This paper presents CMOS VCO circuit, as a part of Automatic Dependent Surveillance-Broadcast (ADS-B) transmitter, operating at 978 MHz, delivers highest output power with highest efficiency ever reported to the best of authors' knowledge. The 0.738 mm × 0.606 mm VCO chip fabricated using 0.18 um CMOS technology, exhibits the differential peak output power of 2.54 dBm at 1 GHz, with the frequency tuning range from 954.3 MHz to 1009.4 MHz and phase noise of -109 dBc/Hz @ 1 MHz offset centered at 978 MHz, consuming 35 mW power from 1.8V supply, thus, showing state-of-the-art-performances.