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Low Power EME Data Collection Experiments

submitted by Mike Cook, AF9Y
http://www.webcom.com/af9y
Work: mwcook@itt.com
Home: mwcook@cris.com
Introduction
The recent MoonNet discussions on low power EME has renewed my interest in development of an optimum waveform for the EME channel. Before the community settles on a particular waveform, we should first determine the channel characteristics. Important characteristics such as coherence bandwidth, selective fading and multipath are best determined by direct measurement with averaging over varying conditions. W3IWI, KA9Q and others have made astute theoretical statements concerning the impact of these perturbations on various modulation techniques. I believe there is an opportunity for the larger EME stations to collect data on the channel characteristics using their radio equipment and a PC equipped with sound card.

Data Collection Approach - Phase 1, Initial Experiments
For the first phase, we need recordings with the strongest signal possible via the EME path. The idea is to transmit various waveforms tailored to allow extraction of key channel characteristics. A one minute transmission is sent by one EME station and the resulting received audio is recorded at the second EME station. Post analysis of the recorded data should provide some interesting insight.

VE7BQH and I ran the first attempt at collecting data on April 28, 1996 using a PC program called TX55a. This program uses the sound card to generate audio waveforms that modulate the transmitter in SSB mode. I am releasing TX55a and its derivatives as a free public program. The program and the data collected from it is available from my webpage at: http://www.webcom.com/af9y.

In its current form, TX55a provides the following:

1 Khz Tone (Transmitted every odd 4 second period)
This allows centering of the signal in the receiver passband and will be used as a reference signal during the test period.
Narrow Band Spread Spectrum BPSK (Transmitted alternating even 4 second period)
This modulation is coded with a 63 bit Maximum Length Sequence (MLS) to allow extraction of channel delays beyond simple phase shifting. It is useful for multipath and channel coherence analysis. I consider this waveform a candidate for weak signal modulation. TX55a transmits the "R" and "O" characters during the test. The main lobe bandwidth is 250 Hz.
Multi Tone FSK (Transmitted alternating even 4 second period)
During this period, three simultaneous tones 1000 Hz, 750 Hz and 1250 Hz are transmitted. This waveform should provide insight into the selective fading aspects of the channel. Multi Tone FSK is a candidate for weak signal modulation and is being considered by W3IWI and KA9Q. The implementation of this modulation mode will likely require an update after W3IWI and KA9Q have had a chance to review it. I may not have captured their intent.

Data Collection Procedure
Lionel and I used the following procedure for the test:

  1. Establish a 2mtr frequency just above 144.100 since this is SSB mode.
  2. Using the program generated 1 Khz tone at the transmitter, tune the receiver for a 1 Khz receive signal
  3. If available, rotate polarity for maximum signal. During our test, Lionel rotated his array through approximately 60 degrees during a 1 minute TX period of the 1 Khz tone. Using the FFTDSP42 program, the signal strength bar showed the best polarity position. This was an interesting side test that dramatically showed the advantage of polarity rotation. (I have include a picture of the results on my webpage)
  4. Agree to a start time and record 1 to 2 minutes of the test transmission from the TX55a program. You can use FFTDSP42 to record the data as a WAV file. Just make sure the "R" button is red (active) and hit "SAVE" at the at the end of the test period before the signal image scrolls off the screen. If you are using another sound program for recording, I recommend you use a sampling rate of 8192 samples per second.

The stronger the signal, the better the analysis. I suggest that we strive for S/N ratios of at least 15 to 20 dB (100 Hz bandwidth). Each band will have different characteristics, so we need recordings for 144, 432 and above. If you email your recordings to me, I will post them on my webpage.

de Mike, AF9Y

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