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1 KW 222 MHz LDMOS Amplifier

These first two photos show the amplifier the way I currently build them; the original article showing the construction of the first prototype is below them. If you are building the RF deck from a kit I supplied, the assembly instructions are here:




 

This amplifier has all the same features as the 2m kw amplifier (SWR/temperature protection, sequencer, etc.). After building that one, I was intrigued by the 230 MHz amplifier example in the data sheet for the Freescale MRFE6VP61K25H transistor...so I made a prototype board using the info in the data sheet, and gave it a try.

The results were just as amazing, with the device performing very much as it did on 2 meters; the only real difference was slightly lower gain, about 23 to 24 db instead of 27 db for the 2m version. Still, it only takes about 4w to drive this one to 1kw out on 222 MHz. Efficiency is over 70%; 50v at 28a, or 1400w DC input.

I did have to make some changes to the input and output matching networks to get things to work correctly with the components I was using, but these changes were relatively minor. The baluns were another puzzle to solve; the data sheet called for them to be made from 25 ohm coax, which I didn't have, so to get 25 ohm coax, I put two pieces of RG-402 (.141 semi-rigid) in parallel. The baluns made in this fashion worked just fine.

After building this amplifier, I tried a a couple of different designs for the RF pallet; the latest design is a bit more compact, efficient, and produces about 100w additional output. Details on this alternate design is at the end of this article.


This photo, taken in the early stages of assembly, shows a clear view of the rf deck. It measures only 4 inches by 6 inches, with the PC board and transistor mounted to a 1/2 inch thick copper heat spreader. This in turn is bolted to a 10" x 6" x 3" high aluminum heat sink.

Also visible is the method for mounting the fans, and for mounting the heat sink to the cabinet floor. The fan bracket was made from .050 aluminum sheet, and the heat sink mounting brackets from .060.

The small board near the SMA connector is an input attenuator; my driver delivers 25w, so I had to use about 6db of attenuation to put the input level into the correct range for proper ALC control. This attenuator is out of the rf path in bypass mode.

A low-pass filter unit is bolted to the copper spreader at the top of the picture, and to the heat sink (3 coils and 2 metal mica capacitors), and this transitions into an N connector. Detail is shown in the inset on the right. The metal bracket is .030 tin sheet, and the mica capacitors are soldered directly to it. The N connector is held to it with two 4-40 screws.

Harmonic content in the output is shown to the right, well within FCC regulations; it was sampled at 1kw out, using a directional coupler and attenuators to keep from overloading the input of the spectrum analyzer.


Note the two small boards mounted vertically, and soldered directly to the ground foil of the main board; these are the bias boards, and each has a trimmer adjustment on it for setting IDQ (idling current).
 

From the left side of the completed amp, the two green boards at the bottom of the picture are the main control board and the rf detector board, the latter providing the signals for the swr lockout and the power meters. It detects forward and reverse power levels from the directional coupler mounted to the right of the heat sink.

There are 4 fans behind the rear panel, and these force air through the heat sink fins at the bottom of the cabinet.

The small fan leaning over the output side of the RF pallet provides additional cooling for the output balun and the output matching capacitors, which run quite hot without the fan in place. In fact, one of these metal mica capacitors actually gets to 100C after just 15 seconds at full power, and while heating is not nearly as severe in SSB or CW, it would most certainly be excessive for FM or digital modes, and thus this extra cooling was required.

Looking at the right side now, the small green board at bottom right is a pulse latching relay driver, used to operate the Dow Key high power transfer switch (antenna relay). This particular relay requires pulsed signals to operate it, and this is what that little board does.

The other smaller green board on top of the heat sink and to the right of the copper spreader is a high current FET switch, which turns on the 50v to the amplifier deck when the sequencer on the control board tells it to. As in the 2m amplifier, the ammeter and all input connections are bypassed and filtered with ferrite beads to minimize rf leakage from the cabinet.

Removing two screws from the bottom of the front panel allows it to be folded away; by unplugging the two connectors shown, one can remove the panel completely, making for easy access to the rest of the amplifier components should they ever require servicing. This also shows clear views of the directional coupler, transfer switch and other parts.

With the panel folded down like this, the method of fastening the LED bar graph meters (center of the panel) is exposed. Two rails run the height of the panel, are held inside it with counter-sunk 4-40 screws, and provide a mounting platform for the two horizontal rails that actually secure the boards. This mounting method allows the displays to be positioned flush inside the cutouts in the panel face without having to drill any mounting holes into the face of the panel itself.
This is the bill of materials for the RF deck, taken from page 4 of the data sheet. The following are the component changes I made:

  1. C1 is a 9-35 pf ceramic trimmer capacitor
  2. C2 and C3 are 33pf 0805 ceramic chip capacitors
  3. C5 is not used
  4. C4 is a 1-4 pf ceramic trimmer capacitor
  5. C14 is a 45 pf J601 metal mica capacitor
  6. C15 is a 47 pf J101 metal mica capacitor
  7. C16 and C17 are a single 500 pf  J101 metal mica capacitor
  8. C18 and C19 are a single 500 pf  J101 metal mica capacitor
  9. C20 is a 7 pf J601 metal mica capacitor
  10. C22,23 and 24 are combined into a single 220uf 63v electrolytic
  11. C26,27 and 28 are combined into a single 220uf 63v electrolytic
  12. Coax 1,2,3 and 4 are each two pieces of RG402 (.141) semi-rigid 50 ohm coax connected in parallel


And last, here's the block diagram for the complete amplifier package:


Alternate design for the RF pallet:

This design is on the same spreader size (3 x 5) as the 2m pallet, and is basically just a scaled version. A temperature compensated adjustable bias circuit is is also used..

I was able to get 1200w + out of this one with about 5.5w drive, and at that point I was limited by the capability of my power supply, but it appeared as if it could produce even more.

For those of you wanting to build this one, a kit containing the more difficult to find parts is available on the parts page.