This paper presents the development and testing of a novel large-area planar carbon nanotube field emission cathode for electrodynamic tether applications. The modular emitter design utilizes a 500 µm CNT yarn with an effective emission area of 4.5 cm². Experimental results demonstrate degradation rates lower than 0.4 V/h at source currents of 4 mA and an efficiency of over 2 mA/W at up to 15 mA.