Harris Flexiva Raises HD Radio Sidebands
     

DAYTON, Ohio — Dayton Public Radio is a CPB-qualified nonprofit corporation and licensee of two noncommercial FM stations, WDPR(FM) and WDPG(FM). Dayton Public Radio provides the only classical music and fine arts programming to more than 1.1 million listeners in west central Ohio, including the Dayton metro (Arbitron market #63). JMS & Associates, with 30 years of experience in designing, building and maintaining state-of-the-art broadcast facilities, was retained by Dayton Public Radio to handle its technical requirements including this project.

Classical music audiences are critical of program audio quality, so WDPR became an early adopter of IBOC HD Radio in 2004 with the installation of a Harris Z6HD transmitter and Dexstar exciter. I later converted sister station WDPG to IBOC with a split-level system using their existing Harris HT-20 and a Harris Z8HD+. DPR also upgraded to accommodate multicasting and enhanced data capabilities.

Limited space

As the initial –20 dBc IBOC levels have proven to not yield coverage parity with the FM analog signal, I closely followed the research and testing regarding elevated IBOC sideband levels being done by John Kean and NPR Labs, as well as CBS, Clear Channel and Greater Media.

It appeared that an IBOC level of –14 dBc was a good compromise for improved IBOC coverage with tolerable levels of interference to the analog signal. The Corporation for Public Broadcasting began offering grants for stations to convert to IBOC HD Radio or improve existing facilities, so I began researching a solution.

WDPG is a 50 kW Class B with a TPO of 12 kW. WDPG utilizes a directional antenna on a leased tower, so combining was eliminated as a reasonable option, as was high-level combining due to the high cost and inefficiency.

The initial split-level system that was implemented for an IBOC level of –20 dBc, which worked quite well at that level, could not adapt to IBOC levels higher than about –18 dBc due to mask compliance challenges. Low-level combining seemed plausible, but there were other limitations at the site.

Space was limited in the existing prefab concrete shelter, and I wanted to maintain the existing HT-20 as an analog only auxiliary. The shelter could accommodate only a six-ton, wall-mounted HVAC unit for the closed system, so heat load and efficiency were factors. Reliability is always important and I wanted to avoid a tube-type transmitter to minimize site visits and lower ongoing costs.

The ideal solution would be an efficient solid-state transmitter with great performance, multiple layers of redundancy, a small footprint and compatible with existing station equipment. I spent considerable time researching new and existing products from the major transmitter manufacturers and queried other station engineers.

My search ultimately led to the Harris Flexiva FAX-20 transmitter. I was familiar with the LDMOS-FET technology that Harris uses in Maxiva solid-state TV transmitters, which allowed a very high-power density in a small physical package. This technology found its way into the radio line with Flexiva. The 20 kW model has a smaller footprint than the Z8HD+ it would replace. Space problem solved.

Each power amplifier has its own power supply for 1:1 redundancy, and the transmitter will run with several failed amplifiers. When a PA or PS fails, they are hot-swappable, minimizing downtime. There are redundant fans as well. The FAX-20 is actually two combined 10 kW PA blocks (which I fed via two separate AC power feeds), so an entire PA could fail and the station would still be on the air at reduced power. Automatic restart eliminates another potential issue.

The transmitter includes a FlexStar HDXexciter, which delivers a high-quality signal. It has grown in reliability since our first HD installation in 2004, when iBiquity was still working on the right coding recipe.

Simple installation

Another important feature is the Harris Real Time Adaptive Correction software, which maintains FCC spectral mask compliance despite varying conditions and the elevated IBOC levels. Changing modes between FM, FM+HD, and HD only, or changing the IBOC injection level from –20 dBc to –14 dBc is as simple as changing a data field. The Flexiva amplifiers change bias on-the-fly to maintain linearity and optimum efficiency for the respective mode.

Installation was straightforward. We slid the PAs and PSes into their slots and the exciter into its rails, and installed the 3-1/8-inch RF line to the antenna switch (with a Bird BPM power meter in line). Following electrical and grounding work, we connected the AES audio and exporter LAN from the Harris Intraplex STL and tested into the dummy load. The transmitter had been tested and burned in at Harris, so the initial turn-on consisted of depressing the “on” button. Cool!

Interfacing to our Burk Systems remote control was a breeze since telemetry, status and command logic levels are standard. The ability to monitor and control the transmitter via a Web-based browser from anywhere with Internet access provides another layer of redundancy.

The Flexiva was ready to be put on the air after upgrading the software and configuring the Harris HDE-200 embedded exporter and HDI-200 importer. The new embedded exporter solves many of the problems the earlier models had due to dependency on the computer hard drives and operating system. The HDE-200 configuration and monitoring is now done via an administrative computer on the network. No changes were necessary to our Intraplex STL or other station systems. The Bird BPM power meter, Agilent spectrum analyzer and Belar FM HD-1 mod monitor verified proper operation.

WDPG is now operating at –14 dBc IBOC level. Digital coverage has increased with no discernible interference to the analog signal. So far we have not had a failure or maintenance issue and are very satisfied with the Flexiva and Harris support.

Jim Stitt is president of JMS & Associates.

For information, contactBrian Clifford at Harris in Ohio at (513) 459-3714 or visit www.broadcast.harris.com.


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