Building the 4SQRP 4-S Tuner/Antenna Coupler

Following up on last week’s successful build of the Four State QRP Group TRF one transistor radio kit, The Murania, this Sunday I thought I would tackle the build of their 4S Antenna Tuner kit.

As I wrote in last week’s blog, I have been a long time fan of the great kits the 4SQRP Group develops and sells, so I was excited to tackle this project. My overall ham radio goal for the spring is to assemble and put a completely home-built portable QRP station on the air this spring, and the 4-S Tuner should prove to be an essential component of my station.

Like the Murania kit I built last week, the 4S Tuner was designed by NM0S, David Cripe, and both kits share several of the same features including “Pittsburgh construction” where the backside of the PCB doubles as the front panel, and a case that is cleverly assembled by soldering six pieces of PCB together.

Construction

The fist step in building the tuner is to construct the inductor which is mounted directly to the back of a rotary switch. For many folks, winding toroids can be a chore. I’ve messed them up myself several times in the past, but as a general rule, I don’t shy away from kits that require toroid windings.

I found that winding the 4S Tuner toroid was actually a pretty straight forward task. For one reason the toroid itself was large enough to manimpuate easily in my hands. Another factor that made the project easier was that the kit comes with BUSS wire instead of enameled wire which is difficult to create taps with as the insulation must completely removed at each tap… a fiddly prospect at best.

But best of all, because the toroid is mounted directly to the back of the rotary switch, “stitched together” if you will by the windings, the overall construction was very straight forward.

The instructions provide two options for the builder, whether to place maximum inductance at the first, “A” position on the rotary switch, or whether to place maximum inductance at the last, “L” lug. I chose the first method.

Once the inductor/switch assembly ws completed it was time to move on to attaching components to the PCB. The kit contains a nominal amount of parts – 6 resistors, 3 ceramic capacitors, 2 LEDs, 2 transistors, and 3 diodes.

“Pittsburgh” construction is sort of a large scale surface mount method where there are no holes and components are soldered directly to pads on the PCB. Pittsburgh native Joe Porter, W0MQY is credited with developing this construction method. and this is my second 4SQRP kit built in as many weeks using the Pittsburgh construction method. I have found it to be especially beneficial when removing components – desoldering is a snap and there is no need for a desolder pump or wick to remove solder from plated holes. In the case of the Murania last week, the ability to unsolder and swap out components made modifying and experimenting with the radio a true joy.

The next steps involved attaching the inductor, tuning caps, a FWD/REV DPDT switch and a pair of BNC connectors. The entire build took less than three hours and was a wonderful way to waste a blustery spring Sunday.

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Theory of Operation

According to the documentation published on the 4SQRP website, “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 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.

How does it work?

You will need to stay tuned for my on air review. As I mentioned above, my overarching project for this spring/summer is to put a completely home build QRP station on the air and while several components have been built, I’m not quite there yet.

Future blog posts will describe some of the QRP transmitters, receivers and transceiver projects I have worked on.

Thanks for reading & 72 de AB1DQ!

©2019 JMSurprenantH