23cm 150W affordable Power Amplifier

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23cm 150+ W affordable Power Amplifer

When I needed a compact and small "driver PA" to power my large 23CM PA for the upcoming 4.8-meter dish for EME activities, I searched for a suitable solution.


The ICOM IC-9700 I use for EME operations can only output 10W on 23cm.Given that my 23CM PA requires significantly more power, and considering the 3 dB loss from the radio to the PA module mounted on the dish, the output power would end up being only around 5W at the input of the PA.


By introducing this small driver PA, the power could be boosted to over 150W if needed, well above the requirement for full output on the large PA, which only needs a maximum of 30W.


Various possibilities and modules are available, and after discussions with Mark 9H1BN, he shared information about an affordable module utilizing a surplus LDMOS device (MRFE6S9160) that he uses.

(This PA can of course also be used as a "final" PA for 150W+ on 23cm!)


This version of the PA design was crafted by Yuan, BG3MDO, and Feng, BG0AUB, drawing inspiration from the original work done by Henning, DF9IC.


Design by BG3MDO/BG0AUB.

Original design by DF9IC.

Another design using same LDMOS from IK3GHY.

500W PA with four LDMOS, design by F5JWF, and his tips and tricks


Datasheet for the LDMOS MRFE6S9160.


I ordered LDMOS devices from EBay from this seller.


The total cost depends on what you have in "your drawer", if you want to make your own copper heat spreader etc. In case you buy everything from new, the list will look something like this, depending on where you source components from:


  1. LDMOS, US$ 9,-
  2. Copper heat spreader from JLCPCB, US$ 24,-
  3. PCB from JLCPCB (at 30 pcs), US$ 1,- (a little more expensive at 5 pcs)
  4. Components from Mouser, €15,-
  5. Capacitors and SMA connectors from RF Microwave, €36,-
  6. A suitable heatsink
  7. A suitable power supply, capable of 28V/15Amp
  8. Various hardware, box, wire etc.


Add to that shipping (Mouser is free shipping above a certain amount), if you do a group buy, most of the items drop somewhat in price.


Design files:

  1. STEP file for copper heat spreader, STP file
  2. Info for threaded holes in copper heat spreader, PNG file
  3. Drilling template for heatsink, can be 3D printed, STL file
  4. Gerber files, package that can be uploaded to JLCPCB directly (select 0.8mm FR4, ENIG surface), ZIP file




Design

The design is pretty straightforward. The only thing missing is temperature dependent bias. The design does not originally have this. Testing will show how much this is needed in my case, I plan only on running the PA at low power, so it might not be an issue. More on this when testing is done.


As Yuan/Feng has published the Gerber files for the 0.8mm thick FR4 PCB, I ordered a small batch of the PCBs from JLCPCB.


I designed a copper heat spreader for the board and had that one produced as well. I added two threaded holes at each end of the copper spreader so a SMA female connector could be mounted at each end.

I also designed a drilling template (orange 3D printed part on picture below) that makes it easy to mark/drill the holes needed to attach the copper heat spreader on the heatsink (file for this available above).

Below are some pictures of the parts, more will be added once I receive some LDMOS devices.


Schematic/BOM

The schematic and BOM information are both shown on the Github page of BG3MDO Yuan and BG0AUD Feng. I have included both below just for refence.


It is a good idea to mount a small Zener diode across C5, I use a 3.6V Zener diode (normal range for the bias voltage is around 3.0 to 3.3V for 1.3 Amp idle current @ 28V according to datasheet.


I ordered parts for the PA module from Mouser and from RF Microwave. 

The BOM list for mouser can be downloaded from here (approximately €15,-)


Besides the components from Mouser, I ordered the two SMA connectors and the ATC capacitors from RF Microwave:


From RF Microwave (approximately €36,-):

1 pcs 100B-100P-J500

1 pcs 100B-2P2-B500

6 pcs 100B-3P3-B500

2 pcs SMA-73-04 (remove the PTFE sleeve and shorten the center pin)



You will also need two pieces of enameled copper wire, 1.5mm in diameter for the two inductors that supplies the drain with power.

Assembly

Assembly of the boards are very easy. There is not much to it, and everything is excellent described on the Github page of BG3MDO Yuan and BG0AUB Feng.


The PCB board is delivered as one single PCB, you need to split it in two. I used a sharp knife from both sides of the board, did a few cuts with the knife and could break the board in two. Do a slight sanding of the edges.


The two SMA female connectors are fastened using two M2.5 screws (4 to 5 mm long) at each connector.

Soldering of the LDMOS followed the same method as I used for the large PA modules, I shot a video of the process.

Bringing up the board

On the Github page of BG3MDO Yuan and BG0AUB Feng there is a description of how to bring up the boards the first time.


DC check and idle current:

  1. Make sure the bias trimmer is at minimum, you need the lowest possible voltage on the gate to start with.
  2. Mount a suitable 50 Ohm dummyload at the output connector.
  3. Terminate the input in 50 Ohm.
  4. Connect a current limited power supply (28V at max 3 Amp) to the board.
  5. Connect +12V (approximately) to the input of V1 (the 78M05 regulator) to turn on bias to the LDMOS.
  6. Adjust the bias trimmer (slowly and careful) so that you see around 1.3A quiescent current (Idq) flowing into the drain.
  7. Remove the +12V bias voltage from V1.


Adjustment of input match:

  1. Re-apply the +12V to the bias input (V1). Confirm that the idle current is around 1.3A.
  2. Using the 3 pF trimmer on the input, adjust to best possible return loss (SWR) on the input. You should be able to achieve a SWR better than 1:1.5.
  3. Remove the +12V bias supply again (V1).


Testing with RF:

  1. Adjust the current limit of the 28V power supply to 10A (if possible).
  2. Apply +12V bias again (check that idle current is around 1.3A).
  3. Apply very low RF to the input, observe the drain current and the RF power output (watt meter).
  4. Slowly increase the RF input, at around 2W input, current consumption should be around 8 to 9A and power out should be 60W or more.
  5. Remove RF and +12V bias.


You can try and move the two groups of 3xATC caps (3.3 pF) 1 mm left and right to optimize the output power (move one group at a time). Retest several times while optimizing the position of the 2x3 capacitors.


Once you go above around 90W output, you will need to increase the current capability of the 28V power supply from the 10A. At full saturated power (around 170/180W), current consumption will be around 14A (or more).

Gain will be around 15 dB.

More info to come...