Ed Murlatt & Associates, Inc.
RF Services for the Broadcast Industry
Challenged with competing in the Philadelphia, PA DMA while transmitting from a location 40 miles to the north in Allentown, WFMZ's General Manager Barry Fisher determined a Single Frequency Network (SFN) would help level the playing field. Barry looked to Linear Industries to supply the over the air equipment required. Among the list of equipment was an upgraded exciter for their main transmitter, a 1 kW booster, and a remultiplexer all supplied by Linear Industries. In addtion, a microwave link needed to be established. Of course an antenna and transmission line for the booster were required.
Larry Will handled the initial engineering and FCC filings. I was contracted to install the exciter in the main transmitter and record proof of performance data. I also assisted with timing the SFN signals. This article is a broad overview of the intial setup and testing.
I conducted some Longley-Rice coverage studies to locate the best test areas for timing the two signals. I needed to find a location where there was sufficient signal from the main transmitter, and the timing would be the same as in the target reception areas. A few locations were identified that were very near the booster location. The proximity to the booster simplified the field work.
The station was concerned that there may be interference at one or more pay provider sites. Studies indicated that most of the sites were essentially collinear with a line drawn from the main transmitter through the booster site into downtown Philadelphia. There was one exception, a site in New Jersey was 10.4 µSeconds closer to the main transmitter in Allentown.
Our first attempt at bringing the system online proved unsuccessful due to a variety of problems. We regrouped several weeks later, and began our testing at the station's main transmitter site in Allentown. We were able to setup a model of the SFN in the transmitter building, and we were even able to use WFMZ's backup microwave equipment to simulate the microwave link (sans path delay). Once succesful in the transmitter building, we were able to move on to field testing.
We used a WFMZ live truck to receive both signals, and using an EFA53 in the ghost pattern mode, we were able to make subtle adjustments to timing. Once the system was timed, we were pleased to discover no interference at any of the pay provider sites. We monitored the site in New Jersey while turning off each transmitter. There was a subtle glitch when the receiver retimed, but we never experienced a complete outage at any of the pay sites. The booster first went online full time on December 14, 2012. There were some issues at the pay provider site in New Jersey when some rather impressive weather fronts moved through, and the booster was taken off the air for a few days. When the New Jersey site presented additional issues, it was determined this site was a difficult location for reception of the main transmitter. Unfortunately, the main transmitter led the booster signal at this site, and when the signal from Allentown faded it became a leading shadow the receiver could not resolve. Ultimately, the station switched reception at this site to a translator that is collocated with the SFN booster.
Once we knew the New Jersey location could not be resolved while maintaining a resonable timing in downtown Philadelphia, we went back to optimize timing for locations in central Philadelphia. A more detailed paper explaining selection of the timing locations, and measurement techniques is planned for the Tech Notes and tips on this site, so check back often.
WFMZ SFN - Allentown/Philadelphia