EME (communications) - EME Communications Technical Details

EME Communications Technical Details

As the albedo of the Moon is very low (maximally 12% but usually closer to 7%), and the path loss over the 770,000 kilometre return distance is extreme (around 250 to 310 dB depending on VHF-UHF band used, modulation format and Doppler shift effects), high power (more than 100 watts) and high-gain antennas (more than 20 dB) must be used.

In practice, this limits the use of this technique to the spectrum at VHF and above.

The Moon must be above the horizon in order for EME communications to be possible.

To determine EME Path Loss we need to know -

1. Moon distance from either the transmitting or receiving station
2. Transmitter station output in watts, expressed as ERP
3. Receive station gain (actual receiver gain minus feedline loss, x antenna gain)
4. The operating frequency of the transmitter and receiver

Free space loss from an isotropic omnidirectional antenna is described by this formula. It calculates the surface area of an imaginary sphere of radius, d, that the radio wave illuminates uniformly:

1. Loss = where pi ≈ 3.14, d = distance and lambda = wavelength, in meters
2. Lambda = c/F F = Hz, c = meters/sec.
3. Lambda = when F is in MHz.

Substituting F into the free-space loss formula and converting to d into km:

• Loss = or
• Loss(dB) =

Adding factors for reflection from the Moon results in

• Loss-eme(dB) = 32.45 + 20Log(F) + 20Log(2*d) + 50.21 - 10Log(.065)

The standard radar path link formula is basis for EME path-loss calculations

• Loss =

After including the factor for surface reflectivity it becomes

• where is the Moon's diameter

Since the diameter of the Moon is ≈ 3500 km

The formula becomes

• Loss-eme(dB) = 20Log(F) + 40LOG(d) - 17.49, F = MHz, d = km

For some reason not specified, Josef has increased the loss by 3-dB producing:

1. Loss-eme(dB) = 103.4 + 20LOG(F) + 40LOG(d) - 10Log(rho) or
2. Loss-eme(dB) = 20Log(F) + 40LOG(d) - 14.49

Note that the distance from the Earth to the Moon varies because the orbit of the Moon is not perfectly circular, it is somewhat elliptical with a mean radius of 240,000 miles. This means there is an apogee (the largest distance) and a perigee (the shortest distance). In addition, the orbital plane precesses with a principal period of 18.6 years.

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, use the radar equation (which defines a two-way path-loss model) and the assumption that the Moon is a spherical reflector.