prior to about 1970, essentially all australian am radio services were implemented using omnidirectional antennas. where spectrum scarcity demanded close channel sharing arrangements, night-time skywave interference as controlled by requiring co-channel services to reduce power at night. this arrangement was less than satisfactory as differences in coverage were apparent. many high-power national broadcasting system antennas were replaced with an "anti-fading" design which, which minimised skywave radiation, easily distinguished by a "top hat" section resembling a wide flat umbrella. a small number of australian am radio services had been commissioned with directional antennas providing pattern minima towards co-channel services. this small deployment was quite effective and the increase in spectrum efficiency was dramatic. the abcb announced in its circular letter b109 of 1975, a changed policy wherein existing services running 2 kw or less would be permitted to increase power to 5 kw, subject to the provision of a directional antenna. the majority of commercial am radio services availed themselves of this option over the next decade.