A Reflection Bridge for the N2PK VNA

An essential add-on tool for your VNA to do RF measurements

A reflection bridge is a must have thing if you want to entertain yourself measuring circuits that you have been using for years without having an idea how they perform from RF Reflection Coefficient perspective. The amount of energy reflected from a circuit input has always been a mystery to most of us. Normally we would only know how much energy passes through. Then we blame the circuit for its high losses instead of trying to tweek its input impedance, or use a matching circuit.

To build my bridge I used the bridge board manufactured as a part of the VNA boards kit. The layout of the bridge board was borrowed from Greg Ordy's article with an exeption - I did not use the extra side connector. I did not use it because I am not interested in working in this range of frequencies, and such an on-board trace appendix would only bring an extra reflection to the system.

The board was designed to fit the Hammond 1590A aluminum diecast enclosure, as was proposed in the above article. It fits in nicely and a variety of connector types can be used to interface the signals in and out. At first I was thinking about mounting the N-type connectors. However, because the VNA board had SMAs, and still was not mounted in its own enclosure at the time, I decided not to mix up different types of connectors, and use SMAs for the bridge also. The ones I had in possession were panel mount threaded SMA femails. Drilling holes was not a problem, however, the back of the connector extends quite a bit into the enclosure interior. Because of that three slots had to be cut and shaped along the board edges, see the picture:

board slots

Cutting the slots out was not a big issue. A junior hacksaw and a fine file set did the job. The board was dropped down to the enclosure bottom onto a string so that the string ends were hanging from the enclosure opening. After the connectors were mounted and their back nuts tightened, I pulled the string, it lifted the board and brough it into position for soldering. Soldered the center pins first, then the string was removed, and the board ground plane soldered to the connectors bodies on both sides from the central pin. Put the box cover in place. Done. Nice RF tight reflection bridge.

finished bridge

For the reflection bridge schematics please refer to the original documentation.


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Contact: miv@makarov.ca