Automatic dependent surveillance-broadcast (ADS-B) system requires higher performance, highly reliable, and smaller size, lighter weight and lower cost transceiver. To reduce the size and cost and to increase the manufacturability it is desirable to integrate the whole ADS-B system into a single-chip complementary metal-oxide-semiconductor (CMOS) technology platform. Watt level power is required from the ADS-B transmitter for transmitting the information about aircraft's precise location at larger distances. This study presents CMOS voltage-controlled oscillator (VCO) circuit, as a part of ADS-B transmitter, operating at the centre frequency of 978 MHz, delivers the highest output power with the highest efficiency ever reported to the best of authors' knowledge. The 0.738 mm × 0.606 mm VCO chip fabricated using 0.18 μm CMOS technology, exhibits the single-ended peak output power of -0.46 dBm and the differential peak output power of 2.54 dBm at the frequency of 1 GHz, with the frequency tuning range from 954.3 to 1009.4 MHz and phase noise of -109 dBc/Hz at 1 MHz offset centred at the frequency of 978 MHz, consuming 19 mW power from 1.8 V supply, thus, showing state-of-the-art-performances. This promising result shows a path towards realising high-power single-chip CMOS ADS-B transmitter.