supply.
In the sky
Perhaps I got a little ahead of myself in talking about transmitters, though. You will base your transmitter purchase on how much TPO you need, and that is based on the antenna being used. Let's take a look at what is available, specifically for the accommodation of analog and digital, from the major antenna manufacturers.
Shively and Dielectric offer interleaved antenna designs. Dielectrc's HDR+ series of interleaved antennas provides several advantages (though they are not advantages in every situation) such as using little additional tower space (as opposed to a completely separate antenna) and high isolation between the antenna used for the analog transmission and that used for the digital transmission — due to opposite circularity (one being right hand, and the other being left hand). Because the antenna elements are similar and located on the same tower element, the azimuth patterns will effectively be identical. The other simple advantage to using a separate antenna — though in this case interleaved — is a high level of redundancy.
ERI has taken a different approach by offering its Lynx dual-input FM antenna. With two inputs (one being 3⅛ and the other 1⅝) some redundancy is built in to the system. All elements of the Lynx radiate the analog and digital signals, and with what ERI calls a controlled mounting configuration, about 30dB of isolation between the analog and digital inputs can be achieved. It is important to eliminate analog RF from returning to the digital transmitter and possibly generating and subsequently radiating intermodulation products, and vice versa.
Jampro offers an extensive line of dual-input antennas that are well suited for analog and digital use. The Jampro Dual-Input HD antenna offers separate and isolated inputs, thus providing a high level of redundancy. This is a high-power system-handling ERP levels up to 100kW. Jampro also offers the JADP-HD, which is an antenna array composed of dual-input, wideband elements that excite a cavity-backed resonator. Like the Jampro Dual-Input HD, all the elements in the JADP-HD are driven by the analog and digital signals.
Focus on multicast
No matter how you chose to get an IBOC signal on the air, it's likely at this point that you want to be able to transmit HD multicast signals. The four transmitter manufacturers mentioned previously provide the equipment necessary to do that. The current method for generating the various HD Radio signals relies on the exgine architecture. (Exgine is a contraction of exciter and engine.) The exgine operation was discussed in the May 2006 Insight to IBOC supplement in Radio magazine, but there are five basic elements of the system: the importer (used to generate a data stream that corresponds to the secondary program service, also known to many as HD-2); the exporter, which generates a data stream that corresponds to the main program service, otherwise commonly known as HD-1, and also combines the SPS (via communication with the importer via TCP) and MPS, along with other data, in to a common data stream sent to your transmitter via UDP; an STL that can carry UDP; and finally the linearized amplifier and antenna that radiate the IBOC carriers.
Broadcast Electronics' legacy HD Radio system can be converted to the exgine architecture. The FSI-10 receives a card upgrade, and is then known as the XPI-10, and thereafter lives at the studio location. Its function is then that of the exporter. The HD Radio exciter also receives a card upgrade, afterward taking on some of the exgine functionality. The XPI-10 will then send the entire data package as UDP from the studio location up to the transmitter site. This simplifies installation dramatically. The exgine architecture is standard now with all HD Radio transmitters from BE.
Nautel's implementation of the exgine architecture makes use of its IPR importer and its XPR exporter. Continental Electronics is currently supplying the Nautel system to its customers that want to use the exgine architecture with Continental HD Radio transmitters.
Harris now makes use of the exgine architecture as well. Its system is called HDX and comprises the HDE-100 exporter, along with the Flexstar FM + HD exciter. The Flexstar has added features such as built-in SCA and RBDS generators and can accept analog, AES or even composite inputs. It comes with an internal harmonic filter should you want to use it in stand-alone transmitter applications. (LPFMs take note.)
A missing piece of the HD Radio puzzle concerns boosters and translators. Creating an HD Radio signal from the ground up for these facilities has its own challenges, and at NAB2006 we saw a practical solution introduced. Instead of generating the signal locally, one approach is to receive the datastream and retransmit it without decoding and demodulating it. The Armstrong TRX-HD series accomplishes this in one unit. Similarly, Crown and Fanfare have teamed to provide a similar system.
This year's advances in broadcast transmitter technology are almost wholly done in conjunction with HD Radio. Early adopters have learned a lot and the trails are already blazed for those just gaining interest in HD Radio and multicast. It's easier than ever to jump on the HD Radio bandwagon.
Manufacturers of transmitters, antennas and RF accessories
Irwin is director of engineering at Clear Channel, Seattle.