45W IRF510 HF Linear Amp

Boy it’s been a long time between posts. Sorry about that. I got real lazy and just starting pumping my content into twitter and really just never took the time to document things.

Anyway, here is a project I built to get me out of QRP land and being heard my frequently on the 20M and 40M bands.


Finally getting around to sitting for my HAM license I started off with my first HF radio, the very useful uBitX v6. It’s a great radio to start with for a tinker like me. It’s open sourced schematic and firmware allows fiddling and updating and there is always a wealth of ideas and projects on the internet for it.

With it’s mighty 5W output and using my home-brew EFHW antenna I would make countless “CQ” calls hoping someone would hear me. I went on for weeks if not months tweaking this, and tuning that but nobody seemed to ever hear me. So I set about creating my first linear amplifier. Inspired by a design in “Experimental Methods in RF Design” EMRFD I set about building myself a linear amplifier to get above the noise.

“Oh the things you learn”

So, I at first figured I’d bash together a prototype on some copper clad and see how it would go…

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I got some output! 4dBm (2.5mW) input netted me 31dBm (1.2W) output from 12V!

Suffice to say I was stoked…. then it started oscillating.

Speaking with my Elmer he suggested my crappy hacked together layout wasn’t conducive to a high power amplifier with fairly high gain, and that perhaps I should do a cleaner layout. So I cracked out my CNC mill and Kicad and made a prototype.

“There ain’t no thang like a CNC”

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Layout done, board turned out nice. Time to build it.

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Using a large chunk of heat-sink I cut down from an old stage lighting dimmer, I mounted the MOSFETs with insulators onto the heat-sink. The PCB was then mounted on top and the board mounted on stand-offs.

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It was about this time I realised I screwed up the schematic and had the gate and drain on one MOSFET arse about. Dutifully I hacked in a modification and continued.

All the components and coils were wound, and the moment of truth

Power up time!

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Powered it up, connected it to my uBitX and whistled into the mic and received 46dBm (39.8W) of output from 24dBm (250mW) of input! I was very very pleased. No oscillations, nice clear output and all looked very good and pleasing.

Now let’s make a proper version this time

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A new layout was made, a new board was milled on the CNC and the components transferred across to the new board

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Worked perfectly. Pushing the input power up to 30V @ 3A the amp will easily get to 50W into 50 ohms, but I limit the supply to 28V and that seems to keep it around 45W output.

Time for an TX/RX switcher

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I needed a way to interface the linear with the uBitX. I pulled 12V from the TX/RX relay inside the uBitX and brought it outside to control a relay block I made for the linear. I really should just create a TX sensor on the input of the linear and have it handle the switching itself (future project) but for now this works.

Put it in a box!

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An old ATX PSU steel case was used to house the amplifier. It was handy because it had a 80mm fan already in the box and it was steel, so provided good shielding. Not much to look at though.

So, how’s it going?

After using it on air for a while and having finally breaking through the noise I have had many successful QSOs from all over Australia, NZ and the USA.

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Suffice to say I’m really very very happy with it.