Common Mode coils for Hot AM towersThe problem of bridging the RF high voltage gap.One of the most difficult problems in building transmitter systems is supplying mains voltage for top lighting to so-called HOT AM Towers. Such a transmitter, were HF-energy is supplied directly to the metal frame of the transmitter tower needs top ligting to prevent collisions with airplanes of all nature.
This kind of tower is positioned on a isolating support, to prevent the HF -current to flow into ground. Almost everywhere in the world, this kind of tower needs some kind of obstacle lighting, most often in the top and halfway down the tower structure. Lamps of 500-1000 Watts are applied, which for reason of durability are power driven with a stabilized (RMS-value) AC power supply. As these lights are often blinking in a slow rhythm, a soft starter is used to prevent in-rush currents and local overheating of the filaments. All this to improve lamp life. As public power supply is usually at earth level - one pole earthed and the other at 115 or 230 Vac- and the tower potential is carrying RF transmitting voltage, several thousands of volts at high frequency must be bridged. A transmitter of 10.000 Watts connected to a transmitting structure (< 1/4 lambda monopole) with a radiation impedance of f.a. 150 Ohms requires a RMS voltage at the transmitting frequency of 1224 Vrms where 100 kW requires almost 4 kVrms. Values which are to be almost doubled when modulation starts. Due to the high voltage combined with a high frequency, extremely low capacitances in the coupling network give rise to high leakage currents, and high dissipation values in ordinary capacitors. Therefore standard filtering networks cannot be used to bridge the gap. In the past ring core transformers with separates winding were applied , but due to their limited core size, leakage currents occurred between their windings (which required to have extremely low capacitance). An old but efficient solution can be offered using the principle of
(very large) common mode coils. Feeding the AC current through a l common mode coil physically
(and electrically) gaps the bridge and efficiently reduces the earth current ( to less then 2-3 Amps
blind current) , which can be filtered by ordinary paralleled Y-type capacitors. ce-test can deliver this kind of common mode coils and the associated filter networks for feeding HOT AM towers between 1000 Watt and several MW in the frequency range of 100 KHz to 30 MHz. The network efficiently crosses the physical AND electrical distance between tower and ground, and can be enhanced by discharge paths for high voltage discharges such as occur when a direct hit of lighting strikes the tower. |
