by Phil Beastall

The advent of digital satellite television has meant that equipment produced for analogue satellite television has become available second-hand at low prices or even free of charge.

A typical TVRO (Television Receive Only) satellite TV system consists of a dish, an LNB (Low Noise Block) and a receiver. The radio waves are collected by the dish and reflected into the feed of the LNB. The LNB consists of a feed and antenna that goes to a microwave amplifier and a mixer that is also fed by a local oscillator. Like any heterodyne system, the signal is output with a frequency that is the difference between that of the original signal and that of the oscillator. Typically a signal of around 12 to 15 GHz (for Ku Band) or 3 to 4 GHz (for C Band) is output at around 1 GHz. This signal is then carried by a coaxial cable to the receiver.

If you happen to have a receiver that has an analogue meter, you are in luck. You can measure the voltage across the back of the signal strength meter, it may even have a signal strength socket. Unfortunately, a typical receiver has neither. Most modern receivers are highly integrated, which means that you cannot even disable the automatic gain control. This is a pity, although you may wish to try a simple detector across the video out socket - the video has a bandwidth of around 4 MHz, whereas the audio is very narrow.

Whereas the LNB is highly useful, most receivers present us with problems and a practical approach is to do away with them altogether. Firstly, we have to replace their other main function, the supply of 15 to 18 volts DC to the LNB. Most LNBs are fed DC up the coax from the receiver. If you are very brave you may wish to take off the cover from the LNB, locate the inductor that acts as a choke blocking RF from the DC input and remove it (it will be on the printed circuit board, and this can be done with the tip of a knife), then solder on a pair of wires to feed in the DC. However you may prefer to take a less drastic option and make a DC injector.

DC injectors can be obtained commercially, but they are very simple. It consists of a piece of board which is essentially a 50 ohm transmission line, there is an inductor (coil) to prevent RF signal going down the DC supply line and a capacitor to prevent DC going the wrong way, towards the amplifier detector. You can make the coil with a few turns of tinned wire wound on a drinking straw, and a chip capacitor or a tiny piece of copper foil insulated with sticky tape will do the trick.

The amplifier needs to work at around 1 GHz. Communications with manufacturers of TV signal boosters have failed to ascertain the gain of these at that frequency, and it would be best to obtain an amplifier made for satellite TV signals or make your own using MMICs.

The detector is very straightforward, and should be made to a design such as this one by Ken Tapping.

The system so far will deliver a tiny DC voltage at the output of the detector, this must be amplified so that it may be measured. You will need to obtain a DC amplifier or make your own. The design suggested employs an operational amplifier chip costing very little, the major part of the cost is in the two multi-position switches. As these will also be multi-pole switches, you could make your DC amplifier into a multi-channel device quite cheaply.

Now we have a much larger DC voltage varying slowly with the strength of the signal. You can display it with a volt meter, and could plot a graph against time. However, one of the great things about radio astronomy is that you can do it whilst you are asleep or whilst you are not even there, so a data logger is an essential.

You could use a traditional paper strip chart recorder, however these are increasingly hard to come by and expensive on paper. Also, it is better to get the data straight into a computer. You may wish to consider an AtoD converter card or a product such as those from Pico Technologies. These may be used with software such as that provided by Radiosky Publishing. The cost of these is not inconsiderable and you may be tempted to either build your own AtoD converter or use a PC sound card to log the data.