Over the past few weeks I have been building up a low power HF transmitter called a QRSS or MEPT (Manned Experimental Propagation Transmitter). QRSS beacons send very slow Morse Code at low power levels on the Ham radio bands. Hams situated around the world have set up “grabbers” – automated receivers that post web pages every few minutes. By looking at the grabbers you can determine if your signal is making it to that part of the world.
My Ham radio call sign is VK5DGR, you can see it spelled out in Morse Code. The signal fades in and out over the day as the atmospheric conditions change. The “dot” time of the Morse Code is about 6 seconds, or a bit rate of 0.16 bits/s!
My little beacon has a power level of just 20mW, but has already been picked up in Tasmania (1200km away) and even Albany, Western Australia (3000km away)! However I am just a beginner at QRSS – many people have sent signals half way around the world using 50-500mW. Look around your room and find a glowing LED. Well a typical red LED has 2V and 10mA flowing through it, or about 20mW. It can barely make it a few metres to be detected by your eyes. Imagine detecting it at a range of 1200km! That’s what I find remarkable about QRSS – tiny power levels that go a very long way.
QRSS is a great example of the bandwidth versus power trade off. Think about the referee’s whistle during a football game. The whistle goes a long way compared to his voice as it has a narrow bandwidth – lots of energy concentrated in a single tone. Compared to human speech a whistle also sends a very slow and simple message – just one piece of information like “stop the play”. If you reduce the size or speed of the message, the energy dedicated to each message is greater, so the chance of detecting the message is higher. The bandwidth of my QRSS signal is about 10 Hz. This very narrow bandwidth signal can be detected over great distances by carefully filtering to pluck the tiny signal out of the noise. It’s like the referee’s whistle being heard amongst 100,000 other voices all yelling at a football game.
For comparison, a voice signal might have a bandwidth of 3000 Hz. This is 3000/10 = 300 times more bandwidth so would require 300 times more power to be detected, or about 6W. It’s actually a little more complicated than this, but you get the idea.
The crystal oven keeps the crystal at a stable temperature and hence a stable frequency. As the bandwidth of the QRSS signal is so low (mine has a frequency shift of 8Hz), the stability of the oscillator is important. My oven is only partially successful – I haven’t insulated my crystal oscillator yet so it still drifts at bit (you can see how wavy it is in the plot above).
I built the circuit “manhatten style” – little squares of blank PCB stuck to a larger piece of blank PCB. This gives a nice ground plane which is useful for RF work. It’s also surprisingly easy to work with, as everything is on one side of the board. I don’t like veroboard much for prototyping as you have to keep turning the board over and connecting wires get broken.
I am using my 80m dipole for an antenna, tuned via a LC impedance matching network at the output of the beacon. I worked out the values of L and C experimentally: trying different values until a good match was obtained using a simple home made impedance bridge, same as that used for my summer ham project. A few hours after that I started getting signal reports!
I was inspired to get into QRSS by the Solder Smoke pod casts. I am also interested in possibilities of HF Mesh Radio for low bandwidth SMS or small email transmission in developing countries. If I can go 3000 km on 20mW, perhaps we can send useful messages over smaller distances (say 100km) using a network of low cost, low power, automated HF Mesh radios. No infrastructure (like cellphone towers) and very little electricity required.
A special thank you to Bob VK7ZAL for kindly firing up his QRSS grabber and phoning me with my first signal report!