23cm 600 W Power amplifer

As I'm waiting for the mechanics to be produced for my coming 4.8 meter dish for 23cm EME, I started looking into building a suitable power amplifier for the system.

Again to "get the most" out of the amplifier and at the same time, save some space in the lab/shack, I decided from the beginning that the PA should be mounted near/at the dish. This will allow me to use the shortest possible coax between the PA and the feedhorn, and also remove the need for flexible "around the rotor" high power/low loss cable to be used. The PA will be remotely controlled and monitored using my REPAM and Dual RF Head modules.

I used the same principle in my 500W 70 cm power amplifier used in my current 70cm EME system.

Kit assembly instructions from Jim W6PQL

Schematic of the PA module from Jim W6PQL

Test procedure from Jim W6PQL

Datasheet for the MRF13750HR5 device from NXP.

The NXP device can be ordered from Jim W6PQL or from f.ex Mouser.

I found the LDMOS at Arrow for 50% of the usual price! Link to Arrow

Soldering the LDMOS, application note from NXP.

So far, the system consists of the following sub modules, some designed by myself, and some purchased:

Cooling system with two fans. I designed a "generic" system that can be used in other PA systems.
Overcurrent detection and on/off switch. Described and documented here also.
Dual RF Head (logarithmic amplifier) that samples the fwd/ref signals from a directional coupler.
Remote PA Monitor (REPAM), allows control and data collection/security of a remote PA.
W6PQL 600W module (I selected the "kit form" of the modules, "assemble yourself")
Power supply from RECOM RACM1200-48SAV/ENC: Datasheet for power supply
Various stuff for mounting, a relay for remote power/on off, a 12VDC PSU (for bias etc.)

Design files

Here you can find the design files/production files for the mechanical parts of the system. Will be uploaded once they are checked for fit etc.

Copper heat spreader (10 mm thick copper, mirror polished), STEP file
- Thread instuctions for copper spreader, PNG file
Enclosing milled alu box (surface treatment is bead blasting), STEP file
- Thread instructions for box, PNG file
Lid for enclosing alu box, STEP file
Drill template for copper spreader and box (for drilling holes in heatsink for both), STL file (for 3D printing), also as a STEP file

PA/Heatsink/Fan assembly

The completed assembly, PA module in milled aluminium box and heatsink with fans will look like this.

This is the exact same heatsink/fan assembly as I use in my 500W 70cm PA module.

Below is a dual module solution, also shown is my 23cm high power 90 deg. hybrid.

Two different ways of mounting the modules are shown.

The side-by-side assembly below shows also my REPAM, Dual RF Head and Overcurrent/switch modules.

Enclosing box

I decided to design a milled aluminium "box" that would contain the PA module, connectors for in- and output and a few feedthrough capacitors. Jim (W6PQL) suggested that it was a good idea to cool the output components on the PA module as they can get warm at full output. I designed the lid for the milled aluminium box so a 60x60 mm 12V fan could be mounted.

I also designed a copper heat spreader for the module and had that manufactured at the same time as the milled box and lid. Shown on the screenshots below are also two small brass strips that will be used to solder the coax cables to, these will then be secured to the PCB/heat spreader with two M3 screws.

The extra M3 screw in the "LDMOS trench" is for a temperature sensor (1-wire DS18B20 sensor that connects to the REPAM module).

The data for the box will be available for download here once I have verified everything. The copper heat spreader design will also be available for download.

Drilling template

Designed a drilling template that will help place all the holes correctly in the heatsink when it is being drilled. This one I just 3D printed as it is a "use once and throw away" type of device.

Mechanical parts received

Received the mechanical parts for the PA module, copper heat spreaders and enclosing boxes, back from milling.

Connectors and heatsink preparation

Finished in- and output coaxes for the two modules. Also used the drilling template to mark the holes needed in the heatsinks. Tried some simple mockups, everything seems to fit as planned.

Cable on input is RG-401 semi rigid, on output it is RG-402 semi rigid.

Connector used on input: Amphenol 082-6099-RFX

Connector used on output: Amphenol 082-6163

3D printed the first of two ducts for the two of four fans. The design for the duct is here.

Soldering of LDMOS devices

Soldering LDMOS is always exciting! You have to be EXTREMELY CAREFUL when handling stuff like this as it is VERY HOT (200+ degC)!!!

The soldering process follows the instructions from Jim W6PQL outlined in his video. This method has proven to work fine over the years in countless PA constructions.

I shot a video of the process.

Carsten Groen's (OZ9AAR) personal place