Adding Receiver Incremental Tuning (RIT) to the SW40+ QRP Transceiver

by... Jim Larsen - AL7FS AK/QRP #003


Visit the Alaska QRP Club HomePage at: http://www.qsl.net/kl7aqc/

In an earlier article I discussed building an 80, 40 or 20 meters superheterodyne (superhet) CW transceiver (2 watts). The theory within the radio was covered in an online course called Elmer 101 and was published in QRPp (NorCal QRP Club). This radio is from Small Wonder Labs and is called the SW40+ (mine is for 40 meters). (If you can access the referenced material at the end of this article, you will find a wealth of additional information including the SW40+, schematic, kits, and other items mentioned in this article.)

The RIT Upgrade

I recently decided I needed the Receiver Incremental Tuning (RIT) add-on for the SW40+ transceiver that I use for business travel trips. I ordered the kit and shortly thereafter, I was melting solder and building up the small PC board RIT. After the modification, I found that my frequency range had changed from 7.009-7.044 down to 7.009 to 7.033. This was what was predicted in the RIT Instructions.

I began to tack on capacitors to the bottom of the board to change the value of C8. I followed the range suggestions in the Instructions sheet but nothing I did seemed to improve the tuning range. I finally gave up on C8 and left it at the stock value even though my tuning range was still reduced.

Again, per the Instructions, I changed C7 (68 pf in my case) to move the range back up the band. I chose to use the next smallest value in my parts drawer (56 pf - I did not go all the way down to the 47 pf in the chart in the SW40+ manual) to give me a range of 7.0226 to 7.047. (Thank you, NorCal Capacitor Kit.) I also tweaked on the spacing on L1 to help bring the range closer to what I needed. I decided this new range was ok as I seldom use my extra class privileges and it gave me a more even spread on the dial. The RIT works as advertised and I am sure I will enjoy this when I call CQ and have to tune around for stations. I have found stations off frequency by over 1 kHz from time-to-time.

There are no new contacts with the SW40+/RIT but I expect some on my next business trip to the Lower 48. This is primarily my travel rig. The power output to a slightly mismatched inverted-V antenna reads 2.8 watts on the Oak Hills Research, OHR WM-2 wattmeter. I suspect it would read closer to 2 watts into a 50 ohm load.

For you Spartan Sprint folks, the SW40+ with the RIT now weighs 11 ounces. I have the SW40+ custom case. Batteries are 8 ounces, Micro-Key is 1 ounce, and Kenwood earbuds are only an ounce or two. The next addition to the SW40+ will be a built in Tick Keyer. With all these upgrades, I now just need to be in the Lower 48 for a Spartan Sprint.

73, Jim
mailto:al7fs@qsl.net
http://www.qsl.net/AL7FS/

References:

AL7FS was originally licensed as WN0LPK in March 1965 (WA0LPK from 1965-1985). Jim is a member of the Anchorage Amateur Radio Club and the South Central Radio Club. For a while he was a Moonbounce (EME) fanatic and earned 2 meter WAS #36. Even then he operated in QRP style, using only about 600 watts output. Jim has participated in HF from 160-10 meters (CW and SSB), packet, satellite, 6 meter, UHF, VHF, ATV, DX, and QRP. QRP has lasted the longest and the strongest - 1970 to the Present.


Permission is granted for reproduction of this article if it is used as written (or changes agreed to by author) and credit is given to the author.

 



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