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Trends in Technology: AM Radio
Hawai’i - KNUI
As we saw from the KRKO/KKXA transmitter example, it’s often times easier to arrange a diplex so that two AM stations can share towers, rather than building two separate transmitter sites.
Are there other ways that AM towers can be used for shared purposes, thus generating some income for the tower owner (presumably the AM radio station)? The answer to that question is yes and it’s been done for years, by one of three different means. Perhaps the most familiar way is by grounding the tower base and shunt feeding the tower; or, alternatively by making use of the folded unipole type arrangement. In either case the tower base is grounded and coaxes can be run up the tower without further isolation. In the case of series-fed radiators, another means is by using iso-couplers that allow the RF signal in the coax to jump across the tower base, while appearing invisible to the RF flowing on the tower itself. Finally, the third method is to run the coax up the tower, arranging the length of the coax outer conductor so that it appears as a 1/4-wave stub to the RF frequency on the tower.
With the proliferation of cellular telephone sites, one would think that more cellular carriers would look to existing AM towers as a way to get around problems with local zoning. If we look at the case of KNUI in Wailuku, HI, we find an example of this very thing happening. Verizon Wireless needed to expand its footprint in the area, but ran into zoning difficulties with the local authorities; so, they asked around, and found KNUI (then KMVI) as a partner station. KNUI had a 450’ tower, but it couldn’t support the antennas that Verizon needed. A compromise had to be made between Verizon and the station owners, though — the new tower could only go up 180’ (with top-loading, it appears to be about 46 degrees). Both parties decided it was mutually beneficial to go ahead with the project. KNUI selected LBA Technology of Greenville, NC, to provide the new ATU, and another LBA product known as CoLoCoil, to allow the installation of the Verizon antennas on the new tower. The CoLoCoil system is a variation on the iso-coupler idea, but it uses parallel-resonant filters, composed of the outer conductor of the coax, and a parallel capacitor, to reject any RF energy from the tower itself. (See figure 1.) According to LBA, the tuned circuits present a “lower capacitive footprint to the tower’s lump base capacitance,” while allowing the RF path for the cellular company, and a DC path for any control systems, or tower-mounted amplifier systems.
AM radio remains a viable means of communications. Certainly we all know that it is not as important as it once was during the 1930s and 1940s, for example, when there was no competing electronic media. But even with its gradually diminishing stature, there are station owners out there putting their hearts and souls into it. During the composition of this article, Andy Skotdal of KRKO/KKXA was busy planning, and later executing a remote broadcast designed to benefit the families of victims of the tragic mudslide at Oso, WA, but still found time to answer my questions. I know there are others who are working just as hard to make their AM stations successful. I hope if you are involved in AM radio that you will give every effort toward the same end.
BW Broadcast has come on strong in the last half-dozen years. It offers the TX300 V2, a stand-alone transmitter capable of 300W. One box houses the exciter, power supply, power amplifier, a stereo generator and audio processor (basically lifted right from the design of the BW DSPX miniFM). Audio inputs are balanced XLR. The internal audio processor can be bypassed if an external unit will be used to drive the BNC composite input of the transmitter. The RF output is on a type-N female. The ac input can accept between 85Vac and 260Vac, which yields some flexibility with the installation. Remote control can be done via RS-232, requiring a computer at the transmitter site for remote access; or, via its new software, Ethernet access to the transmitter can be had via its embedded Web interface. In either case the transmitter site will need network access.
Irwin is RF engineer/project manager for Clear Channel Los Angeles. Contact him at firstname.lastname@example.org.
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