Mar 1, 2008 12:00 PM, By Pat Lane
WKNO-FM, an NPR member station in Memphis, began operations on April 1, 1972, on 91.1MHz with 40kW of effective radiated power. The station launched its HD Radio signal in the fall of 2007. Up to that point, we underwent incremental upgrades and adjustments to the original installation.
The existing Dielectric combiner switch was re-used for the HD Radio installation.
The original transmitter was an RCA BTF-20 with an RCA BFC-6 antenna located 550' above ground level on the WKNO-TV tower. In 1987, a new Continental Electronics 816R-3B transmitter was purchased to replace the original RCA transmitter. The old transmitter remained in service as a backup for a period of two years.
In 1989 a second Continental Electronics 816R-3B transmitter was purchased along with an automatic switcher/combiner unit manufactured by Dielectric, and our power was increased to 100kW ERP. These two transmitters performed quite well until they were replaced in 2007 with two Harris HT\HD+ common amplification tube transmitters for WKNO's HD Radio project.
Prep-work for HD Radio
The HD Radio project was preceded by a series of upgrades that started around the turn of the 21st century. The original BFC-6 antenna had been face-mounted on our Stainless G-7 television tower and was positioned southwest toward the major population area of our market. This arrangement provided good signal coverage to the majority of our listening area until the population began moving northeast of our site.
The original 600' WKNO-TV tower is located about 225' from the G-7 tower and was being used only for microwave antennas. The original Channel 10 batwing antenna was still on the top of this tower and had been unused since 1971. Structural engineering studies were performed and it was determined that the old Channel 10 antenna and the top 100' of this tower could be removed, and a 70' pole installed with a new WKNO-FM antenna. This kept our center of radiation at the same height of 550' as on the other tower, with a net move of 225' to the south.
The original BFC-6 antenna and transmission line were showing signs of age by this time and needed replacement. A new Shively 6810-6 antenna was purchased and installed in 2001 along with new transmission line, resulting in the elimination of the null in our coverage area.
Plenty of copper and brass plumbing.
The original guy wires and anchor hardware for this tower, in service since 1953, were overdue for replacement. Pegasus Tower Company was selected for this project in 2006, as it had successfully changed our TV antenna for our digital television project a few years earlier.
Ready to work
We began studying the possibility of implementing HD Radio in 2007, with the foundation in place from recent tower and antenna upgrades. We had recently sold another station to our south that provided news and information programming to our audience. We felt an HD Radio transmission could provide an additional program stream to replace this service, and add other new services through a multicast configuration.
The cost of high-power solid-state transmitters steered us toward tube models. We opted for parallel Harris HT\HD+ transmitters for several reasons, but first and foremost for its common amplification design. The ability to broadcast HD Radio and analog programming eliminated the need to install a second antenna fed by a single digital transmitter. That proposition concerned us because the HD Radio/analog ratio would not have matched in parts of our coverage area.
We also saved energy by eliminating the need for mid- or high-level combining. These combining methods would have required a massive amount of RF plumbing, and we would have wasted up to 10 percent of our analog power and up to 90 percent of our digital power into a reject load. The use of common amplification also allowed us to retain our system redundancy.
The final Harris HT\HD+ Transmitter installation.
Prior to delivery of the transmitters, Nina Stone, transmitter engineering assistant for WKNO-FM, and I made the trip to Quincy, IL, to witness the proofing and testing of our new transmitters. I consider this a very important step in the overall project. Common-mode HD Radio transmitters tune somewhat differently from analog-only transmitters, and it was a great learning experience to go through the procedures and ask questions with the factory technicians and engineers. We were able to learn the difference in interlock operation from our previous transmitters so as not to have a surprise during installation and testing. Seeing it all hooked up and working and taking pictures instead of relying only on printed instructions during installation was a great asset.
The ac wiring size needed to be upgraded for the new transmitters. Because the power supplies are closer to the electrical service panel than the old transmitters, less of that expensive copper wire was required for primary power for the installation. The old wiring to the previous transmitters was replaced with the three-phase 30A wiring to the new transmitter cabinets for blowers, etc. All of the interconnecting wiring (HV and control) between the power supply cabinets and the transmitter cabinets was run in conduit underneath the floor, making for a neat installation.
For safety and lightning protection, all transmitter cabinets, power supplies and equipment racks are bonded with 2" and 4" copper strap spliced with silver solder. The constant-voltage transformers for the filament circuits were bolted to the floor next to each transmitter. Dampers were retained in the transmitter exhaust ducts to allow the building to be heated by the transmitters during cold weather.
Staying out of the dark
Power supplies for the new Harris transmitters were installed in a room between the FM transmitter room and the TV transmitter room, resulting in a much smaller footprint for the new FM transmitters. The existing transmitters were removed one at a time and the new Harris transmitters were installed one at a time, resulting in our operation at half power level for several weeks instead of going dark during the installation.
Pegasus Tower replacing the tower's guy wires.
The analog and HD Radio combining is done in the Harris Flexstar HDX exciter. The exciter has two outputs: The main output feeds one transmitter directly at up to 55W, and the second transmitter is fed from an auxiliary output through the Flexstar Boost Pro amplifier. The auxiliary output operates at very low power, which is where the Boost Pro works its magic.
The two Flexstar outputs operate independently of each other. If transmitter one goes down for maintenance, the main output is muted while the auxiliary output remains active. The Boost Pro, essentially a smart 55W amplifier, drives the second transmitter. If the second transmitter is taken down for maintenance, the Boost Pro goes dark but the exciter output to the first transmitter remains active.
We use our existing Dielectric combiner to combine the two transmitters into the Shively 6810-6 antenna. The signals are fed through 3?" transmission line to the antenna. If one transmitter is taken down, the Dielectric switcher splits the signal from the other transmitter between the antenna and the reject load, sending about 25 percent of the usual power up to the antenna. That output can be bumped to 50 percent through manual or remote control over our Burk VRC-2500 units.
Transmitter phasing for combining is accomplished through a menu on Boost Pro. A phasing circuit allows us to adjust the phase of the signal coming out of Boost Pro from the front panel, relative to the phase of the signal going into the unit. This allows us to run both transmitters into the Dielectric unit, which by design is a 90-degree hybrid combiner, without having to cut and piece small bits of coax together to phase the two transmitters. A 90-degree phasing section was added to the output of the second HT\HD+ transmitter into the combiner to split any reflected power equally between the two transmitters.
We are using Flexstar Importers and Exporters at the studio to combine and transport our radio programming, which includes three HD Radio broadcast streams. The analog and HD-1 signals are also sent through an Orban Optimod 8500 on-air processor at the studio, while the multicast HD-2/3 channels are processed by Neural Audio NeustarSW4.0 software running on the Importer. All this is multiplexed by the Exporter and sent over a 7.1-mile Microwave Radio Company (MRC) STL system to the transmitter site. The 7GHz Twin Stream unit from MRC carries analog and digital TV, and also includes a Harris Intraplex STL HD Plus system for our on-air radio signals. The Intraplex T1 circuit also provides additional digital control for our Harris Diamond TV transmitter audio through a data stream.
To accommodate HD Radio, we added several modules to our existing Intraplex chassis at the studio and transmitter sites. Intraplex DS-64NC modules were added at each site to handle digital Ethernet traffic with HD Radio signals from the Importer and Exporter. An existing Intraplex LAN extension is used for other IP connectivity, but we elected to keep the HD Radio on a separate LAN to avoid any possibility of data collisions. A PT-153 studio module and PR-153 transmitter site module were also added to handle AES audio signals for interlocking transmissions, as well as D-to-A conversion for FM transmission. These are enhanced Apt-X compressed cards that provide very high audio quality on the FM analog signal while permitting additional capacity to the LAN functions.
Russ Abernathy reviews HD Radio export/importer settings.
All Intraplex modules provide programmed dipswitches for appropriate bandwidth usage, and the upgrades were up and running after a simple plug-and-play installation. The additional bandwidth over Intraplex will eventually be used to connect the studio's Internet server to the transmitter site, using dedicated lines to connect through the studio switching system.
Our signal is very clean. The Harris real-time adaptive correction (RTAC) technology allows us to significantly exceed the FCC mask, reducing our digital sideband regeneration to below 80dB down � well below the 74dB down requirement. RTAC takes a sample of the signal output and feeds back into the Flexstar exciter to cancel distortions on the digital sideband regeneration. It removes the garbage to produce a much cleaner over-the-air signal.
Problems and solutions
Additional equipment at the transmission site includes a Crown amplifier and a set of JBL Control One speakers for live audio monitoring. Digital modulation monitoring is provided through a Day Sequerra M2. This provides everything we need for basic monitoring.
Problems have been scarce since the installation. We connected our exciter rack to the Triplite Smart Online UPS after experiencing some front-panel display lock-up; this was due to lack of clean power direct from the utility company and not the exciter, which remained live and continued to provide the digital and analog signals. We also placed one of the 24V utility power supplies that provided power to the Dielectric relay power on our UPS system to keep the transmitter interlock circuits live when transferring power to our 200kW Caterpillar diesel power generator. The generator provides enough backup power to keep everything running for hours in the event of a local power failure.
We designed an interesting solution to improve bi-directional communication between our Burk remote control inputs and the transmitter status outputs. We etched a circuit board with opto-isolator chips and Euro-block connectors and inserted it between the devices to ensure communication in both directions.
We also had challenges when connecting the modulation monitor to a sample port on the transmission line to read the stereo pilot signal. The readings were incorrect, and running a hand down the RF cable to the monitor altered the readings. A simple common-mode RF choke was constructed out of nine turns of RG-58/U coaxial cable wound on a piece of 1�" PVC pipe and inserted at the RF input to the monitor.
All the heavy work of positioning the transmitters and power supplies was performed by the moving company, and two electricians completed all of the electrical work. Other than that, Nina and I completed the entire RF plumbing, interconnection and remote control wiring. Our pre-planning made the entire installation a breeze, and our on-air signals sound clean, crisp and powerful.
Caterpillar 200kW generator
Day Sequerra M2
Dielectric automatic combiner/switcher
Harris HT\HD+, Flexstar HDE-100 Importer, HDI-100 Exporter, HDX-FM Exciter, Boost Pro, Intraplex STL HD Plus, Neustar SW4.0
JBL Control One
Microwave Radio Twin System STL
Neural Audio Neustar SW4.0
Orban Optimod 8500
Pegasus Tower Company
Triplite Smart Online UPS
Lane is the transmitter supervisor for WKNO-FM, Memphis. All photos courtesy of Brad Broadus, Pegasus Tower Company.