4S-Tuner Assembly Manual
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4S-Tuner Assembly Manual
Copyright 2015 David Cripe NMØS and Four State QRP Group

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Thank you for purchasing the 4S-Tuner. The 4S-Tuner is a versatile random wire tuner that permits nearly any antenna to be matched to 50 ohms. It is a physically small, low parts count, end fed wire or coax fed antenna tuner. With an external balun it will also match antennas with balanced feedlines. It is easy to assemble and easy to use. A Pittsburg style PC Board is incorporated, as is a PCB enclosure. These instructions are for assembling the entire kit, including the enclosure.
Theory of Operation
VSWR Bridge
The circuit detects a mismatch of the antenna using a Wheatstone bridge-type detector. The transceiver drives R1, R2, and R3, three 47 ohm resistors, with the antenna forming the fourth leg of the bridge. If the antenna is 47 ohms (only 6% off from ideal 50 ohms) the bridge is balanced, and the differential RF voltage between the two legs (between R1-R3 and R2-Antenna) is zero.
The diode-capacitor circuit D1-C4 detects any differential RF voltage present and generates a negative DC voltage across the capacitor proportional to the amount of mismatch.
The benefit of using a resistor bridge, as opposed to a more conventional transformer-type bridge, is that regardless of the impedance of the antenna, the worst VSWR ever seen by the transmitter while tuning up is 2:1. For QRP rigs without internal VSWR protection, this should prevent damage to the finals.
Indicator Circuit
The 4S-Tuner uses a unique VSWR detection circuit, employing two LEDs, one green, one red for indication of forward and reflected power, respectively. At infinite VSWR, the red LED shines at full brilliance, and the green LED is extinguished. At 1:1 VSWR, the red LED is extinguished, and the green is at full brilliance. At 2:1 VSWR, the two LEDs are roughly the same brightness. Note: due to part manufacturing variations, your results may vary slightly. However, you will be able to easily detect the dip and achieve a match.
The power for the indicator circuit is derived from the RF power present at the transmitter output. This is rectified, and stored as DC at capacitor C3. This voltage feeds current through R4, LED2 (green) and Q1. A small amount of current also passes through LED1 and D3 into the base of Q1 to bias it into conduction. The current into LED2 is current limited by R4, and is largely proportional to the output voltage of the transmitter.
If reflected power is present, this shows up as a negative voltage on C4. This draws current through R5, from the base of Q2. This is amplified by the gain of Q2, and pulls current through the red LED1. Because both LED1 and LED2 are fed from the current limit resistor R4, any current passing through LED1 is diverted from LED2. This causes one to dim as the other becomes bright.
Initial Assembly
Getting Started:
Note: Dont rely only on the pictures, some of them were taken of the prototype and may not be exactly like the production version.
Inventory the parts, then separate the 2 PC boards into 6 pieces by flexing the joints back and forth until they break. Lightly sand or file the rough edges, and knock off the corners slightly with 2 or 3 light swipes of a file or sandpaper.
You may wish to tin the pads, leaving a slight solder "bump" before installing the components. This allows easier soldering since you won't need to hold the part, solder, and iron at the same time to make the joint - just position the part and melt the solder on the pad.
Building the Inductor:
This is not difficult, but let's get it out of the way first. The inductor is mounted to the switch by soldering it’s 12 taps to each switch position’s solder lug. There are 21 total turns on the core. Build the toroid by lacing the wire through the solder lugs of the switch UP through the core and around and down the outside. The three postions nearest the ground side of the coil have 1 turn between each lug, the rest have 2 turns between each lug. For the first time builder: each pass of the wire through the center of the core is counted as a turn.
( ) Begin building the inductor by viewing and following these photos . NOTE: the following pictures will show magnet wire being used in the beta kits. Buss wire is supplied with the production kits to facilitate soldering to the switch lugs. It has been tested and works as well as magnet wire in this application. Plus its very easy to solder to the switch lugs (no enamel).
NOTE:There are two ways to wind the inductor according to your preferences. One with the most inductance at position A on the front panel, and the other with the most inductance at position L. The most inductance will correspond with matching an antenna on 80M. The choice is yours.
Click here to choose position A as your high inductance tap.
Click here to choose position L as your high inductance tap.