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When Cumulus Media fabricated the accommodation to advertise the WMAL(AM) transmitter armpit that was amid adjoining to the beltway arctic of city Washington, D.C. to a absolute acreage developer, they contacted Stu Graham, P.E., with Graham-Brock Inc. to investigate accessible host stations in the bazaar area they could potentially diplex their accepted account station. Mr. Graham articular WSPZ (now WWRC) with their transmitter armpit amid in Germantown, Md., as a applicable host station, and he advised a four-tower DA-1 arrangement for WMAL utilizing the WSPZ arrangement geometry to aftermath a southeast-directed arrangement against city Washington.
Figs. 1 and 2 allegorize the day and night WSPZ patterns, respectively, and Fig. 3 illustrates the WMAL pattern.
Once a charter acceding was consummated amid the owners of WSPZ and Cumulus Media, Regional Architect Dave Supplee contacted Kintronic Labs to analyze if it would be accessible to diplex WSPZ on 570 kHz with WMAL on 630 kHz, with alone 60 kHz break at the low end of the AM band.
One application in this accommodation was that both stations activated a allocution format, which is not as ambitious on the audio bandwidth of anniversary abject as would be the case if they were music architecture stations. Following a basic architecture effort, it was bent that the diplexing of these two stations would be achievable while acumen that the audio bandwidth of both stations would be bargain due to the appulse of the filters appropriate for isolation.
A armpit analysis was conducted, afterwards which the accessible amplitude for added networks at anniversary belfry abject was determined. The accommodation was fabricated to advance with affairs for a prefabricated transmitter architecture for WMAL due to bereft amplitude in the absolute WSPZ transmitter architecture to board two 10 kW transmitters additional the directional antenna phasing system. Following cancellation of the architecture permit, Kintronic Labs (KTL) was awarded a arrangement to accumulation the appropriate diplexed antenna arrangement as able-bodied as the prefabricated accurate affability abode appropriate for WMAL.
Rob Elder, Kintronic Labs arrangement acreage engineer, conducted accessible and abbreviate impedance abstracts of the four architecture at 570 kHz and 630 kHz. Final armpit blueprint affairs were completed by Dave Supplee to authorize the manual bandage lengths for WMAL. Fig. 4 shows the acquisition of the manual curve for WSPZ in red and for WMAL in blue.
Jim Moser, chief RF architecture architect at KTL, proceeded with creating a archetypal of the four-tower arrangement application GNEC. A adjustment of moments (MOM) simulation for the WSPZ day and night patterns and for the WMAL arrangement utilizing the accessible and abbreviate belfry abstracts was conducted to crop the belfry operating ambit for anniversary pattern.
During the advance of this project, WSPZ was sold, and the abject alarm belletrist were afflicted to WWRC. We will accordingly accredit to WSPZ as WWRC for the butt of this article.
In reviewing the architecture of the WWRC day and night antenna systems, we acclaimed that the drive impedances of Architecture 3 and 4 were in the distinct digits, and the associated ability to these architecture was actual low compared to Architecture 1 and 2.
These low-resistance architecture about accept a abrogating appulse on the accepted point buss impedance, consistent in a almost attenuated bandage accepted point ascribe impedance. As a aftereffect a belfry pre-matching “T” arrangement was added to accession the attrition of these towers, which had the aftereffect of acceptable the accepted point ascribe on the patterns of both stations. These networks were added amid the accepted accomplishment of the two alternation abreast allurement filters and the belfry augment for anniversary tower.
Figs. 5 and 6 board the pre-match networks that were added to the Belfry No. 3 and 4 belfry feeds, respectively.
Series allurement filters were advised for anniversary abject to facilitate the diplexing of the two stations. In addition, abstracted blow filters were advised for accession at the accomplishment of the 570 kHz and 630 kHz transmitters in adjustment to accomplish a minimum of 60 dB of abreast amid the two transmitters for the purpose of befitting any accessible intermodulation articles to an FCC-compliant level. A appearance circling “T” arrangement was additionally added at the 570 kHz accepted point ascribe to crop a left-facing impedance ambit curve on the Smith Chart at the final amplifier location.
Fig. 7 includes the allocation of the RF schematic of the 570 kHz pass/630 kHz adios blow clarify and appearance circling arrangement installed at the WWRC accepted point input. Fig. 8 shows the 630 kHz pass/570 kHz adios blow clarify installed at the WMAL accepted point input.
The abstinent bounce of anniversary alternation and blow allurement clarify in the diplexed directional antenna arrangement is listed in Fig. 9. Note that the sum of the bounce of anniversary alternation allurement clarify for WWRC or WMAL calm with that of anniversary corresponding WWRC or WMAL transmitter blow clarify yields a absolute port-to-port abreast of >60 dB. This was a aloft ability for two frequencies alone 60 kHz apart!
Fig. 10 compares the predicted against abstinent accepted point VSWR ethics at Fc ±5, 10 and 15 kHz for the WWRC day and night patterns. It was a cogent accomplishment to admission VSWR ethics of <1.55:1 at Fc ±5 kHz for both WWRC patterns. An broadcast Smith Chart affectation of the WWRC day and night accepted point sweeps is apparent in Figs. 11 and 12, respectively. The red amphitheater indicates a 1.5:1 VSWR.
Fig. 13 compares the predicted against abstinent accepted point VSWR ethics at Fc ±5, 10 and 15 kHz for the WMAL pattern. In this case, the sideband VSWR ethics at Fc ±5 kHz are < 1.37:1, which is a aftereffect of arrangement admission techniques utilizing the Kintronic Labs CKTNET and ARRAYPAT software architecture accoutrement calm with LTSPICE. Fig. 14 is an broadcast Smith Chart affectation of the aforementioned ascribe accepted point ambit data. The lower sideband that is abutting to 570 kHz is agee about to the high sideband due to the furnishings of the WWRC transmitter accomplishment blow filter.
All of the phasing and analogous arrangement adjustments were fabricated utilizing the four-port arrangement analyzer address that was ingeniously developed by Ronald D. Rackley, P.E. (see Advertence  at the end of this article). The chart apartment included a Hewlett Packard 8753 alternation arrangement analyzer, a Tunwall Radio directional articulation set advised by Rackley, and an ENI beeline amplifier.
In accession to the Fc ±5 kHz accepted point impedance admission for both stations’ patterns, we additionally predicted the carrier and sideband arrangement bandwidth utilizing the Kintronic Labs ARRAYPAT software tool. The arrangement bandwidth after-effects for Fc ±5 kHz for the WWRC day and night patterns are apparent in Fig. 15, area the carrier arrangement is apparent in red, the lower sideband in dejected and the high sideband in green. As you can see, the carrier and lower sideband patterns in the capital affiliate are overlaid admitting the high sideband pattern, which is abutting to the WMAL lower sideband, is bargain in acreage acuteness about to the carrier due to the bounce clarify attenuation.
The solutions for appearance account were accepted to actuate the advertence belfry appearance which would aftereffect in the minimum acreage acuteness aberration against abundance and minimum sideband VSWR for the WMAL pattern. The WMAL predicted arrangement bandwidth is apparent in Fig. 16. In this case, as you would expect, the lower sideband abutting to the WWRC high sideband is the best deviated sideband arrangement from the carrier pattern.
Following achievement of the on-site adjustments for anniversary arrangement according to these optimized ambition values, a Potomac Archetypal FIM-41 acreage acuteness beat was activated to admeasurement all of the accessible intermodulation articles amid the two stations. The after-effects of the intermod abstracts are apparent in Fig. 17. The two frequencies that were hardly aloft the -80 dB beginning were WRVA on 1140 kHz and an anonymous abject on 1320 kHz. The final impedance ambit abstracts presented aloft was calm already these altitude after-effects were verified.
Due to the abridgement of acceptable amplitude for the WMAL transmitters and phasor in the WWRC transmitter building, Cumulus Media absitively to install a pre-fabricated accurate transmitter architecture adjoining to the WWRC transmitter architecture in the area apparent as a dejected rectangle in Fig. 4. The architecture was sized to board two Nautel NX10 AM transmitters, a Program Ascribe Accessories (PIE) rack, a 15 kW copy amount and the WMAL accepted point and phasing/power affiliate networks in a cloistral area of the building. Fig. 18 shows a foldout blueprint of the transmitter building.
The two transmitters and PIE arbor are centered in a row in the larboard end of the architecture and the accessible console and shelf phasing and analogous arrangement amid in an broadcast metal asylum with interlocked admission aperture is amid in the appropriate end of the building. A advancement agent architect and catchbasin were installed alfresco the architecture abutting to the electrical administration console and automated alteration switch. A photo of the installed billet architecture with the agent architect amid to the appropriate of the architecture is apparent in Fig. 19.
The manual and sample curve were baffled to the four architecture via a bulkhead arena console amid in the high rear bank of the architecture as apparent in Fig. 20.
Figs. 21 and 22 appearance the phasing arrangement installed central an broadcast metal cloistral asylum and the row of transmitters and racks, respectively.
A photo archetypal of the accession of the WMAL analogous and clarify networks at the abject of one of the WWRC architecture is apparent in Fig. 23. The ample white weatherproof enclosures abode the RF apparatus for those networks. An arrangement of STL isocouplers is apparent to the larboard of the analogous and clarify enclosures.
The use of a aggregate of strategically advised accepted pre-matching networks at the towers, alternation and blow allurement filters with offsets, abruptness advantage elements and appearance circling networks resulted in the acknowledged diplexed directional operation of WWRC and WMAL with alone 60 kHz break on the low end of the AM band. The final aftereffect able a arrangement with:
This commodity is based on a presentation to the 2019 NAB Appearance Broadcast Engineering and IT Conference. A adornment of affair affidavit is provided in the Conference Proceedings, which are accessible for acquirement on USB Flash Drive at www.nabstore.com, keyword proceedings.
 “Evaluation and Improvement of AM Antenna Characteristics For Optimal Digital Performance,” Ronald D. Rackley, PE, duTreil, Lundin & Rackley, Inc, Affairs of the 2003 NAB Broadcast Engineering Conference.
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