UHF-Satcom.com - UHF preamplifier design (11/06/2007 18:23:59 +0100)
On this page you will find the design details of a good low noise amplifier, tuned for the 240Mhz to 270Mhz band. The preamp is built from parts taken from an old satellite TV low noise down converter.
The picture above shows the finished preamp. As can be seen, it's built on a copper ground plane that is screwed to the lid of a small aluminium box. This method of construction provides mechanical stability and good RF grounding practises.
The input BNC is the connector on the left hand side of the photo. This feeds a 4 turn inductor, tapped at about 1/2 a turn from the start. This provides a good 50 ohm input impedance. The other end of the input inductor is connected to one side of a variable capacitor. This L-C circuit provides fairly sharp front end tuning. The component to the right of the variable capacitor is the gasfet. This was salvaged from an old satellite TV low noise converter and in this application, it provides low noise but high gain amplification. The power is fed to the gasfet by means of a contra-wound matching transformer. The output of the matching transformer is fed through another variable capacitor to the output BNC. This variable capacitor provides an easy way of matching the output.
The gasfet is supported on two ceramic surface mounting capacitors to provide DC isolation from the ground plane, but these capacitors also provide an RF ground. The two trimmers that can be seen are for adjustment of the bias to the gasfet. De-coupling is provided my means of the ceramic disc capacitors, and the stabilised voltage required is provided by the 7805 regulator. An input protection diode prevents accidents from reverse polarity DC input.
The above schematic is of the UHF satcom gasfet preamp. The only changes between the photo and the diagram are that the 100 ohm resistor and R1 have been replaced with 100ohm trimmers to permit accurate setting of gasfet bias. This preamp took about 2 hours to build and has been in service since early 2000.