The practice of using FM series tuned isolation transformers(isocouplers) to facilitate the co-location of FM antennas on an AM/Medium Wave base insulated mast has been utilized in the US broadcast market for many years, but is a relatively unknown practice in the international broadcast markets.
An isocoupler utilizes two magnetically coupled, series tuned loops at the FM transmit frequency to yield an open circuit at the AM frequency when installed in parallel to the AM tower base insulator. The isocoupler, therefore, presents a very high impedance in parallel with the drive impedance of the AM tower such that in most cases the AM impedance is not altered by adding the isocoupler. The isocoupler does have some capacitance associated with it in the range of 80-200pf depending on the FM transmit power rating of the isocoupler. The associated capacitive reactance is typically too high to effect a change in the AM drive impedance.
Figure 1 below illustrates a typical isocoupler installation configuration at the base of the AM tower.
Figure 2 is a photo of an installation of a 30kW FM isocoupler at the base of a base insulated self-supported tower. Table 1 shows a list of the technical specifications for the 1.5, 2.5, 7.5, 20 and 30 kW FM isocouplers manufactured by Kintronic Labs. The two most important parameters in selecting an isocoupler model are the DRM+ (DRM for local coverage) transmitter power (table second row) and the AM peak voltage across the base insulator (table second from the bottom row). If the DRM+ transmitter power is 20kW or less and the AM peak voltage is less than 35kV an isocoupler model can be selected for the application. If the transmitter power is higher than 20kW and/or the AM peak voltage is higher than 35kV Kintronic Labs offers other solutions to co-locate the corresponding FM transmission system on an AM mast.
For further information please contact Kintronic Labs at ktl@kintronic.com or phone us at +1-423-878-3141(US Eastern time zone) or visit our website at www.kintronic.com, click on "Resources", click on "Brochures" and under FM, TV & Cellular click on "FM Isocoupler".
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