A 222MHz Amplifier for the Low Power Transverter
Art Pightling W0BA
While +20dBm seems like a small signal we have found 75+ miles is not a difficult path with good antennas and fairly unobstructed terrain, the time will come when you wish you had “just a little bit more” power. My minimum requirement for an RF amplifier is 10dB gain and preferably more. There are RF modules available (RF Parts) for a relatively reasonable cost that fulfill this requirement, have low parts count, and are easy to build. In fact this amplifier will take us from our peanut whistle 100mW to over 25W output.
Dave (W6OAL) reviewed the module selection and decided the M57712 with 200mW drive for greater than 30W output would be a good choice for this project. (A complete kit is available from Down East Microwave that is very similar to this project.) He built the alpha version of this project and this paper describes the beta version build.
We will be using material (metal) construction techniques, SMD soldering, discrete axial component sub circuit builds, and experimentation to provide ease of operation.
We purchased the module(s) and a small printed circuit board to support the module from RF Parts and an enclosure was selected from my junque box. The greater 222 transverter program will be constructed in the “substrate and bulkhead” configuration for the integrated transverter which allows for easy experimentation and support electronics modification. This amplifier will be constructed in its own enclosure to shield its neighbor modules from heat.
The initial group of components ready for the amplifier build
Since the heat sink and enclosure are recycled we will need some holes and cleanup before proceeding. Dry fitting the board and module allows us to determine the best placement of the major components and the associated mounting holes. A bit of drilling, tapping, and surface cleanup provided a ready to build heat sink and circuit cover. Once we have the PCB anchored firmly to the prepared heat sink we install bypass capacitors on the two Vcc inputs at the module. These caps are 100pF chip, .1uF chip, and a 150uF SMD electrolytic. The bias input is bypassed with 100pF and .1uF at the module pin, a homemade choke bridges the Vcc trace, and a 1uF electrolytic is located at the output of a 78L09 regulator. The regulator can be fed directly via another choke if you are keying Vcc and Vbb simultaneously or a simple PNP switch to allow low current keying of the amplifier via Vbb.
Here is a picture of the completed amplifier to provide that 1000 word advantage. Here’s a close up of the amp by itself. As you can see there are only a few components but a schematic couldn’t hurt!
Note that I specify three capacitors at Vcc inputs but this is not critical or absolute. Some have said I tend to gold plate my designs and that’s probably true. Just make sure your DC inputs are well bypassed and you use a choke to feed Vbb and you should be good to go.
The placement of the brass shield over the module is a precaution I have used for several years. The 78L09 is between the key line choke and the 1uF electrolytic. Use good soldering techniques and make sure the surface of the heat sink is clean. After all the SMD components are installed apply a thin coating of thermal paste to the module and bolt it down, gently but snugly, (don’t warp the substrate of the module by honking the mounting screws) *then* solder the module contacts to the PCB after fitting and trimming its leads appropriately. Solder a power lead to the Vcc/Vbb input at the PCB and install your chosen DC supply connector to the other end. I use PowerPole connectors. After about an hour and a half of component installation you will be ready to connect your RF input and output coax(s).
Test results: +20dBm input from the W7BAS 222 transverter, 30W output peak (or FM), 13.8V Vcc, 8A peak.
Conclusion: This is a good project for builders who like to source and build their own gear. For those who may not have a junque box as well stocked purchasing the DEMI kit is the way to go.
73 and I’ll be hearing you on 222!
