This paper proposes a hardware-efficient time-domain scheme to digitally compensate the I/Q imbalance and LO feedthrough (LOFT) of a sub-GHz wideband transmitter for the IEEE 802.11af WLAN. A simple envelope detector is the only analog part. The parameters are updated by Least-Mean-Square and estimated efficiently in time domain by using Coordinate Rotation DIgital Computer (CORDIC), saving the training time and power consumption. The measured wideband image-rejection ratio (IRR) and LO-leakage-rejection ratio (LRR) are improved from 18.9 to 41.3 dB, and 20.4 to 37.9 dB, respectively.