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Building a simple resistive SWR bridge using Manhattan style

by Diane Bruce, VA3DB

Introduction

Beginning amateur radio builders often make the mistake of trying to use a standard breadboard for quickly laying out a circuit. Sadly, the new builder is discouraged when it quickly becomes apparent this will not work, as there is too much stray capacitance in the circuit. One of the easiest ways of building any RF project is with a PCB that has been made for the project but this is less easy for the beginner building a prototype. An alternative is the Manhattan style in which components are soldered directly or indirectly onto a bare copper clad board (PCB) using isolating pads.

This article will walk you through the steps building a classic QRP resistive-type SWR bridge suitable for low-power radios. It has the advantage of extreme simplicity and low parts-count, but has the disadvantage of needing either a switch or a set of connectors to remove it from the circuit when operating, as the SWR bridge itself dissipates power. The circuit here is from http://www.gqrp.com/resistive_swr_bridge.pdf

Materials needed

Assemble the parts and tools needed, you will need the components, needle nose pliers, wire cutters, metal shears, cleaning pad, measuring rules/tapes, scrap of single sided PCB, soldering iron and a tube of superglue.

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PCB preparation

The beginning hamradio builder would be wise to frequent local hamfests for components. An alternative is to look around on the net for scrap pieces or simply buy a small piece new. Keep in mind a scrap piece will cost you far less and you don't really need very big pieces, ask around your area and/or mailing lists for sources. Scour your piece of PCB clean with the cleaning pad, steel wool is a very effective cleaning tool here.

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Mark a 6m/.25in strip from the end of the board,

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Cut this strip off, metal shears are the easiest way to do this. A paper cutter (guillotine) will do it as well, but will wear down the blade.

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Cut this strip into approximately 6mm/.25in square pieces, it's not critical at all

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Look at the schematic and note where you need to put an end point which is not a ground point.

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Layout each of your pads matching them up with the schematic. Here the first four pads are done as an example.

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Closer look at PCB with components and pads.

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Remove components, roughly glue using the superglue the pads to the bottom PCB.

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Clip the leads to size and start soldering components, more pads can be added later as you go.

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Board completed wired up to a meter and pot, ready for testing.

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ManhattanSwrBridge (last edited 2012-07-12 22:02:16 by DianeBruce)

ManhattanSwrBridge (last edited 2013-02-17 02:22:17 by DianeBruce)