NEQRP Noise Bridge

Contributed by Dave Benson, K1SWL (ex-NN1G), & Seab Lyon, AA1MY

A noise bridge is a device for measuring or comparing impedances on coaxial cables, antennas, components, etc. With appropriate calibration, it allows for the measurement of both scalar and vector (reactive) impedances.

A noise bridge contains a wideband noise generator as well as a bridge balancing circuit, and makes use of the station receiver, tuned to the frequency of measurement, as a detector. The bridge is balanced by changing a bridge capacitor and resistor while listening to the receiver for a noise null. Resistance and reactive values can be determined from the variable R and C positions, and may be converted to an SWR or Z value.

This noise bridge may lack the precision of a laboratory instrument; however, it has sufficient capability for normal amateur use and can perform many of the measurements of more expensive instruments such as antenna analyzers. In addition, this version is only 1.1" x 2.75" and can easily be built into other projects.

The Noise Bridge Project has been discontinued.