SM 5 BSZ - The EME signal bandwidth on 144MHz
(July 29 2001)

Testing the AFC of linrad

Path modulation

Narrow bandwidth is used to receive weak signals. This is because the amount of noise power received is directly proportional to the bandwidth of the receiver filter while the amount of received signal is independent of the bandwidth of the receiver filter as long as the bandwidth of the signal itself is not greater than the bandwidth of the receiver filter.

To communicate at N times weaker signal level one just has to reduce bandwidth by a factor of N and allow the transmission to take N times more time!!

Now there is a limit to how far one can take this strategy. Even with perfect frequency stability of receiver and transmitter the frequency of the received signal will vary at random due to the variation in time it will take for the signal to travel between transmitter and receiver.

The medium between transmitter and receiver is never perfect vacuum - and the properties will vary more or less with time. Even interstellar space is a plasma - and it is moving!!

More significant than the medium itself is multi path propagation. This is valid for terrestrial signals as well as for signals reflected off the moon.

The fading (amplitude modulation) and the phase modulation added to the signal by the transmission path will contain a random component that causes some finite width to a pure carrier. Using a bandwidth below the bandwidth of the phase modulation will cause loss of signal and is no good idea!

To design the optimum communication method that will allow the lowest possible power for a particular transmission path one has to know where the limit is for minimum bandwidth.

The systematic path modulation (doppler shift) can be compensated for but the random component gives a minimum bandwidth.

The EME path

The moon is a very large object and it is obvious that signals reflected from different parts of the moon will arrive after different amounts of time. Since the moon is rocking the relative amplitudes will vary and therefore the phase of the different waves that will compose the total signal will vary at random. Consequently the sum of all the contributions will vary in both amplitude and phase. This is the well known libration fading. Another way to understand it is to say that signals reflected from one side will be doppler shifted differently from signals reflected from the other side. One side is moving towards earth due to the rocking movement while the other side is moving away from earth.

On 10GHz the EME signal has a bandwidth of about 300Hz, clearly much greater than the bandwidth normally used for the communication modulation.

One might think that the path modulation is proportional to the bandwidth and that therefore the bandwidth would be about 4Hz on 144MHz.

As it turns out, the bandwidth is much narrower, by more than 10 times. The reason is that the moon is a rough surface at 10GHz while at 144MHz the moon is a smooth surface. (in terms of wavelengths)

144MHz is probably the amateur band that will allow the narrowest bandwidth and therefore 144MHz is the band where new, extremely narrow banded digital communication modes can be applied. 50MHz will give less libration fading, but probably the scintillation fading due to the effects of the earth's atmosphere will increase the bandwidth beyond that of 144MHz.

Typical signals at 144MHz

Modern amateur equipment has good frequency stability and the frequency is often not drifting by more than a few Hz per minute due to thermal effects.

Figure 1 shows a screen dump of linrad after receiving a 1 minute transmission from IK3MAC (from a DAT tape). The AFC graph shows how the frequency has drifted by 0.35Hz during the transmission. The actually measured frequency for a single fft is scattered by up to 0.2Hz as shown by the green dots. The frequency used to downconvert the signal from 12.843.3 kHz to the baseband is derived as the average of about 10 frequency values, weighted by the corresponding S/N ratio and is shown by the white pixels in the AFC graph.

The baseband graph shows the signal at 12832Hz. The baseband frequency scale is currently assuming a fixed frequency corresponding to the grey center line in the high resolution graph was used for down conversion. The baseband graph has an fft size of 8192 corresponding to 43 seconds so each pixel corresponds to 0.02Hz The graph shows the sum of all powerspectra since start.

Fig. 2 is the same as with fig. 1 with one exception. There is no averaging of the frequencies determined from different transforms. The white points are on top of the green ones in the AFC graph. The corresponding parameter change is that "AFC averaging time" is changed from 10 seconds in fig. 1. to 0 in fig. 2.

The two figures differ in that fig. 1 is produced with a local oscillator that is modulated to follow the frequency of the signal but with modulation frequencies limited to 0.1Hz (average over 10 seconds) while fig. 2. is following the signal about 10 times quicker, corresponding to the bandwidth of the high resolution fft (second fft)

The increase in bandwidth from fig.2 to fig. 1 gives 0.2Hz as an upper limit for the path modulation of the EME path.

Fig 1. Narrow (sub Hz) modulation of the carrier transmitted during a normal CW transmission by IK3MAC. The baseband graph (green) shows that the bandwidth over one minute is about 0.2Hz.

Fig 2. The AFC is unfiltered and has partly removed path modulation from the signal. The same one minute sequence as in fig.1 with all other processing parameters unchanged.

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