EME PATH MODULATION
(Feb 23 2000)
On this page you can see some images showing the signal
transmitted by SM5FRH, Tobbe and received by me,
SM5BSZ, Leif via reflection off the moon.
The transmitted signal is a carrier of several minutes duration
It was received in stereo with 20kHz bandwidth and recorded
on a DAT recorder.
The first figure shows the signal spectrum in several different
ways side by side.
The ground wave as well as the EME signal is in the order of 20dB above
the noise floor. The ground wave is used to extract the momentary
transmitted frequency by a digital PLL loop in the computer.
The first column shows the spectrum of the ground wave with a receiver
following the PLL frequency. In other words the first spectrum
shows how sucessful the PLL was in following the frequency drift
of the fairly unstable signal.
The second spectrum is the EME signal as received with a receiver
using a fixed local oscillator.
The third spectrum shows the EME signal received with the same
receiver local oscillator as the first spectrum, a PLL witch follows
the transmitted frequency.
The fourth and the fifth spectrum graphs show the EME signal
received by a receiver that follows the delayed transmit signal.
These figures come close to what one would obtain using frequency
The fourth and the fifth spectra are with two orthogonal linear
polarisations on the receive side.
The figure below shows the same sequence with higher resolution.
Here each pixel is 0.042Hz.
One can clearly see the two components of the EME signal.
The specular reflection is like reflection of light on a polished
steel ball. It happens on a very small area at the center of the moon
and has a very narrow spectrum. The polarisation is conserved so
this signal is strong in one polarisation while it goes through zero
in the other as polarisation slowly twists due to farady rotation.
The diffuse scattering is like light reflected from a ping-pong ball.
It has a much wider spectrum because it has contributions from different
parts of the moon that give rise to different doppler shifts.
Polarisation is not conserved by the diffuse scattering process and
it is equally strong in both polarisations.
In this experiment the diffusely scattered signal is about 1Hz wide
with a rectangular spectrum while the specular reflection is in
the order of 0.2Hz wide.
This experiment is not good enough for detailed analysis.