SW20+ Tuning Linearity Modification

The SW20+ uses a 100 K linear taper single turn tuning potentiometer to cover over 30 KHz of the band. As a result, the tuning is a bit touchy. A 10 turn pot could be substituted, however a frequency counter such as the FREQ-mite is needed to determine the operating frequency. Fortunately, there is a simple modification which provides a lot more bandspread around the 14060 QRP watering hole frequency (at the expense of bandspread below 14050 KHz) using the stock tuning pot.

The portion of the 20 meter band generally used by QRP operators is about 14058 to 14064 KHz with 14060 being the published calling frequency. FIST CW operators are found around 14058 and digital signals are heard above 14064.

The first step is to use the assortment of fixed capacitors to set C7 at a value that establishes the high end of the tuning range (tuning pot fully clockwise) to just above the QRP area, around 14066 or so. My SW20+ uses 57 pF (a 47 pF and a 10 pF in parallel) for C7. Depending on individual varactor characteristics, the lower end of the tuning range should be about 30 KHz lower in the band.

The modification is a single 10K resistor soldered between the "hot" terminal and the "wiper" terminal of the tuning pot as shown in the schematic. This resistor changes the linearity of the tuning to greatly increase the bandspread in the upper portion of the tuning range.

 

The following chart shows the experimental tuning linearity results for various resistor values. The "DOT" column is the 10 dial calibration dots on the SWL case, starting from the furthest counterclockwise as the tuning knob is turned in the clockwise direction. The "NONE" column is the unmodified frequency calibration. The other five columns show the frequency calibration for five different resistor values, ranging form 4.7 K to 33 K Ohms.

DOT
NONE
4.7K
10K
18K
22K
33K
1
14033.4
14033.6
14033.5
14033.4
14033.6
14033.5
2
14033.9
14040.8
14037.5
14035.8
14035.5
14034.9
3
14035.4
14057.4
14050.3
14044.6
14042.9
14040.3
4
14037.4
14061.5
14055.8
14050.7
14048.7
14045.7
5
14040.1
14062.6
14058.3
14054.9
14053.0
14050.2
6
14044.7
14063.6
14061.0
14057.8
14056.6
14054.0
7
14050.5
14064.2
14062.3
14060.2
14059.4
14057.7
8
14057.9
14064.7
14063.6
14062.5
14062.0
14061.1
9
14064.7
14065.6
14065.4
14065.3
14065.1
14065.1
10
14066.0
14066.2
14066.0
14066.2
14066.2
14066.1

Without the modification, the QRP area is compressed into the area between dot 8 and 9.

With the modification, the 10K value provides very nice bandspread of the the upper end of the tuning range. 14060, the QRP calling frequency conveniently appears with the tuning control at the mid point of rotation, between dot 5 and 6. The QRP "band" is spread out between dots 5 and 8 and tuning is much less touchy.

The drawback of the modification is a much more compressed spectrum below 14050 KHz with 17 KHz of band squeezed between dots 1 and 3.

As always, your mileage will vary, so feel free to experiment to obtain the tuning range you desire.

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