The long-term outlook for AM radio has been the subject of much discussion in Radio World and beyond; but the availability of impressive AM engineering information is alive and well, judging from the offerings one morning at the spring NAB Show Broadcast Engineering Conference.
The session on AM transmission was called the best of its kind in years by many who attended. It featured six timely papers from some of the most recognized antenna systems experts.
Topics included new power dissipation techniques to improve AM DA bandwidth performance; new voltage sampling devices for MoM DA proofs; the rebuilding saga of WWVA after a 100 mph windstorm; the building and rebuilding adventure of KRKO after eco-vandalism; and the construction of an AM DA array in a Phoenix landfill. CBS Radio Senior Vice President of Engineering Glynn Walden chaired the proceedings.
Burn off the negative
Ron Rackley, partner in consulting engineering firm du Treil, Lundin & Rackley, led with “AM Directional Antenna Pattern Bandwidth Improvement Using Power Dissipation.”
The test case was Radio Disney’s WDWD, a four-tower, in-line 12.0/4.5 kW DA-2 array on 590 kHz in Atlanta.
Ben Dawson discussed a sampling technique for DAs that can’t use current transformer or loop sampling devices because of FCC rule constraints. Base sampling using identical custom capacitive voltage divider networks, shown, were used.
The legacy pattern was typical of those designed and installed before about 1970, with the multiple penalties of space phasing deficiency, deep side nulls and a very high RSS/RMS ratio. The imbalanced sideband performance produced very high null distortion. As a result, the station was practically unlistenable over half of its azimuth coverage pattern.
Rackley described his well-honed technique of using computer-modeled iterations of different phasing and branching schemes with incremental load rotation to best match the pattern bandwidth performance of both the upper and lower 10 kHz sidebands with the carrier frequency.
A major challenge to achieving good results in the WDWD pattern was the high amount of recirculated power from the negative tower. The consultants performed a moding analysis of the pattern and modeled to derive a more optimal balance of tower power distributions that lowered the amount of negative power to about 2,400 watts.
FCC rules had always required DA stations to operate with licensed total transmitter power, and any negative power had to be recombined at the power divider. Rackley proved that by dissipating the negative power in a resistive load and not recombining, the result was dramatic improvement in pattern bandwidth performance. He was successful in convincing the commission to allow increased transmitter output power to make up the difference. The team did a Method of Moments proof.
Rackley says, “WDWD can now be heard very well, with almost no distortion around the entire pattern including the deep null areas.” Request a PDF copy of his paper by e-mailing [email protected].
Ben Dawson of Hatfield & Dawson Consulting Engineers unveiled the inner workings of a new sampling technique for DAs that aren’t able to use current transformer or loop sampling devices because of FCC rule constraints.
The test case here was CBS Radio’s KPTK in Seattle, a three-tower 50 kW diplexed array using 160 degree tall towers on 1090 kHz.
The towers are all guyed and of equal height and width, but they do not have the same cross-sectional lattice design, thus eliminating the legal use of sampling loops. The different cross-section designs would result in unequal coupling to loops, and so base sampling using identical custom capacitive voltage divider networks was used instead. CBS Radio Seattle’s RF Facilities Manager Arne Skoog fabricated these on rack-panel bases installed inside the LTU cabinets that they can be removed easily for calibration testing.
Dawson discussed the details of how an accurate mathematical model of the base of towers must be generated to comply with MoM DA proofing rules whenever current or voltage base sampling is used.
Every device connected across the tower base must be included in the model and its reactive shunt loading effect calculated to produce accurate results for the MoM-required impedance matrix calculations. For these 50 kW high-impedance tower bases, relatively low values of capacitance-coupled sampling devices have a negligible effect on the total base loading or the operating parameters of the other diplexed station, but generate adequate voltage levels to drive the antenna monitor.
The high-voltage vacuum capacitors used in this application have proven to be temperature-stable and can be self-healing when subjected to lightning and static discharging.
Dawson reported that KPTK was granted the first FCC MoM proof application for station license using voltage sampling technique just a few days before the NAB convention.
Winds take down history
WWVA, the venerable 50 kW 1170 Clear Channel Radio station in Wheeling, W.Va., lost all three of its original 420-foot, self-supporting Blaw-Knox towers last year in a freak windstorm. They dated to the early 1940s.
John Warner, CCR’s vice president for AM engineering and its designated direction antenna czar, became the point man. His first goal was to restore station operations with an emergency antenna as quickly as possible. His larger challenge was the complete rebuilding and retuning of the array.
Because of the extraordinary expense of replacing such tall self-supporting towers, Warner had to settle for guyed towers in the insurance coverage.
Warner proudly stated that he “put on his ham radio hat” and installed a 150-foot, inverted-L emergency wire antenna to get WWVA back on the air.
Once the fallen tower sections were removed, the team strung the wire between two of the remaining 60-foot upright sections of adjacent towers. The wire was brazed at the top of one and driven at its base insulator into a 5 –j400 driving point impedance. Power was reduced to 5 kW for emergency operations, which continued for several months as construction of the replacement guyed towers commenced.
Using the National Electrical Code, Warner calculated the self-impedances of the new guyed towers at 608 –j449, which was dramatically higher than the original impedances of the self-supporters. Major changes to the LTU and phasing system designs would be needed. To reduce the magnitude of the change and simplify the overall redesign, each tower base was loaded with a shunt capacitor of –j400 ohms, thereby reducing the base resistances to about 250 ohms.
Warner included the necessary redesign and hardware requirements to accommodate the new guyed towers in his insurance claim.
Tom King, president of Kintronic Laboratories, presented the first field application and MoM proofing using the KTL Voltage Sampling Unit device introduced at last year’s NAB Show.
WAOK, the CBS Radio facility in Atlanta, operates a four-tower 25/5 kW DA-2 array that uses two guyed and two self-supporting half-wave towers on 1380 kHz. Chief Engineer Robert LaFore and field consulting engineer Don Crain installed the external VSU boxes at each of the four tower bases and performed the due-diligence testing and system measurements in preparation for a MoM proof filed earlier this year.
Bonneville Phoenix DOE Gary Smith shepherded the forced move of KMVP to a city-owned landfill site.
The VSU also uses a high-voltage vacuum capacitor series network, but then employs a buffered ferrite step-up transformer to be able to accommodate lower-power systems that produce lower sampling voltages. King indicated the units perform to within +/-2 percent magnitude and +/- 2 percent phase tolerance from –40 to +50 degrees C for temperature stability and up to 30 kV of peak voltage capability across the entire AM band.
Crain discovered that the self-impedances of the guyed towers were much higher than the self-supporters, so 40 pF of shunt capacitance was added to better normalize the tower base conditions for the array. He used Jerry Westberg’s WCAP computer program to calculate the transformations and stray capacitances for accurate MoM modeling. The array survived direct lightning hits and extensive beacon light damage just before the NAB Show, but the KTL VSUs have since performed well without problems.
Destruction and reconstruction
Andy Skotdal, president and owner of KRKO in Everett, Wash., is a man who likes a tough challenge. His 11-year journey spent securing the required permits and then building — and rebuilding — his 50 kW station upgrade has been well documented.
Much of Skotdal’s time, effort and finances during those 11 years were spent coping with a less-than-cooperative local government, very unfriendly neighbors and the attack of a self-described eco-terrorist that destroyed two of the station’s four towers.
Skotdal offered an invaluable punch list of critical lessons learned and sage advice for anyone contemplating building or relocating an AM transmitter site and antenna array:
- File your building permits well before your FCC CPs, and expect four to eight years of processing.
- Coordinate a well-planned PR strategy to win over the community to counter opposition.
- Media can and will distort fact and truth to exacerbate and foment opposition.
- Budget adequately for the necessary legal defense and team of experts to support your effort.
- Control the clock as best you can and don’t let bureaucrats stall the permit hearing process to give the opposition time to organize.
- Play an AM radio in the public hearing meetings to show how high noise is making AM radio reception very difficult.
- Don’t give in to shorter towers or give up on full-sized or tall towers.
- Hire an attorney who has successfully sued tower manufacturers to make sure your tower company is worthy.
- Bent anchor bolts can be repaired; anti-terrorism nuts are effective.
Skotdal concluded by observing that “the proposed projects that never get built and are lost to opposition groups usually do not have a good PR plan, adequate community support or financial resources, or the will and perseverance to ultimately succeed.”
Ground nobody wants
Moving or building a new AM DA anywhere near a high-population area can be a daunting challenge in an age of aggressive NIMBYs, restrictive zoning laws and high real estate prices. A number of enterprising AM station owners have resorted to landfills as suitable sites for building AM antenna arrays.
The Bonneville Phoenix cluster offers a fine example of how to do it right. Director of Engineering Gary Smith shepherded the forced move of KMVP, a three-tower 1 kW DA-2 facility on 860 kHz to a city-owned managed landfill site next to Sheriff Joe Arpaio’s prison southwest of downtown Phoenix.
Smith knew that getting city approval for his plan would require carefully executed political maneuvering. Making friends with the mayor and members of the city council, and reminding them how the Bonneville stations had been active community supporters and contributors for many years, were key tactics in that effort. The building permit would also require that there be no visual disturbance to the nearby protected Indian burial grounds.
Landfills inherently are unstable, collapsing about 10 percent every year as the fill settles and deadly methane gas is removed. A 60 mil, high-density polyethylene geo-membrane liner laid underneath the transmitter building provides the necessary gas barrier. Two Det-Tronics gas monitors are installed 18 inches below the building ceiling and above the building floor; they constantly monitor methane gas concentration and are tied into the fire alarm and remote control system. Existing vertically driven pipes collect the methane around the mile-by-half-mile site and route it to a common burn-off flare. Vegetative soil retention and dust control were required to manage erosion and drainage across the site.
All of these installed additions and precautions were required by the station’s modifications to the city’s Type 3 permit, granted by the Department of Environmental Quality. Core drilling, excavation or buried radials were not permitted, thus the guy anchor and tower bases were constructed on steel-reinforced concrete pads measuring 20 x 20 x 2 feet above the surface.
Smith said his total project construction costs were about 15 percent higher than a traditional site as a result; however there have been no increases in ongoing operational costs.
Tom McGinley is director of engineering and MIS for CBS Radio Seattle; he is technical adviser to Radio World and a longtime contributor.