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The XRF-286 is a wonderful part for use on 1296, and one of the few choices available for developing significant solid-state power on that band. It is an unmatched 26v LDMOS transistor that was once made by Motorola, and can still be found in surplus PCS amplifiers made by Spectrian. A copy of the data sheet can be found on the internet in several places.
The Spectrian amplifier output boards contain three of these devices, one 286 driving the other two at 2.4 GHz. There are three of these output boards in those amplifiers, all combined for a total of 40w out. I suppose you could say they were barely used, considering a single 286 is rated at 60w out at 2 GHz.
Unmatched parts like these are the most useful, mainly because the matched ones are set up for different frequency ranges outside of the amateur bands. You can’t really get inside the device package to change the matching network, and trying to re-compensate externally usually results in high losses and inefficiency if it works at all.
On 23cm, an amplifier using just one ‘286’ can be driven into saturation with as little as 3 or 4 watts, and a pair with less than 10, making that last configuration a close match for many of the 10w radios in use today. With 8 to 10 watts drive, a pair of them will easily deliver 150 watts.
On the web, one can also find amplifier designs that appear to have been originally developed by F1ANH, F6DRO and others, implemented on FR4. The basic design is very good, though I did make some changes to the bias circuit and the input and output matching transformers. Our French brothers deserve a lot of credit, though, for pioneering the original design.
Although FR4 is inexpensive and easy to work with, it can be problematic; losses create a practical limit to its power-handling capability on 23cm, and it tends to be variable in specs from one lot to the next; thus the decision to port it over to a more reliable material. I tried two low-loss substrates: Rogers 3006 for the prototypes, and Rogers 4003c for the later versions. Both of those materials produced consistent results.
When prototyping this design, I discovered
that the input matching line was a bit short for 23cm, probably due to the
variability of the FR4 used in the original design. The line needs to be a
bit longer, but I left it alone in favor of the small high-Q ceramic trimmer
capacitor you can see shunting the input stub. For the microwave purist, I
realize this is heresy; however, I like to be pragmatic, and left it that
way, appreciating the ability to optimize input match without having to
strap in snowflake tuning stubs to do the same. This input stub is 4.32
ohms, and 52 degrees long as shown.
Here's the performance of this amplifier.
The hybrid couplers offer isolation and protection for the individual amplifier units; if one should fail, the other is protected by the 150w termination at the dump port of the output coupler.
If you look at the input matching stub on each device, you'll see those rectangular "snowflake" trimmer stubs, there to fine-tune the input match. Again, I chose instead to use a small high-Q trimmer capacitor instead of the stubs. It's up to the individual builder, but I find it easier to adjust the trimmer than to fuss with the stubs; one can do it either way.
The board is
mounted onto the heat spreader shelves using the 4-40 screws shown, and spaced
above the heat sink using 1/4" aluminum bar stock spacers. The 100w termination
also requires a 3/16" spacer for proper positioning.
Removing them from the Spectrian board requires the use of a hot plate and hot air gun; the board is heated from the bottom; then the air gun is used to re-flow the solder around the part. At that point, the part can be picked up with tweezers and deposited elsewhere to cool gradually.
The process of soldering the device to the new spreader requires even more courage, and is best accomplished in an oxygen-shielded environment with special equipment, though I hear from some of the boys in the EU that they have been successful without that precaution. A friend of mine in the semiconductor business handled that latter part for me; after that was done, I was able to treat them like a more conventional transistor, and just bolt them down to the heat sink.
The photo at the right shows one of these parts in the form I like to use them; secured to a thick copper heat spreader. The spreader is 1.75” x 1.75”, and ¼” thick. There is a slot milled into the spreader to space the part at the correct height to accept the PC board, which slides under the lead (s) and onto the shelf of the spreader as shown in the previous photos.
These thick spreaders do a great job of drawing heat away from the devices and passing it into the heat sink below. Even at full sustained output, I can detect no drop in power level, which is a common problem when the heat spreaders are made from thinner material.
amplifier boards can be combined to make some
EME-capable amplifiers. I built
two of them so far (600w out); the first requires about 40w for full output. The
second has a driver stage and needs only 2w drive.
Pictures of other amplifiers built with the XRF kits can be seen courtesy of
Just one final note on these amplifiers:
Corrective adjustments will be necessary if poor construction techniques are used. Leaving too much room between the board and the transistor body (it has to be right up against the ceramic), substituting components (I've seen plastic capacitors used for the input trimmers, for example), leaving wires from coax jumpers too long, and not flow-soldering the devices to the spreaders will always introduce variables. Some of these variables can be overcome with tuning tabs at various spots on the RF traces, but all of them are just fixes for construction errors, so follow the recipe as closely as possible and you'll have no troubles. I built several hundred of these, and found them to perform very consistently; a total of two of them wouldn't make power, and in both cases it was sloppy construction on my part causing the problem (gaps between the boards and transistor bodies).
Schematic for the 2-device amplifier