VSWR bridge for Spectrum Analyzer

A VSWR bridge is a very nice add-on to a Spectrum Analyzer when you want (or need) to measure reflection loss of for example an antenne or load (termination).

Home made VSWR bridge

Home made VSWR bridge

The VSWR bridge fits mechanically on a DSA815 Spectrum Analyzer.

The design of the bridge is straight forward: a VSWR bridge is in essence the same as a Wheatstone bridge but for RF. The display shows the difference compared to a known reference. The better the DUT (device under test) matches the reference, the less reflection is measured. Offcourse this means that the reference should be perfect 50 ohm over the full range (DC-1500MHz).

schematic_1

Schematic of the VSWR bridge

The original idea of mounting the VSWR bridge to the SA was originally from Jos, PA3ACJ.

Mounting POST for N-connectors

Mounting stud for N-connectors by PA3ACJ

PA3ACJ made the mounting studs for the N-connectors, the rest was done by my. Originally the N-connectors have a RG58 crimp connection but I made an adapter so it now holds semi-rigid cable which directly connects to the VSWR bridge.

Inside peek of the bridge

Inside peek of the bridge

The inside shows the mounting of the SMD resistors (2 x 100 ohm in parallel to obtain 50 ohm) and 4 ferrites on the semi rigid cable.

VSWR bridge in front of the DSA815

VSWR bridge in front of the DSA815

Usage it straight foward: connect the bridge to the SA, select frequency range, perform a normalization (without anything connected !) and connect the DUT. Marker reading shows return loss in dB’s but should be positive (the DSA815 VSWR option shows the positive value including reflection coefficient).

Termination

Fifty ohm termination (DC-1500MHz)

23cm gp antenna

23cm gp antenna

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