137MHz LNA Design

Here is a design for an easy to build LNA for the 137MHz satellite band. Looking at the pictures, you can see views of the completed preamplifier - SO239 sockets were used as at 137MHz it's not too critical. The housing that the preamp is built into is the case from an old Marconi LNB. Most of the components are salvaged from that same LNB (fet, chip caps, regulator etc). The circuit diagram is included below.

137Mhz preamp schematic

To build this preamp, firstly cut some double sided PCB to a size that would fit into the case you have selected. Bolt this in and drill the holes for the input and output connectors, and one for the variable capacitor. Next, fit the two sockets and the variable capacitor. The input inductor is made from the earth wire from 2.5mm twin-and-earth mains cable, and it's 10 turns on a 6mm former, with the input being tapped at turn 3 or 4. The end of the inductor nearest the input socket is connected to ground, whilst the other end is connected to the non-ground side of the variable capacitor, thus forming a parallel resonant circuit.

Next is the difficult part - mounting the gasfet. To do this, firstly, about 5mm in front of the variable capacitor, tin the PCB - use some soldering flux if necessary. Next, get a couple of suitable chip caps - use 1000pf's caps as they can be salvaged from the Marconi LNB board. Stand these so one end is soldered to ground and the other end is floating. Mount them about 5mm apart, and also fit a 100 Ohm chip resistor across one of the capacitors. Find a suitable gasfet, again use one from the input stage of the Marconi LNB, and fit this across the chip caps. Solder the two source leads to the tops of the chip capacitors and chip resistor.

Next, take a piece of fine wire about 10cm long and solder one end to the PCB to ground it. Solder the other end to the gate leg of the gasfet - being careful with the soldering iron so as not to fry the gasfet either with high temperatures or static. Next, arrange the thin wire such that it contacts the top of the variable capacitor and solder. Remove the excess wire and desolder the other end.

An output transformer has to be wound, so using some thin wire around .3mm, find a ferrite core preferably a T37 type and wind 12 turns on it. Look at the circuit diagram to determine how to wire it correctly. The purpose of this ferrite is to feed DC to the gasfet, and RF to the output connector.

You'll probably notice that there is a T attenuator in the output chain - this was to prevent overload into the receiver. It's around 4dB with the component values suggested. The power supply section is a bog-standard 7805 regulator and a pot is included to set the drain voltage / current. In this design, power is fed up the coax from the WXSAT receiver to the preamp, hence the input RFC.

(below) component side of 137MHz LNA

137Mhz preamp1

(below) connector side of 137MHz LNA

137Mhz preamp2