An inward‐looking wearable antenna can be used for radio communications with internal transceivers in vivo. The radio transmissions are recorded using an array of electric field sensors on the skin. This paper reports the effect of living tissue on a small cavity‐backed slot antenna pressed onto soft tissue of the human torso at 2.09 GHz. In‐vivo measurements were made on the skin surface at 13 torso locations using eight participants (age range, 22–68 years old), with body mass index ranging between 20.3 and 31.6 kg/m2. Ultrasound imaging was used to determine the skin and fat thickness at every measurement location. The variation in the antenna input impedance measurements demonstrated that the human tissues (fat and muscle) affect the antenna impedance but the mismatch creates field strength measurement errors of less than 2 dB. Fat thickness in the range of 3–30 mm can slightly degrade the performance of these wearable antennas. These effects can be partly mitigated by selective location and antenna retuning to improve transceiver communications. Bioelectromagnetics. 2021. © 2021 Bioelectromagnetics Society