The authors are senior broadcast engineer for Hubbard Radio and manager of broadcast engineering at Xperi Corp., respectively. WWFD is serving as a real-world testbed for the MA3 mode of HD Radio, which the authors say provides more coverage and less adjacent-channel interference than hybrid MA1.
Over the past 50 years, many AM stations struggled to continue to serve their listeners as they moved into the suburbs and exurbs, far from the stations’ transmitter sites. And the weaker their signals became, the more vulnerable they were to noise from power lines, TVs and other electrical sources.
In Part 1 of this article we explored why today’s AM HD Radio technology hasn’t done much to level the playing field with FM, satellite and streaming services such as Spotify. One major reason is because the current system uses the MA1 waveform. Although that provides HD Radio capabilities such as high-fidelity audio and track data, it may do so in only part of a station’s coverage area.
An HD Radio screen display of WWFD’s PSD.
Another drawback is that MA1’s digital carriers require three times more bandwidth than the analog signal, so they create more adjacent-channel interference — an annoyance that’s among the reasons why people choose alternatives such as FM, SiriusXM or Pandora. By providing a better listening experience for some stations, MA1 actually undermines others.
The MA3 waveform avoids those drawbacks because it’s an all-digital signal, whereas MA1 is a hybrid of analog and digital. MA3 minimizes the interference problem and extends HD Radio’s capabilities to the vast majority of an AM station’s coverage area.
Since July 16, 2018, WWFD in Frederick, Md., has served as a testbed that vendors, broadcasters and the FCC can use to understand how upgrading a station to MA3 affects antenna systems, transmitters and engineering practices. Our previous article described the upgrade process in detail, both from a technical and a business perspective.
This article describes the technique and equipment used to measure power coming out of the transmitter. It also discusses the extensive daytime and nighttime drive-test results conducted in summer 2019, which found that both the core and enhanced carriers are received out to the station’s 0.5 mV daytime contour.
These and other real-world tests suggest that there’s a solid business case for implementing MA3. In fact, even though only about 25% of vehicle radios in the Frederick area can tune in WWFD’s MA3 signal, the station already acquired enough listeners to make its first appearance in the Spring 2019 ratings book. The ratings also suggest that listeners are seeking out WWFD because it offers stereo audio, album artwork and other data. Finally, although WWFD is a rimshot into the D.C. market, some weeks its ratings have exceeded those of 50 kW WFED.
ADDITIONAL DRIVE TEST RESULTS
Qualitative field strength measurements used the station’s existing Potomac Instruments FIM-21 meter, which was checked against an FIM-4100, which is specifically designed to handle the MA3 mode. Drive tests used multiple vehicles’ factory OEM radios.
In the initial drive tests:
• Under ideal daytime conditions, the MA3 primary/core carriers could be decoded down to the 0.1 mV contour, as confirmed via reception reports and drive testing at or near Harrisburg, Pa., Breezewood, Pa., and Cambridge, Md.
• Critical hours propagation phenomena typically reduced reliable coverage to the 0.5 mV contour.
• Nighttime MA3 reception generally followed the station’s nighttime interference free (NIF) contour: Wherever an analog carrier-to-noise ratio of 20 dB is achieved, the MA3 carrier will generally be received. Early evening reception goes well beyond the NIF. As co-channel skywave interference increases during the evening, coverage is reduced to the NIF. In the station’s 2.0 mV contour, in-vehicle reception was reliable, without observed dropouts in either the Frederick urban core or underneath bridges. Reliable urban performance is particularly important for competing with satellite, which often has dropouts even in cities with terrestrial repeaters.
The latest round of drive tests, conducted in summer 2019, showed that the primary/core and enhanced carriers are good out to 0.5 mV daytime contour. This coverage area has a population of nearly 2.8 million people.
This means WWFD’s MA3 capabilities — the stereo audio and album artwork that enable aural and visual parity with FM HD, streaming audio and SiriusXM — are effective for attracting and retaining listeners throughout the vast majority of its service area. By extension, those MA3 capabilities also will help the station attract and retain advertisers.
The core carrier typically dropped out at the 0.1 mV daytime contour, with a few exceptions. For example, at one point while driving east into metro Baltimore, the core carrier failed at 0.2 mV due to electrical noise. However, an analog signal would have been completely unintelligible at this point due to the buzz that few listeners would tolerate.
WWFD AM daytime pattern — all digital.
Terrain also proved to be a factor. For example, in the mountains near Rippon, West Virginia, the core signal failed around 0.25 mV. The reason is unclear, but the result is relatively insignificant because at that point, an analog-only signal would have been very weak. So, most listeners might have abandoned the station at that point anyway.
In the latest round of nighttime drive testing, core and enhanced services were received to half the value of the station’s NIF contour. For WWFD the NIF zone extends to the 10.8 mV contour, and half of that value is 5.4 mV. Co-channel skywave interference appears to limit nighttime service to this contour. The core-only carriers, being stronger, may continue beyond this contour but should not be considered marketable coverage, as interference may cause reception to vary both nightly and seasonally.
WWFD AM nighttime pattern — all digital.
WWFD will conduct a second round of drive testing in early 2020 because propagation conditions are significantly different in the dead of winter. The increased skywave interference probably won’t affect the half NIF (5.4 mV) area, but it could reduce coverage beyond that contour.
THREE POWER MEASUREMENT OPTIONS
All-digital power can’t be measured using traditional analog AM practices. For example, MA3’s peak-to-average ratio is significantly higher than that of analog AM, so the transmitter’s power level meter may read inaccurately. Also, if the transmitter isn’t optimized for MA3 mode, the peak-to-average ratio may be reduced, and a different power level reading may result than if the transmitter had been optimally adjusted.
As a result, the WWFD experiments included identifying a new procedure to verify that transmitters are operating at licensed power when in MA3 mode. Three methods were considered:
• A channel power measurement with a spectrum analyzer on the transmitter’s RF monitor port using an unmodulated carrier at licensed power (verified with the station’s existing base current and common point meters), and verifying the same channel power when the transmitter is placed in the MA3 mode.
• A procedure identical to that above, but instead utilizing a calibrated average power meter.
• Replacing the Common Point current meter and each tower base current meter with a thermocouple-type RF ammeter. (Remote monitoring systems connected to pre-existing meters could then be recalibrated to what the thermocouple meter reads.)
AM stations commonly use transformer-coupled RF ammeters, but they aren’t viable for measuring MA3’s OFDM carriers, which use quadrature amplitude modulation and vary by the type of information sent. Sometimes most or all of the carriers are in phase, which would raise the peak power tremendously. Other times, the carriers could be mostly or totally out of phase with one another, thus reducing the power to zero. As a result, average power is the best metric.
The third technique proved to be the best option, for several reasons:
• A thermocouple-type RF ammeter is a device that many AM stations already have. Those that don’t can purchase one for, at most, a few hundred dollars — unlike a spectrum analyzer. In fact, the WWFD tests used a Simpson 0-15A that was purchased pre-owned for $50. These and other models are widely available online from sellers such as test-and-measurement surplus equipment dealers and even at hamfests.
• These devices also are easy to implement. At WWFD, the Simpson 0-15A was mounted on a fiberglass J-plug inserted into the J-plug between the output of the tower ATU and the tower itself. This is where the current transformer for the base current measurement is located.
• Reading and interpretation are straightforward. After the meter was inserted into WWFD’s system, a baseline reading was obtained by operating the transmitter with an unmodulated carrier with no QAM carriers present. The RF ammeter and current transformer readings should match, which means the station is operating at licensed power. Next, the QAM carriers are turned on, and the RF ammeter reading should be the same as with an unmodulated carrier. If the base current meter is a diode detector, such as a Delta TCA type meter, the reading will be slightly lower.
The WWFD transmitter AUI screen while operating in the all-digital AM mode.
WWFD’s tests used all three measurement methods because a power meter and spectrum analyzer were available. All three methods also proved to be accurate in an MA3 environment. For station owners, equipment manufacturers, consultants and other members of the broadcast ecosystem, the bottom line is that the choice comes down to equipment availability, budget and personal preference. But for most stations, measurement at the transmitter output with a thermocouple-type RF ammeter likely will be the most economical option.
OPTIMIZING ANTENNA HELPS WITH SIGNAL AND LISTENER ACQUISITION
Since Part 1 of this series published in October, the daytime antenna system was further optimized using a design provided by Kintronic Labs. The goal was to shift the day pattern from its upward position to the optimal load for the transmitter (“cusp left”), as well as to provide additional broadbanding of the antenna system. This was achieved by replacing the capacitor in the very long transmission line with a second T network.
This change provided several benefits, starting with presenting the transmitter with the best possible load (also referred to as “Hermetian symmetry”), as well as tuning out the transmission line’s inductance. Additional benefits were surprising. Radios were able to acquire the core digital signal faster: within one frame (1.5 seconds). When the digital signal was lost (such as under bridges or near major power lines), it recovered faster.
For stations that decide to implement MA3, these kinds of network changes are worth considering because they improve the listening experience. The less frustration and annoyance that audiences encounter, the less likely they are to tune away. Faster acquisition times help them find a station in the first place as they’re casually tuning around. Large, loyal audiences attract more advertising revenue, which helps make the business case for upgrading to MA3.
Another potential business factor is the possibility of adding HD2 on MA3. This could be particularly valuable for AM stations in smaller markets by providing an additional revenue stream. That income could further offset MA3 upgrade costs. The license fee also will be waived for stations that turn on MA3 full-time.
MA3 WILL HELP REVITALIZE AM
Each AM station has its own set of marketplace considerations and business challenges, which is why there can never be an industry-wide silver bullet. MA3 is no exception to that rule. However, it will be a viable option for many stations.
While an AM station with an existing, profitable analog audience is not likely to be among the first to switch to digital, it should be noted that analog AM broadcasting, in general, is not a growth medium. In-home listening is migrating to streaming devices such as smart speakers, and in-car listening of terrestrial analog broadcasts is being challenged by the new options offered in-dash.
Trends in receiver designs seem to be converging around “tuning” by visual metadata: specifically, a thumbnail preset. A receiver of the future will likely scan the bands for available content and display the available options. When pressing an icon for a favorite station, it may not be immediately obvious whether the source is AM, FM, satellite or a stream. AM stations must be digital to transmit the necessary metadata and achieve the required audio fidelity. All-digital AM is likely to be one option “under the hood” of delivering audio content to future receivers.
For the immediate future, AM stations converting to all-digital achieve aural and visual parity with other services in the dash: FM HD, satellite and streaming. Additionally, having a desirable product with a pleasant user experience in the dash will cause car manufacturers to take notice and include AM (and FM) HD in their standard offerings.
It’s important to note that with the possible exception of electric vehicles, when consumers get AM HD, they get analog AM, too. That “package deal” should benefit legacy stations by keeping the medium in the dashboard. It costs money to keep AM in the car (in terms of hardware and noise-suppression techniques), but by going digital, broadcasters on the “senior” band will cause receiver manufacturers to take notice by showing that AM can be a growth medium, as well. In short, going digital reinforces the presence of AM in the car.
Dave Kolesar, CBT, CBNT, recently recieved the Radio World Excellence in Engineering Award for 2019–2020.
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WWFD: A Station Overview
Owned by Hubbard Radio, WWFD runs an adult album alternative format on 820 kHz. It operates 4.3 kW non-directional during the day and switches to a 430 W two-tower array at night.
WWFD also has a 160 W translator, W232DG, on 94.3 MHz. Most WWFD listeners migrated to the translator after it signed on in July 2017, which made it feasible from a business perspective to replace the analog carrier with MA3 on an experimental basis.
The FCC granted Hubbard a one-year STA to operate WWFD in MA3 mode, a switch that took place on July 16, 2018. Getting to that point took a lot of time, effort and collaboration with Kintronic Labs and Cavell, Mertz and Associates for the antenna system, and Broadcast Electronics, Nautel and GatesAir for the transmitters. Xperi Corp. lent its expertise to set up the digital transmitters, and to verify the operation of the antenna system. The STA has since been renewed.
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