Watercooled 1 kW SSPA for 144 MHz Mark 2

with Freescale MRFE6VP61K25H

This page describes my homemade Solid State Power Amplifier (SSPA) for 144 MHz. It was made primarily for EME work. The construction is based on DUBUS 4/2010 where Lionel Mongin (F1JRD) describes the concept. Later development was done by several radio amateurs, among them W6PQL James Klitzing.  

finished SSPA
The finished SSPA.

My SSPA design approach is simplicity. On the front, I can read DC voltage and DC current on analog meters. I can read the heatsink temperature, and there are 3 bulbs indicating 12V, 48V, and PTT. 

Watercooling was chosen because it separates the transistor from the radiator. The transistor and heatspreader are placed inside the cabinet; the radiator is mounted on top of the cabinet where it is cooled by two large fans. This concept should be more silent than ducted air cooling with high-speed fans. Another reason for choosing water cooling was pure curiousity! I wanted to explore how water cooling actually worked.

W6PQL: Solid State 1 kW Linear Amplifier for 2 Meters (article in QST Oct. 2014)

W6PQL: Notes on the 1 kW 2m LDMOS amplifier (changes since the QST article was written).

W6PQL: 2 Meter KW Amplifier Kit Assembly Instructions


I've modified the SSPA bias circuit slightly as shown below:

revised circuit diagram

Below is a block diagram showing how the different units are connected in the SSPA:

sspa block diagram
SSPA block diagram.

The input attenuator is now -3 dB, not -10 dB as shown above.

Mark 2 changes

This page describes the Mark 2 (second version) of my SSPA. Here are the differences between the first and the second version:
  1. SWR measurement hardware removed (SWR and output is now monitored using a Bird 43 power meter)
  2. LCD thermometer for water temperatur removed
  3. LCD thermometer for heatsink temperature substituted by a LED version
  4. The sequencer was moved to a separate cabinet
  5. Improved attenuator hardware and added shielding
  6. DC-DC converter added (48V in, 12V out)
  7. Added Veroboard for 12V distribution
  8. Low-pass filter mounted in a separate cabinet with forced air cooling
  9. Increased PSU output to 50.4 V


Assembling the cabinet frame, step 1
Assembling the cabinet, part 1. 

Cabinet assembly, part 1
The cabinet frame was assembled from square aluminium elements manufactured by the danish company Porsa System. The elements are intended for DIY aquarium tables! I found the elements suitable for assembly of an SSPA cabinet frame. I was happy to learn, that the Porsa system is easy to work with using ordinary hand tools. 

Each square aluminium element measures 25 mm x 25 mm. The end-pieces are made of black plastic and has a solid aluminium core.

Cabinet width is 30 cm (inner dimension).

Cabinet deepth is 15 cm (inner dimension).

Cabinet height is 15 cm (inner dimension).

Assembling the cabinet, part 2
Assembling the cabinet, part 2.
Cabinet assembly, part 2
The black end-pieces are forced into the alu elements using a hammer.  

Assembling the cabinet, part 3
Assembling the cabinet, part 3.
Cabinet assembly, part 3
The aluminium elements are hollow. 

Assembling the cabinet, part 4
Assembling the cabinet, part 4.
Cabinet assembly, part 4
The aluminium frame is now finished and only the 6 panels remain. 

Front/rear panel: 30 cm x 15 cm

Top/bottom panel: 30 cm x 15 cm

Two side panels: 15 cm x 15 cm

I bought five 30 cm x 15 cm x 2 mm alu sheets for the side panels. Four of them were used for front/back/top/buttom panels. The remaining sheet was cut in two, each part now measuring 15 cm x 15 cm. These two sheets were used as side panels.

Assembling the cabinet, part 5
Assembling the cabinet, part 5.
Cabinet assembly, part 5
The front panel is mounted temporarily.  
M4x12 bolt.

threaded clips
M4 threaded clip.
Mounting the panels
The panels are fastened onto the frame using M4x12 bolts and M4 threaded clips (they are sometimes called "chimney U clips").

The bolts can be obtained in any home improvement store. I bought the clips on ebay.com (stagemotorsports).

1 kW pallet assembly

The copper spreader used for water cooling was manufactured by PE1RKI, Bert Modderman. When I ordered the copper spreader, I forgot to tell Bert that I would use a PCB from W6PQL. The default copper spreader from PE1RKI was drilled for the Freescale PCB, which has a different pattern. This explains why some holes in the spreader seems to be in the wrong places!
Lesson learned:
The copper spreader holes must match the holes in the PCB!

The Freescale MRFE6VP61K25H transistor was delivered by RFham.com (now out of business)

The two PCBs and all passive components for the 1 kW pallet were delivered as a kit from W6PQL.

photo LDMOS transistor
The LDMOS transisor flange is soldered first.             

Soldering the LDMOS transistor 
The flange of Freescale MRFE6VP61K25H is soldered to the copper heat spreader. Soldering is recommended by W6PQL. He has soldered hundreds of transistors without destroying any of them. This instruction video by W6PQL explains the procedure in details. 

Soldering tips: Ensure that the transistor flange is parallel to the copper spreader. All parts of the transistor flange must have excellent thermal contact with the heat spreader. 

I made an error when I soldered the LDMOS transistor: 30% of the flange had no thermal contact to the copper spread. The transistor became overheated when put into service. I had to resolder the transistor to ensure proper thermal contact.

Left side of PCB
The left PCB is mounted.
Assembly and mounting of the left PCB
There is plenty of space on the left PCB. Soldering the SMD components was not difficult.

I have changed the bias circuit slightly. The original circuit diagram by W6PQL is here. My revisions can be found here. Both links will open in a new window. 

Finally, the LDMOS gate terminals are soldered to the PCB.

I used a temperature controlled soldering iron suited for SMD for most of the work. The two LDMOS gate terminals were soldered using a 30 W soldering iron.

Right side of PCB
Capacitors are fitted on the right PCB.
Assembly and mounting of the right PCB, part 1
All capacitors, except the two coaxial capacitors, are soldered first.
I used a 30 W soldering iron for the bigger components, and an SMD soldering iron for the small parts. 
Impedance trafos
Two impedance transformers TC-12 are fitted.
Assembly and mounting of the right PCB, part 2
The two impedance transformers (TC-12) are made of white coax cable.

The coax just visible to the right is a 35 pF coax capacitor.

The two copper coils are each 200 nH. They carry supply voltage to the transistor.

I used a 30 W soldering iron for this task.
Impedance trafo RG-142 is fitted
Impedance transformer RG-142 is fitted.
Assembly and mounting of the right PCB, part 3
The impedance transformer RG-142 is made of brown coax cable. The white 15 pF coax capacitor is visible in the foreground.

The LDMOS source terminals are eventually soldered to the PCB. I used a 30 W soldering iron for this part.


Finished 1 kW pallet
The finished 1 kW pallet for 2 meters.
Finished RF pallet
The pallet is mounted onto a 2 mm alu sheet measuring 150 mm x 175 mm.  

A piece of thin teflon coax connects the RF pallet's input to the N-female ("144 MHz in") on the left side panel.

A short piece of RG400 connects the RF pallet's output to the N-female ("144 MHz out") on the right side panel.


Close-up picture
Securing the heat spread.
Attaching the heat spread
The heat spread is attached to the alu sheet by means of four metal stand-offs.
dig. thermometer
Digital thermometer.
Digital thermometer
The heatspread temperature is measured by a digital thermometer. The temperature range is -55 to +125 oC.

Benefits of a LED thermometer:
  • Display is easy to read from a distance and from different angles
  • Display is easy to read in the dark
  • Powered from 12 V DC (no batteries)
Temperature sensor
Temperature sensor.
Temperature sensor
The temperatur sensor is attached to the copper heat spread. A piece of tinplate holds the sensor firmly against the heat spread.
Hoses connected to the heat spread.
Water tubes connected to the heat spread.
Water tubes connected to the head spread
I've made two holes in the alu sheet of the pallet to allow space for fittings.

The water tubes are connected as shown in the picture. The left tube is connected to the water pump's outlet. The right tube is connected to the radiator.
3 dB attenuator
Input attenuator.
Input attenuator
The 3 dB input attenuator can dissipate 50 W. Input/output impedance of the attenuator is 50 ohm. I bought the attenuator at ebay.com (henryradio). The attenuator is mounted on the side panel and enclosed by an alu-box. The input connector is N-female and the output connector is SMA-female.



I measured some parameters on the finished 1 kW pallet assembly. The low-pass filter and input attenuator were not connected.

RF inputRF outputVccIc GainEfficiency
0.4 W200 W50 V12.0 A27.0 dB33 %
0.7 W400 W50 V16.5 A27.6 dB48 %
0.95 W500 W50 V19.5 A27.2 dB51 %
1.3 W600 W50 V21.5 A26.6 dB56 %
1.6 W700 W50 V24.0 A26.4 dB58 %
2.0 W800 W50 V26.0 A26.0 dB61 %
2.5 W900 W50 V28.0 A25.5 dB64 %
3.5 W1000 W50 V30.0 A24.5 dB67 %

The wattmeter was Bird 43 with a 1000 W 100-250 MHz slug from Coaxial Dynamics (82035).
Wattmeter Bird 43 has 10% tolerance according to the factory..

Low pass filter assembly

Finished Low Pass filter
Low Pass filter for 2 meters. 

Low Pass filter
The Low Pass filter is a kit delivered by W6PQL. It contains a 7 pole Low Pass filter and directional detectors for SWR measurements (not used). As the PCB was not entirely flat, I've added a nylon bolt and mutter to hold it tight against a piece of alu sheet. All components were soldered after the PCB was fitted onto the sheet.

The Low Pass Filter is enclosed in a metal box.
The filter is enclosed in a tinplated box.
Filter shield
The Low Pass filter is enclosed in a RF-tight tinplated box (without lid). The two thick copper-wires are short pieces of Aircom Plus center conductor.

measure swr
Measuring SWR on the Low Pass filter's input.
Adjusting the filter
SWR is measured on the filter's input, and the filter should be adjusted for the lowest value. I used 25 W on 144.150 MHz as test signal, and a Bird 43 wattmeter with slug 25C (100-250 MHz, 25W). The dummy load was able to consume 25 W in the range 0 Hz to 4 GHz.

SWR on the filter's input was measured as 1:1. This is an excellent value, and no adjustment was necessary.

SWR on the filter's output was also measured as 1:1. Perfect value!

LPF cabinet
Low Pass filter housed in alu-cabinet with forced air cooling.
Filter cabinet
In the beginning I wanted to fit the low-pass filter inside the SSPA cabinet. But sufficient space was not available (the water pump occupies a lot of space). So I've decided to house the low-pass filter in a separate cabinet from Conrad Electronics (item 522953). 

W6PQL recommends adding a fan if the low-pass filter is used at power levels higher than 500 W. I've added a small fan as I intend to run 1000 W through the filter. 

The fan is sitting on the rear panel and blows cold air into the cabinet. Warm air is exhausted through 8 holes on the front panel.

Fan data
Sunon, 50 mm x 50 mm x 10 mm, MB50101V2-000U-A99
4300 RPM, 11 CFM, 26 dB(A), 12V DC, 105 mA


left side panel, front view
Left side panel (front view).
Left side panel (front view)
This alu panel measures 150 mm x 150 mm x 1.5 mm.

PTT: Applying +12 V to this connector will switch the SSPA into TX mode. 

144 MHz in: The applied RF power level should not exceed 7 W.

Fuses: Each fuse is rated at 20A T (slow blow). The combined fuse will blow at 40A.

Gnd and +48V terminals: DC input. Power is delivered from an external 48 volt PSU.

Power On/Power Off:
This rotary switch is operated by turning the red key clockwise for "On" and anti-clockwise for "Off".  

Click the photo to enlarge. 

left side-panel, rear view
Left side panel (rear view). 

Left side panel (rear view)

0.0015 ohm Running 50A DC through this resistor will create a 75 mV voltage drop. This voltage creates full scale deflection on the current meter (front panel). .

SW1 This switch can carry up to 100 A.

Attenuator. The label says "-10 dB attenuator", but the true value is now 3 dB 50W.

Click the picture to enlarge it.

Front panel
Front panel.
Front panel (front view)
This panel measures 300 mm x 150 mm. Thickness is 2 mm.

Upper row
Digital thermometer for measuring the heatsink temperature; three control lamps for 12V (green), 48V (green) and PTT (orange).

Lower row
Analog meters for PA voltage (full scale = 100 V DC) and PA current (full scale = 50 A DC). 

front panel (rear view)
Front panel, rear view.
Front panel (rear view)
Click the photo to study the component names.

right side-panel
Right side panel, front view.
Right side panel (front view)
This panel measures 150 mm x 150 mm x 2 mm.

SSPA Out is connected to the LPF.

+12V out
 powers the fan in the LPF cabinet.

The rectangular window displays water level.

Side panel from rear
Right side panel, rear view.
Right side panel (rear view)
The water pump is visible in this picture.

The coax-relay (mounted above the water pump) was replaced by an N-female connector. This connector is the "SSPA out" port.

holder for PSU
This stand will hold the 48V PSU.

PSU stand
The 48 V PSU will be attached to the rear panel using this homemade stand made of alu-sheets.  
PSU edge-connector
PSU wire connection
An edge-connector (included when you buy the PSU) is used for connecting cables for 230 V AC (brown and blue) and for 48 V DC (black and yellow). 
PSU on rear panel
PSU is attached to the rear panel.
Rear panel
The PSU is attached to the rear panel. PSU fans take in air on the left side, and hot air is exhausted on the right. The white cable carries 230 V AC. The black and yellow cable carry 48 V DC.

Water cooling and PSU

water cooling
Water cooling components.

Water cooling components

The picture shows how the water cooling components are connected. The radiator and the fans are on the top of the cabinet. Pump, reservoir, and the heat spreader are inside the cabinet.

Fan data
BitFenix Spectre PRO, 140 mm x 140 mm x 25 mm
1200 RPM, 86.7 CFM, 22.8 dB(A), 12V DC, 0.18A

Radiator: HW Labs Black Ice GTX 280 

Water pump and reservoir: XSPC X20 750 dual bay Reservoir Pump, rev. 2

Heat spreader: Custom made by PE1RKI, Bert Modderman.

Tubing: MasterKleer 15.9/11.1 mm PVC

Fittings: EK - " BSPP (G1/4) - " (12mm) - High Flow

All water cooling components were purchased at Coolerkit.dk

Disassemble cabinet PSU
Removing the upper lid (Eaton APR48-3G).
PSU: Open the cabinet
The Eaton APR48-3G cabinet must be opened before the mod can be done.  

The metal cabinet was assembled at the factory using rivets. They are drilled and removed with a 2.5 mm or 3.0 mm drill.

Special care must be taken with one of the rivets (see photo). If the drill goes too deep, a component near the hole can be damaged.

Cut track and insert Resistor
Cut the track and add the resistor (Eaton APR48-3G).
PSU: Increase output to 50.4 V
PSU output is factory set to 48 V DC. We need 50 V DC for the MRFE6VP61K25H.

A resistor was added to increase the output voltage. Cut the track and solder the resistor as shown in the photo. I tried several resistor values and my final choise was 1200 ohm.

0 ohm47.9 V
500 ohm49.1 V
1000 ohm50.1 V
1200 ohm50.4 V

Adding a resistor in order to increase the output voltage was not my own idea. It was described by Allex in Endless Sphere Technology.  


The building costs are listed in the table below.

Water cooling components 3332500
Copper heat spread1601200
1 kW pallet kit, low-pass filter kit, filter cabinet, attenuator2271700
LDMOS Freescale MRFE6VP61K25H2401800
Cabinet frame and panels, analog meters, 70 A relay, lamps, digital thermometer1741300
Coax cables, connectors 67500
Eaton APR48-3G telecom grade PSU (48V DC, 30A, 1500 W) 400*3000
Total cost1,73513,000

The total cost is equivalent to 1,850 USD.

* The PSU price in 2017 is now less than 1000 DKK on ebay.com.

Written by OZ1BXM Lars Petersen. Latest revision 01-June-2022.