6 HOW do we receive a very weak signal, often well below the noise level?
7 Methods for weak signals 4 Narrow Bandwidth 4 Slow Data Rate 4 Stable Frequency Control 4 Long Time Integration 4 Computer Assistance
8 Communications 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.
9 Example: 12 wpm CW 4 A dot is about 1/10 second long 4 Rule of thumb: 3x dot length or 30 Hz. 4 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. 4 Going narrower that this will cause the dots to run together making difficult copy.
10 Using a SSB filter 4 2.4 KHz bandwidth 4 2400 Hz is 80 times wider than 30 Hz. 4 Will let 80 times energy through it 4 Since our desired CW signal only takes 1/80 of the received energy, most of what we get is NOT the desired signal. 4 This is -19 dB disadvantage!!!
11 Lesson Learned 4 For best performance, we receive with a filter that is no wider than the signal we are trying to receive requires. 4 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.
12 But I can hear a weak signal as well with the SSB filter as I can with the CW filter. What’s the deal here?
13 Psychoacoustics 4 Psychoacoustics - how the brain perceives sounds. 4 Brain/ear capable of picking out narrow bandwidths of signal in presence of white noise or signals removed in freq.
14 But... 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...
15 Back to Communications Theory 4 Lower data rate = narrower bandwidth 4 Narrower bandwidth = lower transmitted power to maintain the “threshold of copiability”
16 Shannon’s Law states that if you are willing to transmit data infinitely slowly, you could communicate with infinitely narrow bandwidth and infinitely low (not zero) power.
17 Obvious 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.
19 What is QRSs? The term QRSs is derived from QRS, an abbreviation that means “Slow Down” QRSs, by extension, would imply very slow sending speed.
20 QRSs is... 4 VERY slooooooow! 4 Agonizingly slow! 4 B-O-R-I-N-G ! 4 Often sent at dot lengths of 20 seconds or longer
21 But, QRSs... 4 Works! 4 Extremely narrow bandwidth. 4 Has been used to accomplish worldwide LOWfer (very low frequency) records. 4 Is almost impossible to decode by ear. 4 Needs computer “assistance”
22 Example: 90 second Dot 4 0.0133 wpm or 0.8 words/hour 4 bandwidth >= 0.033 Hz 4 72,000x narrower than SSB (48 dB) 4 900x narrower than 12wpm CW (29 dB)
23 Needed 4 Computer software 4 Stable and known frequency control of both transmitter and receiver 4 Patience
24 Introducing ARGO 4 Windows 4 Sound card 4 Pentium 200 MHz or better 4 Price is right 4 Works!
32 The 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."
33 The Players 4 AA4XX - Paul Raleigh, NC 4 ON5EX - Johan Zevergem, Belgium 4 WØCH - Dave Seneca, MO 4 AE5K - Don Yellville, AR 4 AKØB - Stan St Charles, MO 4 N4SO - Ken Grand Bay, AL 4 W8DIZ, N3AAZ, KD1YV, VE7SL, VE3FAL,ON6UL, AE4IC, N4HAY, K2UD, VE6KBS
34 Details 4 February-March 2002 4 10.140 MHz. +/- 4 Main beacons: AA4XX, ON5EX 4 Power 250 mw to 50 uw 4 Call, power, “codeword”
35 AA4XX - Raleigh, NC Paul - AA4XX Wilderness Sierra S&S DDS VFO
38 AA4XX - Raleigh, NC 4 Antenna - 30 meter dipole fed with ladderline, about 60 ft. up in NE/SW orientation.
39 My first attempt and results... Paul, Just downloaded Argo, installed it & tuned rig to 10140 (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 1832-1849Z
40 Email comments... 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.
41 Email comments... 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.
42 Email comments... 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?
43 Email comments... 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]
44 Some Results 4 AA4XX -> AE5K (785 miles) 4 200 microwatts good copy 4 100 microwatts missed 1 letter in codeword 4 7.8 million miles/watt
45 Some Results 4 ON5EX->AE5K 4 250 mw - good copy 4 4600 miles 4 18,400 miles/watt
51 Conclusions 4 QRSs is an “experimental” mode 4 QRSs is a very viable mode using very low power and computers. 4 None of our antennas were “spectacular” 4 Signals under about 10-25 mw or so were not audible. (rough estimate, depends on conditions of course)
53 Ugly Transoceanic Hifer Transmitter 4 Built by SM6LKM 4 Crystal 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, +11 dBm @ 14.10111 MHz
54 Accomplishments with UTHT 5mw SM6LKM to W1TAG
55 SOFTWARE http://www.qsl.net/padan/argo Freeware for ham and hobbyist use. Good starting program, easy to use.
56 SOFTWARE http://www.qsl.net/padan/spectran.html Freeware, more extensive and sophisticated than Argo. By same authors.
57 SOFTWARE http://www.qsl.net/dl4yhf/spectra1.html DL4YHF's Audio Spectrum Analyzer ("Spectrum Lab") Due to its “Laboratory” nature, this analyzer is not as easy to use as ARGO. Freeware
58 SOFTWARE ON7YD’s QRS Program http://www.lwca.org/library/ software/qrs/qrs2.htm Freeware, makes easy to send QRSs and DFCW transmissions.
61 My thanks to the following who contributed to this presentation: 4 AA4XX 4 WØCH 4 ON5EX 4 AKØB 4 N3AAZ 4 NL9222 4 SM6LKM 4 KA7OEI 4 W1TAG 4 W5CSJ (my wife who had to put up with all this) 4...hope I didn’t miss anyone (if so, hollar)