Homebrew 20 Watt Linear Amp

20 Watt Broadband Linear Amplifier

20 Watt Broadband Linear Amplifier

QRP is tons of fun on CW, but it gets a bit rough trying to work other stations on SSB with 5W, especially when you are using antennas that are low to the ground. I had been eyeballing the nice RF MOSFETs from Mitsubishi for a while, and since I got a hankering to get a bit more active on SSB, I took the plunge and ordered five of the RD15HVF1 devices. At a current price of $5.25 at RF Parts, they are a bit more expensive than the IRF510 that you see in a lot of 20-40 watt range linears, but these devices have a few advantages over the IRF5xx series. One of the biggest, in my opinion, is that these RF transistors are designed to run off of a 12 volt drain voltage, unlike the IRF510 amps which don't really work well until they get around 24 volts on the drain. These things can also take quite a beating from poor mismatches, and have the convienice of having the source connected to the metal tab on the case, making for a nice solid ground connection.

20 Watt Broadband Linear Amp - Inside

20 Watt Broadband Linear Amp - Inside

Having the appropriate parts in hand and some designs on the internet to steal from, I set out to build my own linear. There isn't a ton of creativity to be used when designing a linear of this class (Push-pull Class-AB). Every design that I've seen looks nearly the same. Not surprisingly, the real focus of the design is in optimising the input and output networks. Feeling lazy and anxious to just get on the air, I pretty much did "cut and paste" from some different circuits to find out what works best. I know, not the best method, but sometimes the desire to just put out some RF trumps proper procedure. I don't have a scehematic to post at the moment, but if you click through on the photo to the right, you can see a close-up with descriptions of major circuit blocks. Below, I've posted links to the two circuit resources that I used the most for this design. I'll have more details about the designs to comment on at a later date, when I can pull some proper notes together.

One of my weakest homebrewing areas is in the mechanical engineering, but now that I have a bit of a real "shop" in my garage, things have been getting better. A bit of scrounging at the surplus stores around town led me to some cheap heat sinks that looked like they might be suitable for this project. After attacking them with an angle grinder to get a lip off of the bottom side, I was able to bolt two of them to the lid of an aluminum Hammond enclosure. I nibbled a nice square area right out of the middle of the copper clad I used to build on, soldered the RD15HVF1 devices to some pads etched out with a Dremel, then bolted the MOSFETs and copper clad directly to the lid of the enclosure. Drilling the holes for the BNCs and the LED was a piece of cake with the aluminium box material.

Without getting into too many details at this point, I was able to get the amplifier working right off the bat. I didn't get quite as much output power as I initially liked (only got about 10 watts), but the amp was working correctly. More troubling was the fact that output on 6 meters was only 2.8 watts. Not too great when you are putting in 2.5 watts. I figured it had to be something with the input or output network. The input return loss measured quite good; -15 to -20 dB across all the bands. So I figured that left the output network. My initial iteration of the amp used a transformer similar to the one in the Pennywhistle amp (this is a configuration that I've also successfully used before in a push-pull class-C CW amp). Without doing any actual measurements and calculations, I dropped in the broadband transformer pair used in the TF3LJ amp, and immediately improved my output power by a few watts. But I was still a bit low on 6 meters. A bit more searching showed that I might need another compensation cap on the output, so I experimented a bit more until I found that a 1200 pF silver mica in series with the drain transformer outputs worked wonders and boosted my power on 6 meters to nearly 15 watts CW. I haven't done any analysis to see why this helped. I know, sloppy...but sometimes expedience wins.

Since there's no output filtering built into the amplifier enclosure, I had to assemble some outboard filters in order to get this thing on the air. I was going to use 7-pole low-pass filters until I realized that everybody else uses 5-pole filters because push-pull amps already suppress the even-ordered harmonics by at least -30 dBc. A bit of work with the new LADPAC software in EMRFD enabled me to crank out a table of filters for all of the bands (160 m - 6 m) using the silver mica caps in my junkbox plus T68-6 toroids. If you click through the photo below, you can get a glimpse of the copper clad enclosure sticking off the output of the amp.

Backyard Linear Test

Backyard Linear Test

Last Monday, after a bit of checking of the signal purity with my dummy load and scope, I was satisfied that everything was working OK and took the amp out for a spin on the back porch. I set up the Buddipole in Versatee Vertical configuration with the Low Band Coil. It tuned right up on the upper end of 75 meters, and I had no problems at all checking into the Oregon Emergency Net. One watt out of the 817 gave me about 25 watts out of the linear on 75 meters. I was too busy to do much else with the amplifier until today (the following Sunday), but I was excited to give the amp a try on 6 meters, since that was one of my biggest motiviations for building the thing. The Buddipole was set up in a simple 6 meter dipole configuration about 10 feet above the ground and I parked the 817 on 50.125 MHz. It didn't take long before I heard N6OR booming into Beaverton from Southern California (grid DM12). I snagged him on the first call, getting a 57 signal report in return and a report of good, clean audio when asked. He was running 100 watts into a quad, which you can see on his QRZ page. I was really thrilled since this was not only a victory for my mad homebrewing skillz, but was also my first 6 meter QSO!

I've been parked on 50.125 for most of the afternoon here at the NT7S shack and have picked up a few more QSOs. So far, all reports of the audio quality of the linear have been FB, so I'm satisfied that it (and the LPF) are working as they should be. I think I've about worn out my keys on this post, so I'll wrap it up for today (I always start with modest ambitions on these posts, they they grow exponentially). I'm having way more fun than I should be, and I'm very pleased to be back out of my ham radio funk.