Small Wonder Labs SW+ 40m QRP Kit

Dave Benson, operator of Small Wonder Labs, put together a monoband CW amateur radio kit that is easy to build and provides excellent results on the air.

With about 2 Watts of power, I have consistently had radio contacts with stations putting out 20 to 50 times my power rating. The tone has been reported as very good, and I generally get a report that my QRP rig is doing a "gud job", although signal ratings tend to be low. I assume this is because my signal is weaker than other signals on the band, however other stations seem to be able to copy my signal well.

The radio dial covers a band of about 35 khz. You can adust the frequency within the 40m band two ways: by squeezing or expanding a wire coil wound around a toroid (a small "donut" shaped inductor) and by adding a capacitor to the board. The kit includes a number of calibrated capacitors and explains how to calculate which value will drop the frequency to your desired level. As a new ham, I selected a 35 khz spread around 7.12 Mhz. This ranges from about 7.095 to 7.130 and includes a large part of the Tech 40m frequencies. There are a lot of slow code folks on or around 7.12 Mhz, and the procedural errors which all beginning hams make seem to be tolerated. Also, a number of older hams drop down and help out and encourage the new operators. When I improve my code and procedure skills, I may modify the radio to cover the 7.040 Mhz area, which seems to have more action and is where more DX might be around. But for now, I think I chose the right part of the band to start out in.

I am pretty new to electronics and ham radio, although I have wanted to be a ham for many years. Why did I decide to build a kit as my first radio? I never seemed to have an extra $700 lying around for a big rig. And without some basic experience with a radio, I didn't have the confidence to try to take the ham test. I also thought that the coolest way to be a new ham operator would be to build my own radio. So after doing some research on the net, I took the plunge and ordered the kit. At the same time I was studying CW and the ham exam test questions.

My method of building the SW+ was to stuff one part at a time, solder, then inspect the work. In particular you have to be careful with diodes and some capacitors, which are polarized and have to be installed with the positive and negative leads in the right direction. I had to remove a diode and turn it around. It was one of the first parts that I installed, and I am glad I caught it. Soldering was easy, using the smallest diameter solder I could find at Radio Shack. A small package of solder was more than enough for this kit. My soldering iron was also a basic model with the standard tip. By the time I found the special small chisel tip that I wanted I was done with the kit.

For a beginner, it was important to get both the board kit and the case kit. The case kit has the connectors and knobs and dials that make it easy to complete the radio. It is a nice looking product and with both kits, it looks professionally done when it is finished.

When I was finishing my kit, I tuned it up very quickly and rushed to power it up to see if it worked. It did, and I was able to start listening to CW immediately. I didn't have my license yet so I didn't try the transmitter. Well, actually I did do some testing with a small antenna to see what frequency I was on. I used a cheap digital SW broadcast receiver and scanned the band to see which frequency I was sending on. As originally constructed according to plan, 7.040 was right at the middle of the dial. I removed a turn from a toroid and adjusted the coil of wire on the toroid until 7.12 was in the middle of the dial. I still have the option of adding a capacitor to the board to drop the frequency, if needed.

After I got my license, I tried sending CQ. For hours, and nobody on the air seemed to be able to hear me. I had got my license on the first day of the Iraq war, and the weather was not very good, so I thought that there were just bad conditions and everyone was watching TV. I posted a plea for help on the QRP-L forum, and someone mentioned that with a SW+, it does not put out much power unless tuned correctly. So I decided to go back and read the directions (!) and tune it in the manner specified. My only instrument was a voltmeter. The plans show a small circuit you can build out of a few diodes and capacitors that acts as a peak voltage detector. I twisted a few wires together and used some alligator clips to connect the circuit to the antenna output, the board ground, and the pos and neg terminals of the voltmeter. While sending short tones on an open frequency, I noted the voltage. The tuning procedure is to adjust T2 and T3 (two transformers) which have a screwdriver slot on top until you get the maximum voltage. Then you adjust a resistor to bring the voltage back down to about 12V. I found that just a hair difference in T2 and T3 made for a big increase in power. You have to adjust these carefully and take time to keep trying different settings for maximum power. Once T2 and T3 are optimized, then go to the resistor and turn it so that you have about 12V. I think my final reading was about 13V. The resistor is also very picky, just a small movement makes a big change so be gentle. This resistor is how you adjust your output power should you want to reduce it or increase it. I have heard that some have pushed it over 3W, but they warn of heat build up, so be cautious.

Once I had my rig tuned, I answered a CQ and was amazed to get a reply from Texas (TX to MI). I was pretty nervous as this was my first QSO. Mercifully, on the hour, a broadcast station kicked in and stopped me from making a lot of beginner mistakes and failing to copy the code. After having tried for days to get a reply, I was surprised and a little shaken when someone actually did reply! Since my first QSO I have made many more and have often been complimented about the quality of the signal, especially when the other operator finds out that I am QRP. Usually the others are running 40 to 100 watts of power compared to my 2 watts burning up the atmosphere. Actually I may be above 2 watts, I have a feeling I am a little above the specified 2 to 2.5 watts, maybe I am even approaching a smoking 3 watts (hey, I can dream). Watch out for RF burns in my shack.

In this beginner's opinion, the key to success with QRP is getting the signal from the radio into the ether. This means avoiding any loss in the feed line, and using an effective antenna that is resonant on your frequency. Every connector on the coax adds a little bit of loss to the signal. I do not have a SWR meter, or antenna tuner. If I had one, I would use it to test my antenna one time to check if it was resonant, then remove it. With a monoband transciever, you should be able to put up a resonant antenna. I have a single piece of coax from the radio to the antenna. My antenna is a dipole at about 20 ft high, which should be higher, but for now it will have to do. I am very happy with the results, so I may not change it for a while.

My operating strategy is to look for a quiet spot on the band, where there is little or no broadcast interference, and listen for a while. Eventually someone will start tuning up and call CQ. I try to respond quickly. I have also successfully called CQ. I think the key for QRP QSO's is to choose a frequency with little or no noise or interference. Times of the day when there is not much going on on the band seem to be the best time for me to snag a QSO. Please remember that these comments are coming from a beginner and I may just be making assumptions here.

In conclusion, I would say that if a new ham has some basic radio knowledge, he should be able to put together one of these kits and use it as his main radio and be very happy with the results. I like CW, and I am thinking about choosing one or two more bands, and just building SW+ kits for each band, rather than buying a big rig. I might also want an amplifier to bring the signal up to about 20 watts to work DX on the 15 and 20 m bands. I know that this does not fit the QRP definition of 5W, but it does meet the requirement of using the lowest amount of power that will allow reliable communication. These little transcievers seem to have the right filters for CW and the circuits are optimized for the specific band you are working. I suspect that when it comes to CW on a specific band, that the SW+ is better than an entry level big rig, and that the big rigs only advantage is the extra power. I'm new at this so I could be wrong, its based more on comments that I have heard. In order to get a quality signal out with low power, you have to have good radio design. Add to it an effective antenna that is optimized for one band and you have a combination that is hard to beat.


April 1, 2003

SW+ 40m dual band mod, Tech and General "bands"

I built my SW+ to cover a 35khz range around 7.12Mhz, the Tech CW band. I also wanted to get the freqs around 7.040Mhz, the General/Extra CW band. The Elmer 101 notes had a comment about adding a cap and a switch at the C7 location to allow for dual "band" operation. For those who haven't built the kit, C7 is where you put a cap to move the rigs frequency to the desired range. Usually you solder the cap in and that determines the freq where your rig stays.

I tried it out, installing a small switch on the front panel, and connecting the grounded side of C7 to the grounded switch pole, then soldered a 47 cap to the ungrounded switch pole, and connected the cap to the other C7 hole.

I was a little concerned about adding long leads to a resonant circuit. I emailed Dave and asked if this would reduce performance of the rig at all and he said just keep the leads as short as possible.

I think there may be a small amount of added noise, but it is hard to tell. If I were to do it again I would try using a piece of small coax and use the shield as the ground and the inner wire as the connector to the cap. Maybe this would help with noise(?). I would also try to get a better cap type than ceramic.

The mod seems to work well. When I want to move from one band to the other, I just flip a switch. I have gotten QSO's on both bands now, it seems like it did not affect the rig negatively. Now my simple and effective transciever covers both CW sweet spots on the 40m band. With the cap switched in it runs about 7.038 to 7.074 MHz, and with it switched out of the circuit, the coverage is from 7.103 to 7.137 MHz. The SW+ is my only rig so I appreciate being able to use both the Tech and General bands.

Update: In order to drop the frequency to cover 7.030, the QRP calling frequency, I replaced the 47 pf cap with a 56 pf cap. With the 56 pf cap, the range is 7.024 to 7.058.

A mod that I have not tried is said to extend the coverage from about 35 kHz to 65 kHz. The mod is to add a 130 pf cap in parallel with C8. The person suggesting this added it on the bottom of the board. I have not tried this mod.


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