Reflection and transmission properties of embedded dipoles and PIFAs inside Concrete at 915 MHz

The possibility of using antennas inside a concrete bridge pier is investigated in this work. Dipole and planar inverted-F antennas were simulated both in the presence and absence of steel rebars inside a cylindrical concrete block. Both air dried and saturated concrete samples were considered. It was found that dipole antennas when placed inside a cylindrical concrete block have the desired return loss characteristics. The transmission loss between two dipoles inside dry concrete is between -30 dB to -20 dB for a distance of 251 mm. The loss in free space for the same distance is about -20 dB. The transmission loss is significantly higher when the antennas are placed inside saturated concrete ((-38 to -35 dB). The two dipoles when placed next to steel rebars have very poor return loss characteristics. This is completely alleviated if planar inverted-F antennas are selected instead. The transmission loss with PIFAs for the same distance is about -20 dB for dry concrete and -30 dB for saturated concrete. Clearly if a sensor contains a PIFA and transmits 0 dBm of power the other sensor antenna will receive -38 dBm at a distance of about 10 inches in saturated concrete. Needless to say that sensor sensitivity is usually much smaller than -30 dBm which will allow transmission over greater distances within concrete.