HOW FAR CAN YOU TRANSMIT DATA WITH A TEN MILLIWATT CARRIER?
Turns out - quite a long way if you do about it the right way.
I've mentioned before that I am interested in very long range wireless at very low power levels. It has been a forty year hobby. It is getting cheaper and cheaper to do it these days with tiny micro controllers available for under a tenner - even less if you buy the Chinese clones. I built a whole setup last week for twelve quid.
I have been messing about with this one shown above which was designed by someone to be used as a balloon tracker radio, to hang under a couple of party balloons full of hydrogen or helium transmitting telemetry about itself and its position as it travelled around the world. I know people who have these floating around the planet for months. A polish guy I am in contact with has one with his own board that he designed himself, that has been sauntering about for eight months at about 35000 feet and it is reporting its position whenever the sun shines on its tiny solar panels.
You might remember that during the Chinese spy balloon fiasco, Biden sent a million dollar missile to shoot down one of these over Alaska...
They never found it because the radio is about the size of two postage stamps and weighed about 3 grams and the two balloons just vanished with the radio. They definitely shot down two party balloons and a minute solar powered radio transmitter, because the group who launched it were tracking it and it disappeared in the exact location of the USAF interception of a 'dangerous enemy intrusion'.
I have been running my new tracker in my back garden for a few days powered by a salvaged vape battery I picked up in the street. This one is a bit power hungry because I just bought a cheapo GPS on Ali Express. It is drawing 67 milliamps! I have to swap out the battery every day or it goes off air.
But look where it has been heard over the last few days - it only has 10 milliwatts transmitter power using two of the GPIO pins of the Raspberry pico micro controller. It transmits for four minutes every ten minutes.
It works flawlessly as long as the GPS has a satellite lock and the battery is still running.
Most of these green lines represent multiple decodes of the telemetry transmission.
I remember having a discussion with Saneagle a few months back about whether it was possible to transmit data from the moon to earth back in 1969 with a ten watt radio - which they did. The answer is yes - as long as the data rate is slow enough. Ten milliwatts to get a fully decoded packet across the Atlantic containing a valid callsign, transmitter location and power level. Followed by another transmission containing Altitude, board temperature, bus voltage and tracker position.
Ive done this for years with more power - 150 milliwatts, but never been so low as now. 150 miliwatts can get to Australia if the ionosphere is right.
This below the result from the 12th May with my more powerful weak signal transmitter in Newcastle. That one is about 140 milliwatts - still only a tenth the power of your cellphone which struggles to get three or four miles, the answer of course is that the weak signal radio is transmitting a tiny amount of data very slowly, so software defined radio can dig the tiny wobbling carrier out of the background noise. My radios are all home made. That one has been running 24/7 for about 11 years.
This is only possible because of the work of some really clever guys who invented the ultra slow baud rate signalling protocols (1.6 bits per second ) and set up the servers to run a world wide network of thousands of hobbyist receivers connected to the Internet which listen on specific frequencies on all the ham radio bands and decode what they can hear and upload the spots to the central server. I run a receiver 24/7 as part of that network. Right now there are about 4,800 receivers, all over the world, connected to the WSPR server uploading their spots. Not all continents are well covered. Not many in Africa. I rarely get spotted there just because there are very few receivers there, Not many in China either, or Iran. WSPR stands for Weak Signal Propagation Network. The guy behind its invention is a Nobel Prize winning radio astronomer, called Joe Taylor. The originator of this tracker with the Pi Pico is a guy in Russia called Roman Piksaykin. The project, originally publicised on Git Hub has been taken further and improved by others - i got my stuff from someone on GitHub going by the name of EngineerGuy314. Look here for details if interested.
https://github.com/EngineerGuy314/pico-WSPRer/wiki/pico‐WSPRer-(aka-Cheapest-Tracker-in-the-World™)
Here is the lashed up beacon doing a shake down test of a few days running outside on an old metal garden table. When I am happy with it. I am going to put it on a pole about 18 feet up in a weather proof box with a few tiny solar panels on to power it. Then it can run permanently whenever it has solar power available.
I might even put one up somewhere in the middle of Kielder Forest on a sunny hillside and see how it does over the changing seasons. Ar twelve quid a pop - why not?
The long wires are part of a twenty metre loop of wire for the transmit antenna. You might be able to make out the tiny 95mm GPS dipole at the bottom of the GPS unit. Gps is needed to set accurate time for the transmissions to start and end and for disciplining the clock in the Pi Pico micro controller. Very narrow band signalling protocols like this need to be absolutely stable and cheap clock generators in computers and micro controllers are not good enough so while it is operating the GPS adjusts the frequency to keep it absolutely rock stable. The signal is a frequency shifted carrier and symbols are indicated by tiny movements in the frequency of four tones which are one point four htz apart.
The message is contained in the tiny changes in the frequency of the carrier visualised here below.
The software of course is the core of the project and I wrote none of that. I just dragged and dropped the .UF2 package some clever person wrote and dropped it into my pico and then configured my details into it and settings using putty and a serial connection. The people who devised this scheme have made it really easy to do. A few bits of wire soldered to a few pins, a trivial bit of computer wrangling, a suitable low pass filter on the output to get rid of harmonics, and its done.
eyes.nasa.gov
library.fiveable.me
