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1KW 70cm LDMOS Amplifier

 

This one is similar in size and function to the 2m and 222MHz KW amplifiers.

There are some small differences in appearance; the front panel was machined and engraved for me by Front Panel Express, and though it doesn't quite match (in color) the other kw amps in the station, it's quite close.

I also decided to add a voltmeter on this one just for fun.

 


The RF deck combines two of the Freescale MRFE6VP5600H devices used in the 500w LDMOS amplifier, onto a single board. It uses a small commercially-made coupler on the input, and for the high-power side, the output coupler is printed right onto the board. The material is microwave substrate with 2oz copper both sides, necessary to handle the higher RF currents this amplifier produces.

The bias circuit is regulated and temperature compensated.

A high-resolution photo is here

The heat spreader used in this deck is 5.5 x 4, and .500 thick C110 copper.  Mill the slot along the 5.5 dimension in the middle of the 4 inch width; align the template slot with the spreader slot and mark the hole locations for drilling. Here are two drilling  templates (top and bottom) for this spreader.

.5 x 5.5 aluminum board support spacers (3), and a .5 x 1 spacer (under the hole in the middle of the coupler) can then be aligned under the heat sink drilling template and marked for drilling (.156 pass-through holes).  The 250w termination requires a .375 inch thick aluminum or copper spacer to put it at the correct height.

The copper spreader is mounted to the heat sink using the three #8 screws shown in the slot of the spreader, and by 4 additional #6 screws passing through the heat sink on the other side, and into the bottom of the spreader. A little more detail on this mounting method is here.

Here is a video showing how to do flow-solder your LDMOS to the spreader.

If you are building this RF deck from a kit I supplied, here are the assembly instructions.


Looking inside the cabinet, the RF deck is mounted to the chassis floor with .050 aluminum brackets.

This rf deck is quite a bit larger compared with the VHF amplifiers, so I had a bit of a challenge figuring out how to fit everything into my standard sized cabinet, but I managed to get it all in there.

The other devices requiring heat sinking are mounted to the rest of heat sink surface that was available; this includes the input attenuator, FET switch boards and the voltage reducer used to supply 12v to the control board and LNA power feed.

There are four fans mounted to the back of the heat sink; these draw in cooler air from the rear vent, forcing it through the heat sink fins and exiting through the vent in the top cover of the cabinet. One additional fan is mounted topside center, and this one ensures the output components remain cool. This isn't really necessary for ssb/cw work, but it's helpful on the digital modes (WSJT) where the duty cycles are much higher.

Note the use of conformable .250 coax for the high-power connections, necessary at this power level and frequency.

On the back panel, next to the power connector, is the inrush current limiter circuit used to protect the AC power switch on the front panel. On the far left side is the LNA power connector and fuse.

On the far left side of the rear panel, in order from left, is the input connector, ALC detector PCB (with through-panel adjustment), the ALC and PTT connectors and the attenuator jumper quad.

This last set of connectors is used to configure the amplifier input power level. I can drive it directly with 10w, but since I normally use an IC7000 (30w) I jumper in a 3db attenuator and allow the alc to reduce the 15w remaining to 10w. The attenuator is switched out on receive.

Here's a peek behind the front panel, showing the mounting methods and locations of various components; beginning from the left:

reverse-polarity protection diode

above that, front panel connectors

relay pulse board is below those

antenna transfer switch

cooling fan

AC switch inrush current limiting resistor

Next photo shows the low pass filter/dual directional detector assembly just prior to mounting to the right side of the heat sink.

This 3-in-1 assembly is a space-saver, and cuts down on a bit of wiring and insertion loss.

The capacitors in the filter are printed right onto the board substrate, and the inductors are made by winding #14 magnet wire to the appropriate size. This filter reduces the second harmonic by more than 40db, and the third by more than 50.

The dual directional detector provides the signals for the SWR lockout, and also drives the LED forward and reverse power meter bar graph displays.

This 3-in-1 assembly is held to the chassis floor with #6 hardware, and to the side of the heat sink at the top. The connection to the output port of the rf deck is done with a small tin angle bracket soldered to the coax, and then secured to the output board spacer with #4 screws. The center conductor is soldered directly to the output port once the coax is firmly held in place.

The other side has an N connector attached, and this screws directly into the antenna transfer switch.

Here's a look at the assembly after installation. Note the last fan at the back of the heat sink is positioned so some air is forced across the filter coils (they run warm during high duty cycle modes).

Here's a look at the front opening, showing the side of the fan peeking through from the back, and how the air is directed through the openings.

This photo of the left side of the amplifier shows the locations of the following (from the left):

the 3db input attenuator

below that, the LNA fet power switch

to the right of that, the control board

above that, the VDD fet power switch

This is the back side of the front panel, and shows the mounting method for the LED bar graph displays, switches, and the shielding on the panel meters.

The panel is easily removed for servicing the amplifier by uplugging the Molex-type disconnects shown.
Here's the rear panel layout

Finally, all done and fitted into it's rightful place in the station