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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.
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.
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.
HF antennas Multiband - Monoband - WARC - Vertical Antennas - Beverage Boxes
Models for the pure contester (20m, 15m and 10m), and for DXing on the traditional and WARC bands (20m, 17m, 15m, 12m and 10m). Input power limited by feed line only. Wideband operation. Low SWR, excelent F/B. Heavy duty construction
Beverage box 1.8, 3.5, 7 MHz
Beverage box 1.8, 3.5, 7 MHz with common mode current choke For low band DX-ing. - Wid..
39.00€
Beverage box 1.8, 3.5, 7 MHz two directions
Beverage box 2 directions. For low band DX-ing - Integrated Common mode current choke. ..
46.00€
Beverage box complete set 1.8, 3.5, 7 MHz
Beverage box complete set for 160m, 80m and 40m Beverage box come with integrated common mod..
46.00€
Beverage box 1.8, 3.5, 7 MHz two directions complete set
Beverage box 2 directions complete set. - Wideband operation 1.5 - 7.3 MHz. ..
88.00€
Termination resistors for Beverage boxes 8 W
Termination resistors for Beverage boxes 8 W - 450 Ohms 8W - In plastic box for outdoor moun..
7.00€
Power supply for Beverage boxes with RFI filter
Power supply for Beverage boxes with integrated RFI filter - Power su..
26.00€
Inverted 'V' dipole antenna 3.535 / 3.74 MHz with relay switches
Inverted 'V' dipole antenna 3.535 / 3.74 MHz with relay switches - Resonance at 3.535 MHz - ..
140.00€
3500W Inverted 'V' dipole antenna 3.535 / 3.74 MHz with relay switches
3.5kW Inverted 'V' dipole antenna 3.535 / 3.74 MHz with relay switches - Resonance at 3.535 MHz ..
179.00€
Relay Switch with tuning coils CW-SSB 3535 KHz-3760 KHz for 80m Vertical
Relay Switch with tuning coils CW-SSB 3535KHz-3760KHz for 80m Vertical Switch CW and SSB frequen..
119.00€
3B-222 3-Band HF antenna
HF 3-Band antenna 3B-222 for 20m, 15m and 10m band Economical antenna with excellent perform..
399.00€
3B-223 HF 3-Band antenna
HF 3-Band Yagi antenna 3B-223 3B-223 Triband antenna for 20, 15 and 10m band uses a common f..
590.00€
3B-334 Triband antenna
Triband antenna 3B-334 3B-334 Triband antenna uses a common feed line. High efficiency, dire..
870.00€
3B-334-4O HF Triband STACK antenna
3-Band STACK antenna 3B-334-4O Tri-Band antenna with central wide spaced elements that can f..
840.00€
3B-345 HF Triband STACK antenna
High performance 3-Band STACK super Yagi antenna 3B-345 Tri-Band antenna 12 elements with ce..
990.00€
3B-456 Super high performance 3-Band Yagi antenna
Super high performance 3-Band super Yagi antenna 3B-456 Input power limited by feed line onl..
1,390.00€
5B-11-5 HF WARC 5-Band STACK antenna
High performance 5-Band STACK antenna 5B-11-5 Five-Band HF - WARC antenna 11 elements for 20..
790.00€
6B-15-5 HF - WARC - VHF 6-Band STACK antenna
High performance HF-WARC-VHF 6-Band STACK antenna 6B-15-5 Six-Band HF - VHF - WARC antenna 1..
840.00€
PA1824-4-2.5 Two band WARC antenna 17 and 12m
HF 2-Band WARC antenna PA1824-4-2.5 for 17 and 12m Cheap WARC antenna PA1824-4-2.5 covers 17..
349.00€
PA1824-5-4 2-Band WARC antenna 17 and 12m
HF 2-Band WARC antenna PA1824-5-4 for 17 and 12m WARC antenna PA1824-5-4 covers 17 and 12m b..
430.00€
3B-WARC 3-Band WARC antenna 30, 17 and 12m
High performance 3-Band WARC antenna 3B-WARC Three-Band WARC antenna. 5 elements, for 30m, 1..
540.00€
40m Full size rotary dipole antenna
40 Meter Full size rotary dipole antenna Full size dipole antenna. Excellent mechanical prop..
390.00€
Three band rotary dipoles 3B-Dipole
![Homebrew Homebrew](/uploads/1/2/5/8/125860526/372546603.jpg)
Three band rotary dipoles 3B-Dipole Full size dipole antennas 20m, 15m, 10m. Excellent mecha..
249.00€
Three band rotary dipoles 3B-Dipole-21-24-28MHz
3-Band Rotary Dipoles 3B-Dipole-21-24-28MHz Full size dipole antenna 15m, 12m and 10m. Excel..
249.00€
Three band rotary WARC dipoles 3B-WARC
Three band rotary WARC dipoles 3B-WARC Full size dipole antenna 30m, 17m, 12m. Excellent mec..
269.00€
40m 2 elements Full Size Yagi Antenna PA7-2-6
40m Full Size Yagi antenna 2 elements PA7-2-6 Full size 2 element beam antenna. ..
980.00€
40m 3 elements Full Size Yagi Antenna PA7-3-12
40m Full size heavy duty Yagi antenna 3el PA7-3-12 Full size 3 element Yagi antenna. Heavy d..
1,690.00€
40m 4 elements Full Size Yagi Antenna PA7-4-18
40m Full size Yagi antenna 4elements Heavy duty construction PA7-4-18 The best antenna for 4..
2,599.00€
20m monoband Yagi antenna 3el. PA14-3-6
20m monoband Yagi antenna 3el. PA14-3-6 Very high gain, wideband operation antenna, low SWR,..
399.00€
20m Super Heavy Duty Monobander 3 elements PA14-3-6SHD
14MHz Super Heavy Duty monoband Yagi antenna 3 elements PA14-3-6SHD Super Heavy Duty, ..
899.00€
20m monoband Yagi antenna 4el. Heavy duty PA14-4-8
20m monoband Yagi antenna 4el. Heavy duty PA14-4-8 High gain antenna, wideband operation, lo..
649.00€
20m monoband Yagi antenna 5el. Heavy Duty PA14-5-12
20m monoband Yagi antenna 5el. Heavy Duty PA14-5-12 High gain antenna, wideband operation, l..
849.00€
15m monoband Yagi antenna 3el. PA21-3-5
15m monoband Yagi antenna 3el. PA21-3-5 High gain antenna, wideband operation, low SWR, Exce..
280.00€
15m 4 Elements Monoband Yagi Antenna PA21-4-6
15 m Monoband Yagi Antenna 4 Elements PA21-4-6 High gain antenna. Wideband operation, low SW..
399.00€
15m monoband Yagi antenna 5el. PA21-5-9
15m monoband Yagi antenna 5el. PA21-5-9 Very high gain antenna. Wideband operation, low SWR,..
649.00€
15m monoband Yagi antenna 6el. PA21-6-12
15m Monoband Yagi antenna 6el. PA21-6-12 Very high gain antenna. Wideband operation, low SWR..
849.00€
10m monoband Yagi antenna 3el. PA28-3-4
10m monoband Yagi antenna 3el. PA28-3-4 Very high gain antenna. Wideband operation, low SWR,..
279.00€
10m monoband Yagi antenna 4el. PA28-4-5
10m monoband Yagi antenna 4el. PA28-4-5 Very high gain antenna. Wideband operation, low SWR,..
329.00€
10m 5 elements antenna
Electrical Specifications Frequency Range:28 - 29 MHzFree Space Forward Gain:10 dBiForw..
399.00€
10m monoband Yagi antenna 6el. PA28-6-9
10m monoband Yagi antenna 6el. PA28-6-9 Very high gain antenna. Wideband operation, low SWR,..
649.00€
10m Super Yagi Antenna 7 elements PA28-7-12HD
10m long Boom Yagi antenna 7elements PA28-7-12HD Wideband operation, low SWR, excellent F/B. 10m..
849.00€
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