Birth of a hombrew SOTA rig

Since the mid 90’s, I’ve wanted to build the Rick Cambell’s (KK7B) direct conversion and phasing receivers/transmitters. I spent a few years in the part accumulation phase, and finally etched some PCBs for the r2pro version in EMRFD 2 or 3 years back. They were populated and tested briefly with a VCO and LC phase shift network. My initial plan was to use them in a 2m IF as suggested by the articles, however the key attraction of the receiver is low distortion reception (passband full of cw signals) not really something which would be noticed on the microwave bands. 

Since being totally addicted to SOTA and wanting a rig slightly higher powered than the FT817 , a sensible option seemed to be to turn it into a triband (20,30,40m) rig for SOTA using RF16HHF mosfets for the final allowing up to 20W output.

Full requirements:

  • At least 10W output, hopefully 20W
  • Big screen 🙂
  • Big tuning knob
  • Trail friendly design, i.e. display/knobs on top.
  • CW keyer memories

Of all my current projects, this is a background one, working on it from time to time, and the last few months were spent working out how to fit everything into the smallest possible box. Having more or less settled on something, I cut the first sheet of aluminium as the chassis and built up the screen and microcontroller board as I needed to free up some breadboards and an arduino. A custom built box is going to be a PITA but it allows for the smallest possible size and best form factor. 

I etched the boards as a set of modules for experimentation purposes, however this does mean it won’t be as small as it could be. Modules are the nemisis of compact design.

The various board from left to right: 

  • common gate rf preamp + mixers + Si5351 oscillator as LO
  • Diplexer and AF preamps
  • Audio amp out of EMRFD (ne5532 with 2n3904 as drivers) underneath the diplexer
  • Phase shift network and 4khz filter.

Works fairly well, reception is maybe a few dB down on the FT817. Some problems include mild motorboating with no signals (obviously some better bypassing/powerline filtering required), and some microphonics around the connectors. The final version will halve the number of connectors, direct connections for many of them. 

The gap in the aluminium is for the speaker. There is still some board space left for the tx audio phase shift network and hopefully an audio filter board to allow 3khz and 500hz filters. All the RF (including the exising rx mixer board will go on the rear of the chassis, and the display/controls ontop of the audio. 

2 thoughts on “Birth of a hombrew SOTA rig

  1. Good project, I also gathered a number of the parts for an R2 and eventually built it, but got stuck on something, can’t remember what it was, it was quite a few years ago. Must get it out and have a second look. I have been toying with a version of Drew Diamond’s TCF ( twin crystal filter) project with the aim of building a really simple but effective SOTA radio for ssb. it is ne602s and a 4th IF. Even considered building a free running vfo with one of the cheap Chinese freq counters on it. Just for old times sake!

    Which the arduino and si5351 circuit/configuration are you using???

    • Hi Paul,

      I’m using an uno, but I reimplemented it on breadboard. I use the original board with some jumpers for programming. The only critical thing is the amount of storage – the graphical lcd with fonts consumes a lot of memory. 32k is required.

      The si5351 is the adafruit board and I think I’m using a slightly modified version their library. It offered an easy way to set the 90 degree phase shift for the iq mixer.

      The two things I noticed with the r2 is the wideband nature of the “roofing”‘ filter and it’s sensitivity to microphonics. There is a little noise while tuning the si5351 but nothing too objectionable. I’m hoping it’s noise off the i2c lines which would be easy to fix with some rc filtering.


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