Tuesday, June 3, 2014

Sam Wetterlin Reflection Bridge

Having bought a cheap ($20) Chinese reflection bridge off the internet (NOT ebay) which had appalling directivity (10dB), I decided it was time to build my own to be used with my HPSDR VNA.
A survey of the internet and some recommendations pointed me at the designs by Sam Wetterlin. There were two possible designs, I chose the three bead balun version (mainly on size grounds) This used a Minicircuits transformer and a ferrite bead Balun. I bought some "TC1-1-13" off ebay but these had a pinout different from the Minicircuits catalogue and didn't work well. N1JEZ provided a couple of the genuine articles from his collection which worked perfectly The rest of the parts were bought from Mouser and I designed the two tiny PCB with 0805 components and had them made by ExpressPCB 
Assembly took five minutes. The two PCB are separated by a brass strip which supports the balun.  I had some Glass fibre tape which I used to stop the Balun moving around on the support. I soldered an smt resistor to the PCB as the reference load to save using a 4th SMA for an "external load.
Complete Reflection Bridge
Closer view of the input circuit. The TC1-1-13 is the small square on the right
Directivity was very good, exceeding 30dB from 0.47MHz to 1GHz, more than adequate for my present needs

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