Ingested radio transmitters are used to search for irregularities in the gastro-intestinal (GI) tract. Thin insulating layers (interstitial tissue) around internal organs effects the propagation path the transmission. This effect was investigated using a small, battery powers 433 MHz transmitter located in a saline solution (conductivity 2 S/m) and surface field measurements on the outside of the container. Thin, water-filled, PVC pipes were positioned variously near the transmitter and the electric field strength profile of the surface EM field was measured at different pipe locations. The presence of the insulating layer distorts surface field by up to 3 dB. The variation in field strength depends on the conductivity of body organs. This has implications for location algorithms used for the 24-72 hour wearable tracking system.