Note that the distance from the earth to the moon varies becasue the orbit of the moon is not perfectly circular. It is somewhat egg-shaped. This means there is an apogee, the largest distance, and a perigee, the shortest distance. Depending on the position of the moon with respect to the earth, apogee can be as much as 406,700km, while perigee can be as little as 356,400km. This translates to as much as 2.25dB difference in path loss from apogee to perigee. The mean distance from earth to moon is given as 384,400km.

These calculations consider the fact that the moon is only 7% efficient as a reflector and, using the "radar equation", the fact that the moon is a spherical reflector. This last factor varies slightly with distance, and this calculator makes the adjustment; for this reason, the path loss results are valid only for distances between 356,000km and 407,000km.

The calculator is initialized for a 1296MHz EME path with distance to the moon of 240,000 miles, 1KW transmitter output into a transmission line (coax) of 1dB loss, 10' dishes (55% efficient) at both ends, and the receive preamp connected directly to a coaxial relay on the receive output of the feedhorn. To run the default calculation, click on "Compute". To run different numbers, simply highlight the applicable number and enter a new value. Make sure units of distance, frequency and power are properly selected, then click on "Compute". To run the default calculation again, click on "Reset Input" and then on "Compute".