SWR Bridge

Circuit Diagram

Description
I found this circuit to perform very well in my 136 kHz transverter and have subsequently used it in my Loop ATU and my 400 W Power Amplifier with great success.  It easily copes with 400 W of RF, and probably a lot more besides.  Dave, G3YMC has also used this design in his 136 kHz transverter. 

Each of the two transformers uses a 25 mm OD 3C85 ring core, wound with 60 turns of very thin insulated wire.  I used PTFE-insulated wire-wrap wire.  The 2 turn coupling is wound using wire obtained from 13 A mains flex.   Both windings are held in place using a few dabs of  'Loctite' Super Attak universal instant glue (my favourite brand of cyanoacrylate glue). 

Here is a picture of the two transformers, as fitted to my Loop ATU.   (Click on the thumbnail to enlarge.)

By carefully checking the number of turns on the transformer prior to final assembly; and by selecting two diodes with similar forward resistance, excellent balance should be achieved without any trimming of the 'Adjust On Test' resistors.  For initial test purposes, resistors values of 1k can be used.

Feedback
Following some interesting correspondence with Larry Bettencourt, WA6LUT, I should point out that poor results may be experienced with this design if the transmit signal includes a significant amount of harmonic content.  It is good practice to use a low pass filter at the output of an LF transmitter, especially when using a FET power amplifier.