A reliable knowledge of radio signal amplitude and phase characteristics is required to design and maintain communications and navigational circuits at VLF and LF. The ability to accurately calculate signal levels as a function of frequency, position, and time is of considerable importance in achieving reliable assessment of communication coverage. Detailed computer models based on multiple mode waveguide theory have been developed. These models have been found to produce good comparisons between measurements and calculations of signal variations as a function of propagation distance. However, results can be very sensitive to the ionospheric inputs to these computer models. This paper has two purposes. The first is to present the results of a systematic comparison of a set of measurements of signal strength from various transmitters over a number of propagation paths using a simple model of the ionosphere. The variation of the parameters of this simple model with basic propagation parameters is examined. The second purpose is to examine the built-in version of this simple model of the ionosphere as implemented in the Long Wave Propagation Capability. This model is found to adequately represent a set of in-flight signal strength measurements. It is also clear that there is still room for improvements in this ionospheric model.