6HOWdo we receive a very weak signal, often well below the noise level?
7Methods for weak signals Narrow BandwidthSlow Data RateStable Frequency ControlLong Time IntegrationComputer Assistance
8Communications Theory The faster the signaling rate, the more bandwidth you need.The more bandwidth you have, the more energy you need to keep your signal above noise.
9Example: 12 wpm CW A dot is about 1/10 second long Rule of thumb: 3x dot length or 30 Hz.So this means that if we want to receive a 12wpm CW signal, we’d need to use a filter no narrower than 30 Hz.Going narrower that this will cause the dots to run together making difficult copy.
10Using a SSB filter 2.4 KHz bandwidth 2400 Hz is 80 times wider than 30 Hz.Will let 80 times energy through itSince our desired CW signal only takes 1/80 of the received energy, most of what we get is NOT the desired signal.This is -19 dB disadvantage!!!
11Lesson LearnedFor best performance, we receive with a filter that is no wider than the signal we are trying to receive requires.If you have a wider filter than the signal, you are receiving “extra” energy from the noise that is “diluting” the desired signal -- making the effective S/N ratio worse.
12But I can hear a weak signal as well with the SSB filter as I can with the CW filter. What’s the deal here?
13Psychoacoustics Psychoacoustics - how the brain perceives sounds. Brain/ear capable of picking out narrow bandwidths of signal in presence of white noise or signals removed in freq.
14But...We are going to try to receive very weak signals that are below noise level...so let’s look at how we can do it...
15Back to Communications Theory Lower data rate = narrower bandwidthNarrower bandwidth = lower transmitted power to maintain the “threshold of copiability”
16Shannon’s Lawstates that if you are willing to transmit data infinitely slowly, you could communicate with infinitely narrow bandwidth and infinitely low (not zero) power.
17Obvious Practical Limits It should go without saying that there are practical limits to how slow you would go to convey useful information in a reasonable amount of time.
32The start of it all... On 9 Feb 2002 at 22:51, Paul Stroud wrote: Hi Gang,You are invited to listen for weak signal beacon AA4XX/B Sunday, Feb10th, from 20:00-02:00 UTC (3:00-9:00PM EST) on 10,140,000 hz.This beacon will be running slow speed CW, with 10 second dots and 30 second dashes, commonly referred to as "QRSS10."
33The Players AA4XX - Paul Raleigh, NC ON5EX - Johan Zevergem, Belgium WØCH - Dave Seneca, MOAE5K - Don Yellville, ARAKØB - Stan St Charles, MON4SO - Ken Grand Bay, ALW8DIZ, N3AAZ, KD1YV, VE7SL, VE3FAL,ON6UL, AE4IC, N4HAY, K2UD, VE6KBS
34Details February-March 2002 10.140 MHz. +/- Main beacons: AA4XX, ON5EX Power 250 mw to 50 uwCall, power, “codeword”
38AA4XX - Raleigh, NCAntenna - 30 meter dipole fed with ladderline, about 60 ft. up in NE/SW orientation.
39My first attempt and results... Paul,Just downloaded Argo, installed it & tuned rig to (with 800 hz. bfo offset) and there you are! Just got the complete call.Very exciting for this old timer! Just wonder what power you are running right now? Time of reception has been Z
40comments...It was a real treat to hear from six stations who copied the five letter codeword "GREEN" during last night's 30M QRSS10 beacon session. The beacon was running 1 milliwatt output into a 30M dipole up 60 feet. Congratulations to the following stations who confirmed the codeword: N4SO, W0CH, AE5K, ON5EX, ON6UL, AK0B.
41comments...The most common comment that is being received from the QRSS mode listeners is that they want to try to copy lower power levels.With that in mind, the AA4XX beacon will be running 500 microwatts QRSS30 mode tomorrow (Wednesday). The longer characters will hopefully facilitate reception of the 1/2 milliwatt signal.
42comments...Next week we will continue our downward plunge as long as at least one of our listeners decodes the code word. This is really getting interesting, isn't it?
43comments...Do you think I ought to leave my antenna half down on the ground? ;-) (good ground wave?)[AE5K, after receiving AA4XX’s 200 microwatt signal]
44Some ResultsAA4XX -> AE5K (785 miles)200 microwatts good copy100 microwatts missed 1 letter in codeword7.8 million miles/watt
45Some ResultsON5EX->AE5K250 mw - good copy4600 miles18,400 miles/watt
50Difficulties Observed Propagation - no path?QSB, QRM, QRNMultipathFrequency stability/accuracySoftware crashes
51Conclusions QRSs is an “experimental” mode QRSs is a very viable mode using very low power and computers.None of our antennas were “spectacular”Signals under about mw or so were not audible. (rough estimate, depends on conditions of course)
53Ugly Transoceanic Hifer Transmitter Built by SM6LKMCrystal oscillator BF245A (similar to MPF102) JFET buffer with resonant tank 74HC00 one gate for keying, remaining 3 gates in parallel as "Power Amplifier" Series tuned tank 4:1 transformer BNC output connector, MHz
61My thanks to the following who contributed to this presentation: AA4XXWØCHON5EXAKØBN3AAZNL9222SM6LKMKA7OEIW1TAGW5CSJ (my wife who had to put up with all this)...hope I didn’t miss anyone (if so, hollar)