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Build log: 60W 2m linear amplifier

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Some may recall me asking about heatsinks a little while ago.


The one I ordered turned up on Wednesday, after a mishap caused a minor delay. As people asked me to post pictures when I had everything built, I thought I'd post a bit of a build log. Perhaps it might inspire some to take a radio license and join in the fun.


The following is a duplicate of http://stuartl.longlandclan.yi.org/blog/20...-2mlinear-day0/




A few months back, I grabbed the trusty FT-290R II ready to do my weekly run from The Gap to Tarragindi. Quick test to check everything’s okay… the power meter swings to full scale, but strange, I’m not hitting any repeaters.


Okay, grabbed the FT-897D instead, and I just did my weekly radio duties with that instead. When I came home that evening, I had a closer look. The FT-290R II was emitting a signal, the hand-held was picking that up. It was also receiving just fine. On a hunch I took off the FL-2025 linear, and hooked the antenna up directly to the radio. Bingo… the radio works, the linear does not.


So, the linear had died, and thus I was in need of a new one. Hand helds really don’t have much punch for mobile use, in fact, the FT-290 has been brilliant on the bike. Not menu driven, so it’s real easy to drive while riding, simple, no frills, and sufficient grunt to get out of a bad area. It also does SSB (and CW, but I’ll leave that to LY2KW).


I could buy a new set, in fact, I may get a FT-857D, as the 897D is a heavy lump of a radio to lug around, and there are times when HF capability is useful. It is less than ideal on the bike however due to its size and weight. There was nothing wrong with the FT-290, just its linear was dead, thus I was limited to its barefoot transmit power of 2.5W, even less than most handhelds.


So, I decided I’d try my hand at a semi-homebrew linear amplifier.


The concept


I wanted an amplifier that could achieve at least 25W of transmit power using SSB. As I’d likely use it for things like WIA broadcasts, I wanted one that would also handle transmitting for a long period of time.


Designing a full blown amplifier on 2m is a bit beyond me with my limited homebrew experience. It is also an issue sourcing the PCB material needed for VHF projects. A lot out there call for FR4 grade fibreglass PCBs. I have no idea what Jaycar sell. So this was going to be a potential minefield. Thus, I opted for a kit.


Minikits sell one based on the Mitsubishi RA30H1317M. The same kit, can also take the 60W module, which sounded good to me. Most of the time I’d be running it at 30W, but having 60W capability sounded good. I purchased this, along with the 30W module as well just in case.


I also thought a pre-amp would be nice. The same supplier sells this preamp kit. The kit also offers RF sensing, which would allow the amplifier to auto-detect the radio transmitting, and switch into transmit mode automatically. This also allows for filtering, to prevent reception of pagers (not fun copping an earful of one of those when you’re wearing a helmet-embedded headset riding a bicycle).




Minikits recommends using a Pentium 4 heatsink for 30W modules, however it wasn’t clear if this would be sufficient for 60W modules.


I wanted the amplifier module to stay below 100?C while operating with ambient temperatures at 40?C. Pretty sure I don’t want to operate a radio under such conditions, so it should work fine in all conditions that I’m likely to encounter.


The amplifier module is about 45% efficient, thus about 135W is dissipated when operating at full power. By my calculations, I was looking for cooling that can provide 0.22?C/W.


A quick search revealed that I could get one via Conrad which in the open air achieves 0.84?C/W. Combined with a fan, it can achieve 0.24⁰C/W. Jaycar sell this fan, which is quite capable. In fact, two of them will fit across the back of the heatsink, so with dual fans, I should be well and truly within limits.


I placed the order for the heatsink a fortnight ago. Due to a mix-up, I didn’t get it until Wednesday, but that’s fine, I wasn’t in any hurry. With the heatsink now in my possession, I today headded to Jaycar to pick up some of the bits and pieces I’d need for this project, starting with the enclosure.


One thing I did neglect to procure today, were the fans… but no biggie, I’ll get those later.


Prior work


Well, technically day one was some time ago. I had already mostly built the amplifier kit, and the preamp. The preamp got built way back when I first obtained the kits.


The power amp was built later, however the instructions suggested that I wait until I have the amplifier module mounted on the heatsink before I go soldering it to the PCB.


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Day 1


Having got the heatsink, enclosure and tools, I set to work. Initially I positioned, drilled and tapped the two M3 holes for mounting the amplifier module. I haven’t tried putting the amplifier in place yet, but it looks like the holes are positioned pretty well.


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My plan, is to bend the pins on the module at 90⁰ and mount the PCB horizontally. Both module and PCB would be passed through the side wall of the enclosure, with the heatsink outside. I originally wanted the heatsink inside the case (with vent holes), but of course, Jaycar are not good at providing internal dimensions, and I soon discovered it’d be awkward to fit.


It took a bit of experimentation to cut the hole in the side. No, I won’t be winning any prizes for my metal work, in fact, it never was one of my best subjects.


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Next steps:


My immediate next step will be to mount the amplifier module, solder it to the PCB, and mount the PCB inside the case. Then I mount the heatsink and fans to the case.


I have a controller that I have designed at digital logic level, however I’ll need to do some further design work to make sure it’ll do what I intend, before procuring the parts and building it.

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Yay! Thanks for writing it up, quite a fun read. Can't wait to see how it turns out. Nice find on that heatsink supplier, too! I'll keep them in mind if ever I need something more specific :)

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The following is a duplicate of http://stuartl.longlandclan.yi.org/blog/20...-2mlinear-day1/


Well, today I did some more work on the 2m linear. Earlier this week I ordered some SMA connectors and some 1N5711 diodes for the project.


Two 1N5711 diodes will be used to make a voltage peak detector, to detect when the amplifier is subjected to power above 60mW. The SMA connectors will be the interconnects between the modules. This afternoon's effort was spent soldering the SMA connectors onto two of the boards, and mounting the 2m amplifier module onto the heatsink.


The EME157B2 preamplifier kit was originally intended to be mounted in a masthead box, with BNC connectors soldered to the PCB, and stuck up a pole near the antenna. In my application, I wanted it to be in the same enclosure (with suitable shielding) as the power amp, so that I could use its RF detection to automatically switch the power amplifier on. I will also be using different relays, mounted on a separate board.


Instead of mounting the SPDT relays for the kit on the EME157B2 board, I've instead left these off. I also omitted the 2N7000 MOSFET which turns on the relays, and L4, an RF-blocking choke which permits the preamp to run from a 12V source supplied up the coax. I instead will power the preamp directly.


Since the relays will be on a separate board, the plan is to run wires from the gate and source connections where the 2N7000 belongs, and run those out to a controller board. With the relays gone, the RF detection and the preamp are essentially two distinct circuits. So 3 SMA connectors will be needed. Here is the completed board with the SMA connectors fitted, and suitable jumpers installed, ready for tuning.


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Completed 2m preamplifier. Connectors going left-to-right: Antenna input, Amplifier output, RF detector input.

Next, I finished off the power amplifier board, mounting it to the heatsink. I have left one EMC filter disconnected for now, as the instructions say to power it up first with it disconnected to set the trim pot for 4.5V bias. Rather than mounting the board flat on the heatsink, I have instead opted to mount the PCB at 90⁰ to the module. I had to make the supplied eyelets a fraction longer to accommodate this. I also mounted SMA connectors on this board.


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Completed 2m power amplifier. RF input is on the left.

The plan is, I'll route RG195 coax on the left side to a small module which will contain the overload detection circuit and two SMAs for an external attenuator module. On the right, a low-pass filter will be connected. I also had a stab at tapping holes into the sides of the heatsink for mounting a bracket. This bracket would hold the fans on top, and would bolt onto the main enclosure. In doing this, I managed to bugger up two of the 8 holes, and thus what I'll probably do, is buy a M4 tap tomorrow, drill them all out to 3mm and tap them to 4mm. These are structural holes, so bigger is probably better anyway.


Much of today though was spent designing the controller. I'm still finalising the design, but a rough schematic is below.


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2m amplfier controller

So, not yet going, but big parts are built now.

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