Presentation on theme: "QRSS (Send slower, no really, slower) Communicating at.8 WPM Welcome to the slow lane of Ultra QRP March, 2011 by Michael Seedman, AA6DY With special thanks."— Presentation transcript:
QRSS (Send slower, no really, slower) Communicating at.8 WPM Welcome to the slow lane of Ultra QRP March, 2011 by Michael Seedman, AA6DY With special thanks to Hans Summers, G0UPL
QRSS – The basic idea The slower you send, the less information you’re transmitting, the less bandwidth you need. The less bandwidth you need, the better the SNR. A good way to think of this - as you minimize the bandwidth at your receiver, the less noise makes it through along with the signal of interest. The result of this is increasing the distance you can communicate with a given amount of power. We all know that it takes less power to communicate in CW that in SSB. Take this a step further and it makes sense that by using slower CW, it would take less power to communicate with 1 second dots, or 3 second dots, or 10 second dots… So milliwatts sent with 3 second dots and 9 second dashes can be heard around the world – even with simple antennas. Think about this – milliwatts traveling around the world! In the world of QRSS, one watt of power would be considered extreme QRO!
How does it work? Shannon’s Law – Considering all possible multi-level and multi-phase encoding techniques, the Shannon–Hartley theorem states the channel capacity, meaning the theoretical tightest upper bound on the information rate (excluding error correcting codes of clean (or arbitrarily low bit error rate) data that can be sent with a given average signal power S through an analog communication channel subject to additive white Gaussian noise of power N, is: C is the channel capacity in BPS: B is the bandwidth of the channel in Hz (passband bandwidth in case of a modulated signal; S is the total received signal power over the bandwidth (in case of a modulated signal, often denoted C, i.e. modulated carrier, measured in watts or volt^2. N is the total noise or interference power over the bandwidth, measured in watt or volt 2 ; and S/N is the signal to noise ration (SNR) or the carrier to noise ration (CNR of the communication signal to the Gaussian noise interference expressed as a linear power ratio (not as log db)
Most QRSS activity is beacon operation – know as MEPT MEPT stands for “Manned Experimental Propagation Transmitters” Sending “599 IL AA6DY K” would take about 7 minutes. The other side of the Q would take another 7 minutes or so, making the entire QSO about 14 minutes long. So, we don’t really do too much of this. Although there are people that have ‘true’ QSO’s, much of the QRSS operation is as a beacon. All of my QRSS operation has been on 10.140000Mhz to 10.140100Mhz (100 Hz wide band) using 7 Hz frequency shift keying
How do you QSL? A QSL in QRSS land is a screen shot of my signal in some far away land.
The Receiver Any receiver with a 1Hz filter will work. The problem is that receivers with 1Hz filters are hard to come by. So, we use a computer with some spectrum analysis software. The output of the receiver is fed to the input of the sound card. The computer does FFT’s on the signal (many times too faint to hear with your ears) There are quite a few capable software packages available. The one I’ve used is called Argo. Simple and it works. Argo accumulates the signal strength in sub-Hz buckets, displaying it on the screen as the tape slides sideways.
CW is one mode, but there are others Plain old CW is the classic QRSS mode, but there are others: A.Slow CW – actually ON OFF keying of one frequency. B.Slow Hellschreiber – shift carrier to generate the pattern C.FSK-CW – CW encoded with two frequencies about 6 or 7 Hz apart D.Soundcard Modes – sent as SSB signals – reverse of a Fourier Transform E.Pattern F.Pattern – Easy to generate pattern. Easy to spot, but no ID G.Shows a ghost of the image due to propagation H.Notice drift and the signal bouncing off something above.
The QRSS Transmitter They’re typically simple home built “handful of parts” transmitters that generate 500 milliwatts or less, and can transmit two tones around 10.140 MHz – 7Hz apart. Most QRSS transmitters are crystal controlled, and the small frequency shift is controlled by using a varactor diode – in fact, a reverse biased LED makes a fine varactor diode. Drift is a big deal. Crystals aren’t exactly ‘rocks’ as we know them. Temperature can make crystals drive a few tens of hertz, which shows up in the receivers display. Many builders put their transmitter in some kind of insulated container to keep the drift down. Even Crystals ‘Chirp’ (although not much). At 1Hz, it can be clearly seen on the display. The only input to the transmitter is a single bit. Transmit the low frequency, or transmit the high frequency. Where am I transmitting? Even a crystal can be off by more than the 100 Hz band where people are watching. An accurate frequency counter is pretty helpful.
QRSS Transmitter Rev 1.0 I’ve built three QRSS transmitters. My first one, using a 2N5109 in the PA, puts out about 500 milliwatts.
Here’s the signal from QRSS Rev 1.0 10.85V p-p X.5 = 5.4V p-p X.707 = 3.8V rms Squared = 14.7 Divide by 50 = 290 mW P = E^2/R
Arduino as the Controller I used an Arduino as a controller for the QRSS transmitter. If you’ve not heard of it, or played with one, go get one. There are libraries for just about everything. You can go to Sparkfun.com and purchase the board for about $30. You can download the environment free at arduino.cc.
Arduino Shields as QRSS Transmitters An Arduino shield is a daughter board that fits on the Arduino computing platform. There are lots of prototype shields available. I used one from Sparkfun to build Rev 2.0. Same circuit, different form factor.
My Latest QRSS Transmitter Arduino + DDS + Amplifier + Filter = fully programmable QRSS Transmitter. I use an Arduino to speak I2C to the direct digital synthesis (DDS) board built for a different project to generate a square wave at the right frequency. The DDS square wave feeds to a schmitt trigger, and into 4 small 2N7000 FETs. The voltage for the PA is controlled by an LM317 adjustable voltage regulator.