The Antuino has been in use for over a few months now. We had produced 100 of them that were sold at the FDIM. These work quite well for an SWR meter and Antenna Analyzer, but they were sub-optimal for serious RF work. For those who bought it for SWR measurements, you can continue to use it. For those who want to improve it, read on.
It was noticed that the db scale was not accurate. Antuino was designed to be an accurate and precise instrument. The db readings should be within +/- 1 db accuracy. However, they tended to vary by as much as 2 db on the upper range of power measurments. This was finally tracked down to having too much gain. I had prototyped the original with 2N3904 transistors but in production we used BFR93W as I guessed it would have 'better' performance. It turned to be a bad choice. The higher gain resulted in compression of signals above -30 dbm. This restricted the useful range of the Antuino to about 50 db.
I am attaching the new (Version 2.1, it is a minor edit of the V2 that was sold at the FDIM). In summary, these are the changes:
2. Replace the Q1 and Q3 transistors to 2N3904. Although the PCB pads are SMT, you can solder the leaded type by twisting the legs around.
3. In the first IF amplifier (that immediately follows the mixer), we decrease the emitter resistor (R18) from 100 ohms to 51 ohms (you can also parallel a regular 1/4 watt 100 ohms with the original 100 ohms to get to 51 ohms). We also parallel the (R19) 220 ohms collector load with a 10 uh inductor (you can use 10 turns on FT 37-43), the exact value is not important.
4. In the second and third IF amplifiers, we replace the 100 ohms emitter resistors (R7, R34) with 10 ohms and remove the 4.7 ohm resistors (R32, R4).
5. In the last IF amplifier, we change the load resistor(R33) from 51 ohms to 220 ohms.
You need a few 10 ohms resistors and a 220 ohm resistor. You can resuse the R31 at R18.
Attached are the images and the circuit.