Ben Dawson, P.E., principal engineer of Hatfield & Dawson, wrote to tell us about KNUV, an AM station licensed to Tolleson, Ariz., a suburb of Phoenix. The station was required to vacate its longstanding transmitter site.
This is not an unusual situation for an AM station to find itself in these days, given the trends in urban development, the ever-rising value of real estate and the business challenges facing all AM stations.
With advice from Ben’s firm, the licensee and her technical contractor conducted a thorough search and decided to relocate to the site from which Phoenix station KXEG(AM) broadcasts.
KNUV now broadcasts on 1190 kHz with 640 watts day, 22 watts night. KXEG is on 1280 kHz with 2.5 kW day, 49 watts night. The KXEG site has an equipment building that had more than adequate space for KNUV’s equipment.
The single 180-foot KXEG tower is close to the building, so they chose to install the diplexing filters and antenna tuning unit matching networks in an equipment room, and the tower is fed through a bowl insulator in the building wall via a well-supported copper tube to the tower base.
The filters and match networks for multiple frequency use of an antenna require segregated enclosures to electrically isolate the individual networks from one another.
A three-cabinet phasor, formerly used at KXEG’s three-tower directional site, was chosen to house the antenna equipment. One cabinet was used for each frequency’s filters, and the remaining cabinet was separated with a shelf into an upper and lower compartment, one for each of the ATU matching networks.
Fig. 1 shows a view of the cabinets before stripping components.
Thomas Driggers measured the antenna impedances at both frequencies, and Steve Lockwood prepared a design for the system networks. Network values were selected to provide reasonable sideband loads to the transmitters of the two stations, using various software tools and with attention to the dictates of Ron Rackley’s “VARs matching” technique (described by Rackley in the 11th edition of the NAB Engineering Handbook, Chapter 7).
Once the design was in final draft form, the component list was prepared, and the team created an inventory. Components from the former phasor and ATUs were used where appropriate, and parts from available surplus collections were gathered to complete the inventory.
The phasor itself and the two ATUs were removed from service at the original KNUV site, with one left for STA operation.
The final system was constructed by Driggers entirely of surplus material and components except for a few feet of new 4-inch copper strap. Fig. 2 shows some of the surplus parts used.
The surplus items were inspected and measured carefully. Because filters often require high-voltage capacitors and high-inductance coils, the team found it necessary in at least one case to obtain the necessary inductance by using two available coils in series, shown in Fig. 3.
The vacuum capacitors that were to be connected in series parallel to obtain the necessary voltage-handling capability in the filters were evaluated with a hi-pot tester.
Given the cost of replacement “new” parts, the package of additional components was packed up with care and insured for shipment to Phoenix.
When all parts were on site, the phasor cabinet was stripped and a shelf installed in the cabinet for the ATU networks. Parts were installed to minimize interaction. In particular the KNUV 1190 pass 1280 reject filters were connected to the KNUV ATU cabinet with a coaxial cable to avoid any coupling to the 1280 pass 1190 reject filters in the center cabinet.
(You can read a paper by Ben Dawson, “Isolation Requirements in Diplexed MF Antenna Systems.” At Hatfield & Dawson’s website, scroll down to “isolation requirements.”)
Fig. 4 shows this coaxial cable from filter cabinet to ATU cabinet on the right side.
Though the resulting system is far from physically elegant, it is electrically straightforward and performs well. Fig. 5 is a rear view of the completed system.
The trends I mentioned at the beginning have driven a virtual explosion of diplexed, triplexed and even quadriplexed installations, some at sites with as many as nine existing towers.
Under these circumstances, the project cost for multiple use of existing antennas requires careful consideration. In some cases simple systems can be implemented with lowest cost by “hand-made” or “composite” systems, if qualified design and construction personnel are available.
Ben adds that in his teenage years, half of the local AM stations had handmade transmitters, and many had handmade audio console equipment. “Not something often seen in modern installations,” he said.
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