CQ CONTEST: MAKING THE SW-40+ COMPETITION READY

My "Trail-Friendly" Small Wonder Labs SW-40+ has been a great companion in the wilds. I especially enjoy the SW-40's VFO, which in my case tunes from 7016 to 7052 KHz using a standard linear-taper potentiometer as a tuning device.

The relatively wide frequency range of the SW-40 is great for making a quick QSO when I am in a hurry to get off a cold mountain top, or when I want to find someone willing to chat on a starry night when I am camped far from civilization.

But in addition to being a backpacking and hiking rig, my SW-40+ also serves as a light-weight contest station. I use it for Spartan Sprints, QRP to the Field, Flight of the Bumble Bees and other outdoor operating events.

During contests, I find the SW-40's wide tuning range becomes a detriment. In the heat of competition I become excited and can't touch the tuning knob lightly enough to separate stations that share a contest-congested band! Faced with this dilemma, I decided to modify my SW-40 to be more QRP contest friendly.

I reasoned that a contest rig only need tune about 5 KHz on each side of QRP calling frequency (7040 KHz in this case) to be useful. I wanted to use as much of the 270 degrees or so of "dial" provided by the tuning potentiometer as possible so the tuning would not be "touchy."

Further, I wanted any modification to be quickly reversible so that my the rig would remain "trail friendly."

After pondering the SW-40's schematic I decided to try a very simple fix. It has worked very well, and may be worth your attention.

The SW-40's tuning device is a potentiometer which is used as a voltage divider to supply 0 to 8 volts to a varicap diode. The diode (in turn) acts as a variable capacitor to change the frequency of the VFO.

In this circuit, the exact resistance range of the potentiometer is not critical; in fact almost any potentiometer will work. I use a 100K linear taper pot to tune my rig. Of course the value of the pot you are using affects the "fix," but does not affect the principles involved in making the modification.

Using a Volt-Ohm meter, I first measured the voltage output between the "wiper" of the tuning potentiometer and ground and compared it to the frequency output of the rig. I found that the tuning range I wanted for QRP contesting corresponded to a voltage output of 4 to 6 volts to the varicap.

Next, I "padded" my tuning potentiometer (pot) to supply only this voltage range by adding fixed resistors on each "side" of the pot. I determined experimentally that a 100K (Brown Black Yellow) resistor added to the "top" of the pot (the side connected to the 8 volt regulated source), and a 220K (Red Red Yellow) resistor added to the "bottom" (grounded end) of the pot did the job perfectly.

You will note that the added components greatly increase the total resistance of the voltage divider circuit, but the divider itself still works exactly the same. The smaller current flow to the varicap diode due to the increased resistance makes no practical difference.

I used 1/8 watt resistors from my junk box and placed a miniature DPST slide switch on the front panel to switch them in and out of the circuit. This allows nearly instant reversal of my "contest mode" modification.

In practice I have confirmed that the modification does not affect the operation of the rig other than reducing the tuning range. When in "contest mode" 7040 is exactly in the middle of the "dial" and my rig tunes from 7035 to 7045 KHz using most of the range of the pot. The "dial" is therefore much less touchy, and stations are easier to tune. A flick of the switch returns the VFO to it's original range.

Total added weight of this modification is only that of two 1/8 watt resistors, a bit of hook-up wire, and the miniature DPDT slide switch. The cost was about two dollars.

I anticipate this modification will work as well with similar Small Wonder rigs designed for other bands. It should be easy to reduce the tuning range of your rig to any subportion of the band your rig operates using this technique.

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Dr Bob Armstrong, N7XJ, an avid builder and expert outdoorsman, is a family doctor living in Manti, UT. His e-mail address is: barmstrong@mail.manti.com