NPR Sheds Light on HD Radio Coverage

February 1, 2008

For every radio station that signs on with a new IBOC digital hybrid signal, you can pretty well bet there's at least one person who's likely to spend a lot of time behind the wheel of a vehicle with an HD Radio, driving criss-cross patterns across the market in an effort to empirically determine how well the digital signal matches up with the pre-existing analog coverage. Until now, that person would almost certainly have been an engineer. But with HD Radio products beginning to find their way to consumers and station owners looking for a return on their digital investments, you can bet this scenario is about to change in a big way. As HD Radio integrates with the existing business model, station owners, programmers and clients will all want to know where their new digital signal will be heard.

Reliable coverage modeling

As a singular entity, probably no radio group has invested more time and effort in trying to fully understand and develop the potential of IBOC digital radio than National Public Radio. Through its engineering department and NPR Labs division, this organization has made a substantial investment in HD Radio, so it's no surprise they began looking for an accurate way to calculate digital service areas early on during the technology's deployment.

The test setup used by NPR Labs.
Click image to enlarge.

From the beginning of the HD Radio rollout, most engineers assumed that an FM digital hybrid signal with the specified 20dB D/A ratio fed to a common antenna would provide digital service contours likely to mimic those of the station's analog coverage, albeit with a slight reduction in total service area. Thus, as with analog FM signals, field strength would be the principal predictor of digital service coverage. But the roles fading, multipath and interference might play in disrupting digital reception was not clearly understood or appreciated. As anecdotal reports of unexpected variance in digital coverage began to surface among NPR member stations, a need to better understand reception issues became apparent.

With this in mind, attempts to evaluate IBOC digital coverage at multiple sites began as early as 2004, using a portable test rack consisting of an HD Radio car receiver, field strength monitoring equipment and a GPS-equipped laptop computer to record data while radials form various IBOC transmitter locations.

In a paper delivered at the 2006 NAB Broadcast Engineering Conference, an NPR Labs study of digital coverage involving 26 different IBOC hybrid FM signals clearly demonstrated that field strength by itself was not a an accurate predictor of digital service area. Wide variation existed in the amount of digital signal required for reliable acquisition among the stations sampled.

Clearly, existing analog coverage models were not going to work for HD Radio, and further research was needed.

Better measurement systems

Among the salient issues that became evident during NPR's early mobile signal surveys were intervening variables imposed by unique RF environments and limitations in the measurement platform. So when the Corporation for Public Broadcasting (CPB) released an RFP in early 2006 to research and document predicted digital coverage for around 850 public radio stations, it was already understood that developing a functional model would require new technology and a comprehensive approach. NPR Labs won that grant, and an enormous technical undertaking began.

One unknown was the performance characteristics of the HD Radio receivers themselves. While the available choice in digital receivers was somewhat limited in 2004, an increasing number of products were coming to market. Characterizing an average receiver required the creation of a fixed test bed that could repeatedly manipulate complex variables with the unit under test. The NPR Labs team responded with the construction of a system that could vary desired signal FS, independent levels of co-channel and adjacent channel interference, and simulate Rayleigh (multipath) fading. With such a spread of variables, automated testing and data collection were essential to the process.

Changes in the mobile measurement platform were also needed. A standardized monopole antenna with integral ground plane was fabricated for attachment to test vehicles, greatly reducing pattern/gain variations presented by the mag-mount antenna used in earlier studies. Multiple field strength monitors were added to allow simultaneous measurement of the desired carrier, as well as those on two adjacent channels. With these improvements, detailed measurement data sets needed to verify modeling performance became available.

Once the new technology and methodologies became operational, the research team was left with the massive task of data collection and analysis that would provide the basis for predictive modeling of hybrid IBOC digital coverage.

The emerging model

The ground plane antenna used on the test vehicle.
Photo credit: Photo courtesy NPR Labs

After testing and characterizing 15 different HD Radio receivers, the data suggested that much of the variability in IBOC digital service coverage can be accounted for by a couple key variables. While field strength of digital carriers is an obvious factor, it was determined that first adjacent channel interference plays a major role in determining whether a desired digital signal will be successfully captured and decoded. Most notably, it was observed that while FM IBOC digital signals can survive a considerable amount of interference from either an upper or lower first-adjacent channel, simultaneous interference both above and below can degrade receiver performance by as much as an additional 10dB beyond that of a single interferer.

The test sequence also established parameters for the interplay between desired signal levels, co-channel interference and fading/multipath variables.

Based on the extensive data set generated by receiver tests, NPR succeeded in developing a predictive computer model for IBOC digital coverage. In order to test the programs utility, a new set of measurements was taken for WJFK in Manassas, VA, using the improved mobile test platform. A comparison of the predictive versus measured data showed a very strong correlation of 94 percent between data sets, affirming the model's ability to anticipate coverage based on signal and terrain data for both the station under study and first-adjacent channel stations. Since that time 10 more stations have been used as points of comparison between measured and modeled coverage, with correlations ranging between 85 and 95 percent.


The fruition of NPR Lab's effort to build an FM IBOC digital coverage model had its debut at an IEEE presentation in Washington, DC, last October, as John Kean presented a paper detailing his group's work. A complete portfolio of predicted coverage for approximately 850 public radio stations, as well as publication of receiver test data, are forthcoming.

Krieger, Radio magazine's technical consultant on digital radio, is the director/general manager of WJCU-FM, and a contract engineer in Cleveland.

An in-depth examination of the work done by NPR Labs on the HD Radio coverage project in online: