Automatic Magnetic Loop Tuner – SWR Bridge

While reading the frequency from the radio, and having the “antenna characteristics” in the tuner’s memory, there is no real need for an SWR bridge to be used.
However, we can add a basic power/SWR-meter and auto-tune function if we do so. For this SWR bridge, Loftur proposed to use a Tandem Match over a Bruene Bridge.

He wrote the following in his Automatic Magnetic Loop Controller BOM and building instructions:

A Tandem Match coupler may be a bit more accurate than a Bruene Bridge, however it is more difficult to achieve very good
isolation between the Forward and Reverse outputs, as this is dependent on how similar the two transformers are. On the other
hand, this is not intended to be a super-precision instrument, the below circuit is certainly as good as the average HAM grade
Power/SWR meter.

The picture here below is a partially finished kit from KitsAndParts.com with the adaptation proposed by Loftur
Swr bridge

Modifications to the kit from Kits and Parts:
20 turns wound on the two ferrite cores. This will make the meter suitable for 100W (power dissipation in R1 and R2 is 0.25W max
at 100W), should be able to handle 200W SSB without upgrading the resistors to 1/2W type.
R3 and R4 are shorted, C1 and C2 are exchanged to 4.7nF capacitors (example: Digikey 445-4746-ND) to speed up the response of
the outputs. This is necessary for fast SWR tuning. C3 and C4 are omitted.
Note the different order of the pins at the J1 connec tor, when compared with the VSWR sense connector on the Controller PCB.

Resistor values R15, R16 and R17, R18 on Controller PCB:
For a useful range of 0.5 – 200W, R15 and R16 on the Controller PCB should be 18 kohm 1/4W (example: Digikey 18KQBK-ND) and
R17, R18 should be 22 kohm 1/4W (example: Digikey 22KQBK-ND).

Due to the number of components that needed to be replaced or tossed away, and since there was some interest from other OM’s of our club, I designed an SWR Bridge kit with the correct components and sufficient copper wire for the 20 turns per toroid. Since the calibration resistors are omitted from this design, it’s important to make the toroids as equal as possible. A small deviation is not an issue. The tuner will tune to the lowest SWR point. Even if the reading is a bit off, the antenna will still be tuned to the minimum SWR.

SWR Bridge Kit
Drop me an email at on5ia@uba.be if you are interested in this SWR kit specially made for the Automatic Magnetic Loop Tuner.

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