Running interference

WOR Radio, the 50,000-watt powerhouse in New York City, adopted Ibiquity's HD Radio (IBOC) system at 9 a.m. on Oct. 11, 2002, and at that time became New York's first digital AM radio station. WOR was initially chosen as a test station for IBOC because Ibiquity was looking for a high-power signal near a large city for testing of IBOC coverage and compatibility. WOR's licensee, Buckley Broadcasting, believes that IBOC can only help AM radio, and adopted IBOC transmission on WOR the day after it was approved by the FCC.

So far WOR has encountered many positive results during the transition to IBOC. Foremost is our daytime digital coverage. At the suggestion of Ibiquity, WOR runs its IBOC signal 6dB lower than the original IBOC specification. This is because the secondary lobe of WOR's directional pattern sends an 85kW signal towards Philadelphia. Philadelphia is 75 miles away from the transmitter site in Lyndhurst, NJ. The reduced digital power level reduces the power toward Philadelphia. Even while operating the digital carrier 6dB lower, the coverage to the Philadelphia city line, the 2mV/m boundary, is solid. With this in mind, the station was curious to investigate the IBOC nighttime coverage and examine any resulting sideband interference.

On the evenings of Dec. 2, 3 and 4, an Ibiquity test van made several trips around the New York metropolitan area. As station engineer, I rode in the van with Russ Mundschenk of Ibiquity. We ran three tests on each night. On the first night we tested our own analog compatibility. On the second night we tested the digital coverage with the presence of a first-adjacent channel digital interferer. On the final night we tested the nighttime digital coverage without the digital interferer. WOR operates on 710kHz. The station we used for the first-adjacent interferer was WLW on 700kHz in Cincinnati.

Let the games begin

To establish a baseline reference, we drove west from New York on I-78 to a point 51.7 miles from the WOR transmitter site. At this point WLW's skywave signal is 10dB (on average) below WOR's carrier. This was measured with a Hewlett-Packard spectrum analyzer with a roof-mounted 31" whip antenna in the Ibiquity van. We chose the parking lot of a state highway maintenance post in an area away from any streetlights. This location proved to be noise-free, and the skywave conditions could not have been better at 10 p.m. The spectrum analyzer, set to show stations from 660kHz to 770kHz, showed carriers neatly spaced every 10kHz. It was amazing to see the skywave phenomenon displayed on the spectrum analyzer. Hearing skywave effects on a single station is one thing, but observing the atmospheric changes on one signal while another continues without effect is fascinating.



The racks at the WOR transmitter site. The IBOC exciter is mounted at the bottom of the rack on the right.

Using a Potomac FIM-41, the WOR signal was measured to be about 0.75mV/m, which is roughly WOR's 0.5mV contour. This is outside the station's secondary lobe, but not yet in the null at 342 degrees. The WOR signal was fairly constant, indicating that a groundwave signal was present, while the WLW signal would vary greatly, indicating that we were receiving skywave coverage.

The IBOC exciters at WLW and WOR were set to cycle on and off for one-minute periods over a 10-minute window. While WLW's IBOC carriers were on for one minute and then off for one minute we monitored WOR. We then swapped the process and monitored WLW during a similar cycling of WOR. We used six radios to monitor these signals. Five were standard consumer models that you would find at any consumer electronics store and included a Technics tuner and a GE SuperRadio III. The sixth was an Ibiquity test receiver.

When the WLW carrier was 10dB below the WOR carrier and WLW's IBOC carriers were on, I observed some minor noise in the analog WOR signal. By minor I mean that the receiver volume had to be turned up to hear it. If WLW's carrier met or exceeded WOR's carrier level (and WLW exceeded WOR by 10dB at times), the noise level in WOR's signal rose, but it was far from objectionable. Clearly, the WOR analog signal within our 0.5mV contour would be useful.



Close up of the rack-mounted IBOC exciter.

Conversely, when WOR's IBOC carriers were on, and WLW's carrier was 10dB down, the WLW analog signal was rather noisy. It was not bad enough to make me want to tune WLW out, but it was annoying. If WLW's signal decreased to a level of 15dB below that of WOR, the signal was unlistenable. If, however, WLW's signal level was equal to or exceeded WOR's signal level, the noise was only slightly audible and the analog signal was useable. When the WLW signal was unlistenable and WOR's IBOC carriers went off, the WLW signal was still unlistenable, either because of another station on 700kHz coming in, or the sideband from a station at 690kHz splattering. In general, I would say that it was not strictly the IBOC carriers that made the signal unlistenable.

Wanting more

After an hour of listening, we packed up and drove to a location about 72 miles from the WOR transmitter, near Bethlehem, PA. At this point, the WOR and WLW signals were nearly equal, and judging from the signal variations on the spectrum analyzer, were both predominantly skywave. When both signals were skywave, the noise level on the desired signal was somewhat annoying when the interfering carrier was 10dB lower. The noise level increased if the interferer increased to a level that was equal to or exceeding the desired carrier, but I considered the signals to be listenable. Annoying, but listenable.



The courses taken on the second and third nights of testing.

On the second night we took four routes. One took us out I-78, another took us south down the Garden State Parkway, a third took us through Manhattan, the Queens Midtown Tunnel and east out the Long Island Expressway, and a fourth took us up WOR's null north on New Jersey Route 17.

During all four routes, the digital signal held its own, finally falling apart 50 miles to the west, 52 miles to the south (which is in WOR's minor null), 53 miles to the east on Long Island (which is in the major lobe, but the ground conductivity on Long Island leaves much to be desired), and 20 miles north through our null. When driving through the null, we noticed that the farther we were from the transmitter, the less upper sideband we received from WOR. Yet the digital signal still decoded properly, and in most cases fell apart close to the 0.5mV/m contour.

In Manhattan, we drove down 40th Street. There are several areas on 40th Street before Broadway where the WOR analog signal is fairly noisy. Despite the interference, the digital signal remained strong. The only place where the digital signal was undecodeable was at the corner of 40th and Lexington. There are several large buildings there, and probably due to a combination of reflections and shielding, the entire AM band from 660kHz to 770kHz as shown on the spectrum analyzer all but disappeared. Naturally, when there is no signal, neither analog or digital will play. Once we moved out of this area the digital signal locked in again.

The Lincoln Tunnel and Queens Midtown Tunnel have a leaky coax system that the MTA can insert traffic advisories on all New York AM stations. This system completely strips the IBOC sidebands. However, when sitting under the roof at the toll plazas at both tunnels the digital was going strong.

The hat trick

For the last night of testing, WLW ran only an analog signal while WOR ran analog and IBOC. The WOR coverage mostly duplicated the same results, as it did with WLW as an IBOC interferer. This leads me to believe that this interference does not affect an IBOC signal.

Listening to the digital signal while on the road was amazing. WOR's Joey Reynolds had a guest singing live Christmas carols with an acoustic guitar. It almost sounded like the guest was in the van with us. It was that good. It was also amazing to realize that this was an AM station. It wasn't getting fuzzy under bridges and overpasses.

My opinion is that if the IBOC carriers were reduced by 3dB to 6dB for all stations at night, the digital coverage would obviously be reduced, but there would be less effect on the analog signals. Certainly, digital coverage is adequate at night on the WOR signal.

Another way to look at this is to consider a Class B FM station. Depending on terrain, it is expected that signal will be lost about 50 miles from the transmitter. No one thinks anything less of the system because of this. Maybe we should start considering AM in this same vein. The audio quality of the digital signal, in my opinion, far surpasses the analog quality. Is IBOC the savior AM radio has been in search of? I don't know, but I know that I would rather listen to the digital audio rather than the bandwidth-restricted, scratchy analog.


Ray is corporate director of engineering for Buckley Broadcasting/WOR Radio, New York.


Map image courtesy of Chip Morgan and Realcoverage.com.


The tour of the WOR transmitter site.
SBE Chapter 15 meets at WOR

The January 2003 meeting of the Society of Broadcast Engineers New York City Chapter 15 was dedicated to IBOC. The program began with tours of WOR's transmitter facility and an Ibiquity test van.

Later, Tom Ray of WOR, and Jeff Detweiler and Russ Mundschenk of Ibiquity gave a presentation on the transmitter installation, the upcoming consumer radio rollout and general discussion on implementing IBOC.


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