Long range communication using low power shortwave transmitters is possible with very low data transfer speeds. Every time speed is reduced range is increased, this is used by ham radio enthusiasts with morse code/CW. Normal morse code is often sent at 20 words per minute, by reducing this speed to 1 word per 10 minutes range can be increased, or needed RF output power can be reduced. I am have a ham radio license, in my country that is needed for this.

Transmitter

The Joy-It PSG9080 80MHz signal generator is capable of putting sine wave 10Vpp into a load of 50 ohms at 10MHz, this makes it a low power radio transmitter. 250 mW is just enough power needed to send a short message for for a few thousand kilometers.

Calculated RF output power of PSG 9080 signal generator

For this project I only need 100 mW, and after some adjustment I get 6, 5 Vpp dialed in, using a oscilloscope and a 50 ohm load.

Modulation

The method of sending extremely slow morse code is called QRSS, and there are grabbers that listen to this radio spectrum and publishes images of it every 10 minutes. The CW signal is not sent as on-off, in stead it is sent as two slightly different frequencies. Key down is normally 8 Hz higher than key up. This way a its easier to decode the message if the signal has poor reception. This is also called DFCF, Dual Frequency Continous Wave. Modulation of +/- 8 Hz puts a lot on the transmitter, the oscillator cant drift during transmission, or the signal will be unreadable. The PSG9080 has a very stable oscillator, once its warmed up after 20 minutes.

I have done some tests on the frequency and how it changes with temperature, once it has warmed up its pretty stable.

Controlling the signal generator

The signal generator can be controlled by a Serial USB interface from a computer or a Raspberry Pi, but for this project the control will be mechanical. The signal that is going to be sent will be coded into a rotating wheel that turns at 0.1 RPM.

Prototype using a JGY370 geared DC motor, 12v around 100 rpm

The signal generator has two channels, and can be set to 8 Hz from each other, a simple micro switch has NC/NO contacts and will switch which channel is connected to the antenna. The wheel has a ham radio signal coded into it, and pushes the arm of the switch. There is around 20 dots or dashes in a ham radio signal, putting them on a mechanical memory like this is handy.

For an antenna not much is needed, I used a 5 meter wire in my garden, with a ground wire around 2 meters long.

Gearbox

Reducing rotation from 130 rpm to 0.1 rpm using a gearbox needs a reduction of 1300:1. The gears are all made in Onshape that has a good function for creating spuhr gears, they are 4 reduction gear sets, each gearing down in average 5:1.

Fine adjustment of speed can be done by changing voltage on the electrical motor, it draws around 100 mA.

Final gearbox assembly, 1300:1 driven by a 130 rpm 12v dc motor

The final assembly uses RG58 coaxial cables two from the signal generators channels to the micro switch, and one from the output of the switch to the antenna.

Complete transmitter

Testing

Testing the transmitter from my location I can see my signal appear at the lower part of the spectrum, at 10, 139, 820 Hz. The reciever is in Kent England, around 980 km from my home in Sweden, this puts it at 9800 km / watt.

There are a few tricks that can be used to increase readability, several of these images can be stacked on top of each other using Photoshop, since the signal will be present on the same place every time, but the noise is random.

Spectrum from receiver in Kent, UK. This shows radio spectrum during 20 minutes on horizontal axis and frequency on vertical axis.

Links

For anyone interested in controlling the Signal Generator with USB Serial the manual in the link will be usefull.

http://68.168.132.244/PSG9000/TXXY_EN_manual.pdf

Grabbers are available online at the links below

https://swharden.com/qrss/plus/#LA5GOA-30M