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The SK0CT Rover

Modification of a 100W 2,1GHz PA for 2,3GHz

Typical data on 2,3GHz after modification :
Gain: 9-10dB, Output power 100-120W Active device is Motorola MRF 21120
VDD 28V at 10A, bias +12V.

MRF21120 is a Gemini enhancement LDMOS transistor. It has a power gain of typically 10 dB at 2,1GHz and about the same performance is possible on 2,3GHz, but some adjustment is needed to squeeze the last couple of tenth of dB out of the transistor. This PA was originally made for 2,1GHz. It is a cut-out from its original board.   The VDD is  +28V and the bias input is +12V.   For full performance, a good size heat sink must be used. If the temperature on the transistor wafer is allowed to rise above about +100 degrees Celsius, the LDMOS will reduce gain up to several dB. If the heat sink is too warm, a fan must be used to cool the heat sink.  Make sure that you have terminated both input and output into 50 Ohm Then adjust each half of the Gemini to 0,8A idle current with no RF drive applied.  This assures linear operation of  the PA.

It is very important that you provide a perfect ground at your input and output terminals of the PCB. In this example, a small piece of copper sheet is soldered on the heat sink side of the PCB under the input and output terminals. The sheet is protruding outside the PCB, thus providing a ground pad for the connecting coax cable. Please note the nonexistent length of centre conductor from the coax at the soldering point of the PCB.
In the original, the input match is not at optimum on 2,3GHz. This can easily be adjusted by increasing the pad size close to the input on the PCB. The pad showed here may not necessary be the same size on your board as it is an unique adjustment for each PA. About one dB was found, bringing the gain on 2,3GHz from 8,5  to 9,5 dB on this sample PA.
To provide a PTT line, a relay or similar can remove the +12V bias. The idle current will then go to zero without need to remove the VDD when PTT is in RX.

When you connect VDD to the PCB, take care not to short  it to ground as one end of the trace is feed to the heat sink side of the PCB. In the PA used here, I have cut of the copper trace close to the decoupling capacitor and soldered the VDD cable close to the capacitor (the orange cable).


Make sure that the transistor has a slot 1,3mm deep to allow the PCB to be absolutely flush with the heat sink. All provided screws holes must be used if you want the available power into your output coax. All microwave design rules apply!! A note of caution, to not drill out the through plated holes in the PCB, to make room for bigger screws than the original size of 2,5mm.


 For a 400W PA see the OH3LWP solution http://oh3tr.ele.tut.fi/~ftp/pa/13cm_400W_LDMOS_PA/