Observations of the redshifted 21-cm hyperfine line of neutral hydrogen from early phases of the Universe such as Cosmic Dawn and the Epoch of Reionization promise to open a new window onto the early formation of stars and galaxies .
We present the first upper limits on the power spectrum of redshifted 21-cm brightness temperature fluctuations in the redshift range z = 19.8 - 25.2 ( 54 - 68 MHz frequency range ) using 14 hours of data obtained with the LOFAR-Low Band Antenna ( LBA ) array .
We also demonstrate the application of a multiple pointing calibration technique to calibrate the LOFAR-LBA dual-pointing observations centred on the North Celestial Pole and the radio galaxy 3C220.3 .
We observe an unexplained excess of \sim 30 - 50 \% in Stokes I noise compared to Stokes V for the two observed fields , which decorrelates on \gtrsim 12 seconds and might have a physical origin .
We show that enforcing smoothness of gain errors along frequency direction during calibration reduces the additional variance in Stokes I compared Stokes V introduced by the calibration on sub-band level .
After subtraction of smooth foregrounds , we achieve a 2 \sigma upper limit on the 21-cm power spectrum of \Delta _ { 21 } ^ { 2 } < ( 14561 \text { mK } ) ^ { 2 } at k \sim 0.038 h \text { cMpc } ^ { -1 } and \Delta _ { 21 } ^ { 2 } < ( 14886 \text { mK } ) ^ { 2 } at k \sim 0.038 h \text { cMpc } ^ { -1 } for the 3C220 and NCP fields respectively and both upper limits are consistent with each other .
The upper limits for the two fields are still dominated by systematics on most k modes .