In full-duplex (FDX) multiple-input and multiple-output (MIMO) systems, the self-interference between each transmitting and receiving antenna, caused by finite isolation between antennas, limits the obtainable signal-to-interference ratio of the system and thus the total capacity. In this paper, the design of planar wavetraps for isolation improvement between multiple antennas is introduced, with an emphasis on back-to-back relay structures. General guidelines for the design of planar wavetraps are deduced which could be applied also in isolation improvement of other antenna structures. A prototype back-to-back relay antenna with two dual-polarized patches for FDX MIMO is presented with the frequency of operation of 2.6 GHz. The prototype antenna is manufactured and the simulated results are validated with measurements. The minimum isolation among each antenna pair on the opposite sides of the relay is improved by 20 dB across a 18-MHz bandwidth, while 15-dB improvement can be obtained across 167-MHz bandwidth, yielding total isolation levels of 70 and 65 dB, respectively. The radiation pattern of the relay antenna is measured with and without wavetraps to show that the wavetraps do not have significant effect on the main lobe radiation properties.