Building HF Power Amplifier - Part 1

I decided I want to try building HF power amplifier myself. It seems like an interesting project where I will have to learn a lot of new things. It also looks like spending around $1000 will get me 1000 watt LDMOS amplifier. Which is cheaper than buying used one which costs $2000+. I am not even looking into buying a new one, because they are way too expensive.

Power Amplifier Board

Since I am not a hardware engineer, I decided I won't be building the amplifier board from scratch but will buy one that is already assembled. There are various options available out there.

1. Chinese boards from eBay. I did read a lot about them and watched videos on YouTube. It is hard to make a conclusion. Some say they are okay quality, some say that the ferrites are overheating and there are various quality issues. I am not that advanced to make a decision on this.
2. vk-amps.com is a good option, but it is on the expensive side.
3. w6pql.com is a well known resource, but is also expensive.
4. dxworld-e.com is in Greece and is well known. I have seen various people use their boards.

At some point during my research, I found out AK6AB built an amplifier. He kindly shared his PDF with lots of well organized information. He did test the Chinese board, and eventually ended up getting rid of it and bought board from DXWorld-e.

DXWorld-e seems like a good option because everything can be bought from them - amplifier board, delay board, LPF board, protection board, etc. After some reading and research I decided to go with DXWorld-e. So I placed an order.

As you can see, I got nine boards from DXWorld-e:

1. LDMOS 1.2KW HF/6m LINEAR AMPLIFIER BOARD 1.8-55MHz V3.0
2. HF/6M LPF FILTERS 160-6m 1800W 5 POLE
3. PROTECTION BOARD 1500W
4. RX/TX & ANTENNA SWITCH 2000W
5. SWR PWR TANDEM MATCH 1.5-54MHz 2500W
6. LCD PWR SWR METER 1200W
7. RF DELAY BOARD
8. LED PANEL PCB BOARD
9. STEP DOWN VOLTAGE REGULATOR 12V / 2A

It cost me $712!

Transistor

Since I was done with board selection, I started looking into transistors. Transistor price on legit US websites like Mouser and DigiKey is around $300. Since my research showed that the chance of blowing up my first transistor is pretty high, I decided to go with Chinese transistor from eBay. I got MRFX1K80H for $75.43.

Power Supply

This was a straightforward choice. Many ham radio enthusiasts in Europe use Eltek Flatpack2 power supplies. They are known to work well and produce no noise that interferes with RF. They are also relatively simple to be programmed through CAN control for specific voltage. So I got one on eBay. I gathered all of the materials I used in materials and tools post.

CAN Communication

For CAN communication, I bought Arduino Nano and MCP2515 CAN bus module.

But there was an issue - I needed some kind of board that I can plug in the back of the power supply. At first I crafted following ugly board myself.

Here is the Arduino code:

#include <SPI.h>
#include "mcp_can.h"
#include <mcp_can_dfs.h>

// https://www.thebackshed.com/forum/ViewTopic_mobile.php?TID=12035&P=4

// This is for logging into your flatpack. Must use your serial number.
// My power supply serial is 124371158440.
unsigned char login[8] = { 0x2, 0x43, 0x71, 0x15, 0x84, 0x40, 0x00, 0x00 };

const int SPI_CS_PIN = 10;
MCP_CAN CAN(SPI_CS_PIN);  // Set CS pin

// 48V:
// unsigned char setdefaultvolt[5] = { 0x29, 0x15, 0x00, 0xC0, 0x12 };
// 50V:
unsigned char setdefaultvolt[5] = { 0x29, 0x15, 0x00, 0x88, 0x13 };
// 53V:
// unsigned char setdefaultvolt[5] = { 0x29, 0x15, 0x00, 0xB4, 0x14 };

void setup() {
  Serial.begin(115200);
  pinMode(3, INPUT);
  pinMode(12, INPUT);
START_INIT:
  if (CAN_OK == CAN.begin(MCP_ANY, CAN_125KBPS, MCP_8MHZ)) {
    Serial.println("CAN BUS Shield init ok!");
  } else {
    Serial.println("CAN BUS Shield init fail");
    Serial.println("Init CAN BUS Shield again");
    delay(1000);
    goto START_INIT;
  }

  CAN.setMode(0);
  delay(200);
  CAN.sendMsgBuf(0x05004804, 1, 8, login);  // id = 1 so XX = 04
  delay(200);
  CAN.sendMsgBuf(0x05019C04, 1, 5, setdefaultvolt);
}

void loop() {}

For me, very important part was to connect power supply CAN bus ground, to the Arduino GND. That made it all work. I was able to program the voltage into the power supply.

Power Supply Board

I wanted the board to look nice. Because this board was small, I decided to try it out and build it. I built it with easyeda.com and ordered it at jlcpcb. They had some kind of promotion going on, so with shipping, it cost me $11.33. When the board is made at jlcpcb, they do send you five boards by default. Components for the board were ordered at lcsc.com and they were $16.91. It was kind of an expensive, but it was interesting to go through the process and get my first PCB worked out. After soldering the components, the board looks like this:

You can download the Gerber files for the board here: Gerber_Flatpack2-Power-Board.zip

Heatsink

ChatGPT was very useful to calculate the size of heatsink that will be enough. It took me a while to find something. They are pretty expensive. I checked eBay, AliExpress, Facebook Marketplace. And after a lot of searching, I ended up buying 14" x 9" x 3" aluminum heatsink for $80.81. Which I think is a very good deal for 18 lbs heatsink.

Copper Heat Spreader

Copper is an expensive metal. At first, when I started looking for a plate that will fit the power amplifier board and will act as a heat spreader, it was looking like a $100 purchase. I was again monitoring eBay, AliExpress, Facebook Marketplace, and eventually found a very good size plate on eBay for only $40.25. It was 1/2" × 8" × 6" and it was a little bit rough. But since it was relatively cheap, I was happy with it. My idea was to use wet sand paper and some flat surface to polish it. Now, when it arrived, I reviewed it and my initial idea was to polish it on a piece of tile from Home Depot. I went and I checked tiles at Home Depot and Floor and Decor, but I couldn't find any tile that is perfectly flat. My other idea was to use a piece of glass, on which I would've put sand paper and wet sand the copper. While looking for glass, I talked to a relative, and he took copper to some shop and face milled it. Now, the bottom of the copper plate is very nice and flat. The top does have a notch from milling. I will have to position the transistor in a way where it won't lay on top of that notch.

Enclosure

Initially I bought "Rosewill RSV-R4100U 4U 3 Fans Rackmount Server Case" on eBay. But it is way too flimsy. The heatsink weights 18 lbs, there is no way this case can hold it. So, I gave up on this idea and started searching more.

On Amazon, I found steel electrical boxes, like this one. But they are very heavy - 25-30 lbs.

That brought me to looking into aluminum angles and how people in Europe use that to build cases. This seemed like and expensive idea because I initially checked prices on aluminum 1 inch angle at Home Depot. But then I checked Facebook Marketplace, and I ended up buying 21 ft of 1.5 by 1.5 inch 1/8 thick aluminum angle. That was enough to build the enclosure of the size that I need.

Talking about the size. Because my heatsink is huge, and I was planning to use 140 mm fans, I ended up with size looking like 16 by 19 by 7 inches. I understand there is a way to make it all fit tight and make it smaller, but I thought for such project, I am okay if that turns out somewhat big. I have enough space on or under my table in the shack - so that should be fine.

To cut the angles, I got blade for miter saw. You can check details at materials and tools post.

Once it was cut, to assemble it, I 3D printed guide that helped me make it square and drill holes in the same place for all the corners. Here are some links to the 3D files:
Aluminum Angle Corner Union Guide OnShape Link
Aluminum Angle Corner Union Guide STL Download

Here are some pictures.

For enclosure walls, I was looking to get 1/8 thick aluminum sheet like this one. But when I was picking up aluminum angle, the guy sold me a big 3 ft by 4 ft sheet for $30.00.

For screws, I went with M4 size. So I got this, this and this. Update: I later also purchased star washers.

Last thing I did for enclosure so far is printed the rubber feet.
Rubber Feet OnShape Link
Rubber Feet STL Download Link

Fans

For fans, I figured the best way is to blow the air along the heatsink fins. I had an idea of putting bigger fans, but make them run slower and hence, quieter. From what I figured, here were my options:

1. Noctua NF-F12 iPPC 3000 PWM - the best option.
2. Noctua NF-F12 PWM - the good option.
3. Noctua NF-P12 redux-1700 PWM - the okay option.
4. ARCTIC P14 Pro - the okay option, and it is also cheap. This is what I went with.