The wide availability of radio signal strength attenuation information on wireless radios has received considerable attention as a convenient means of deriving positioning information. Although some schemes have been shown to work in some scenarios, many agree that the robustness of such schemes can be easily compromised when low power IEEE 802.15.4 radios are used. Leveraging a recently installed sensor network testbed, we provide a detailed characterization of signal strength properties and link asymmetries for the CC2420 IEEE 802.15.4 compliant radio using a monopole antenna. To quantify the several factors of signal unpredictability due to the hardware, we have collected several thousands of measurements to study the antenna orientation and calibration effects. Our results show that the often overlooked antenna orientation effects are the dominant factor of the signal strength sensitivity, especially in the case of 3-D network deployments. This suggests that the antenna effects need to be carefully considered in signal strength schemes.