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This article describes a simple SWR meter that I have built using readily available parts. Note that no attempt has been made to ensure accurate power readings, as I am usually aware of the approximate power level I'm running. I simply wanted a circuit that would fit in a small enclosure, and give me an indication of 'minimum' swr. This circuit does the trick. It is based on the ever popular WM-1 made by Oak Hills research, with a few changes.
I built this circuit up, as I do most of my homebrew stuff, dead-bug style. A little planning results in a compact, and symmetrical layout. (symmetry is important for the section containing L1/L2).
Note that T1 and T2 are somewhat unusual. Start with a FT50-61 core. Wrap 24 turns of enameled wire on it (size not critical as long as it fits). Spread the turns out to cover most of the core. Next take a 1 inch long piece of RG-58/U and strip about 1/8 inch of the jacket off both ends. Remove the braid covering the dielectric on one end, and twist the braid on the other end into a pigtail. Slip this prepared cable through the prepared toroid, and connect one end of the enameled winding, as well as the braid to ground. The other end of the enameled winding connects to the anode of D1 as shown. One side of the center conductor of the coax attaches to the input connector J1, while the other end goes to the output connector, J2.
Take another FT50-61 core, and create another assembly as above. Connect one end of this core's enameled winding to the output connector, while the opposite end goes to ground. One end of this core's length of coax attaches to the anode of D1, while the other end connects to the anode of D2. Don't forget to ground the pigtail of this core's coax braid. This whole process is easier to do than to describe!
The purpose of the lengths of coax, is to provide a single turn primary (a wire passing through a toroid acts as a single turn) while the shield grounded at one end only, acts as a Faraday shield, (see any older ARRL Handbook for a description of this forgotten technique) which reduces the 'capacitive' coupling between windings.
I used an LM324 IC as it can be had at Radio Shack and runs off a single supply. The values shown in this part of the circuit are for a 200uA meter movement I happened to have. A different meter may require changing these values somewhat. The rest of the parts can be had from Radio Shack or Dan's Small Parts.
To use, connect your TX to J1, a 50 Ohm load to J2, set the switch to 'forward' and adjust R for approximately full scale deflection on key down. Switching to 'reflected' should show very little or no deflection. If not, check your wiring.
This circuit should work with QRP rigs in the HF bands, although I have made no attempt to try it on other than 20 and 40.