The Sun's Thermal Radiation
Use a Satellite Dish to Observe the Sun's Thermal Radiation
Equipment
- Satellite dish with Ku band LNB
- Satellite finder
- 100 microHenry Choke
- Power supply 13-15V
In this project you will learn about the Sun's thermal radiation and there are instructions to enable you to construct a working radio telescope using easily obtainable equipment. This is the first project that amateur radio astronomers often complete and is known as the IBT, which is short for "Itty Bitty Telescope". The primary components are a satellite TV dish, a satellite finder and a power supply.
A satellite finder is a signal strength meter. It is connected to the satellite dish by a length of coax cable. A DC power supply is required for the satellite dish receiver and satellite finder circuits. One extra component that is necessary is a choke. This is a passive electronic component that consists of a coil of wire wound around an iron core. It is inserted between the battery and the satellite finder to prevent radio frequency waves travelling back into the battery and causing a short circuit.
The satellite dish can be one that is normally used for TV reception. It should be an 18" dish or larger operating in the "Ku" band, so it can pick up microwave emissions at 12Ghz. Radio emissions received at the dish are focussed onto a Low Noise Block (known as an LNB). The LNB has a coax output connector: if you have a quad receiver, it has four outputs but you are only going to use one of them, so it is a good idea to screw a 50 ohm terminator like the one below onto each of the remaining three outputs to reduce signal loss.
A satellite finder is a signal strength meter which is used to detect emissions from TV satellites. It is normally used to help position the antenna to point to a TV satellite but as it gives a direct reading of incoming radiation it is useful for detecting high frequency electromagnetic radiation from sources other than just satellites.
A battery is required to power the meter and the amplifier in the LNB, and this is usually around 13-15V, but check with your particular dish instructions. The incoming radiation is very high frequency and the battery is DC. Unfortunately the high frequency signals can enter the battery and cause shorting problems, so a choke is connected between the battery and the satellite finder. The choke's value should be at least 100 microHenries.
Very importantly, make sure all connections are made with coax cable designed for high frequencies and not just any old wires. Also the central core is the signal wire, the outer sheath is the Earth so make sure the +ve supply and the choke are connected to the signal wire.
When all the connections have been made as shown in the illustration, simply point your radio telescope dish at the Sun and the needle will rise. It is measuring the temperature of the Sun's radiation.
Counterintuitively, if you point the dish to the ground, we measure an even greater temperature, shown by a rise in the satellite finder needle. The needle rises even if people cross the path of the dish. This happens because the sky's temperature is at about 3 degrees Kelvin. Kelvin is the temperature scale generally used in astronomy in preference to the Celsius (Centigrade) scale. Kelvin graduations are identical to the Celsius scale, but the zero point of Kelvin is different to the zero point of Celsius. Zero degrees Kelvin is what is known as absolute zero, the coldest temperature in the Universe whereas zero degrees Celsius is the freezing point of water. Absolute zero is equivalent to -273 degrees Celsius.
The ground is about 300 degrees Kelvin (27C) yet the Sun has a temperature of 6000 degrees Kelvin, but because the Sun occupies just 0.5 degrees in the sky whilst the dish has a capture of 3 degrees, the Sun's rays only occupy a small portion of the dish surface at any time, and therefore the total received radiation across the dish is low, whereas someone standing in front of the dish occupies more of the dish surface and this gives a higher reading. This project is therefore a highly informative and practical way of learning about dish capture area and thermal radiation.
Another experiment to try is to point the dish so it is just slightly ahead of the Sun and leave it fixed in position. As the Sun moves across the surface of the dish the temperature and hence the reading will initially increase to a peak and then plateau before decreasing as the Sun moves out of the line of sight of the dish. The duration of the scan shows the duration that the Sun crosses the dish. This is a technique used in radio astronomy called a "Drift Scan". If you plot your readings against time you will see something like this:
If you enjoy making things with electronics, you might like to tap-off the signal voltage from the meter and feed it through an analogue to digital convertor (ADC) where a computer e.g. Raspberry Pi etc can plot the graph in real time.