The New England QRP club is pleased to announce the continued availability of the simple and inexpensive HF noise bridge. The kit, which includes circuit board and parts (no enclosure)  is available from Dave Benson at Small Wonder Labs.  Please click here for additional information.

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.  The 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.

Click here for the layout and instructions on building and using the Noise Bridge.

Have you ever wondered what the real power output of your QRP rig is? How about being able to measure QRPP?

Here's a circuit using two junkbox transistors and a garden variety op-amp that will let you accurately measure your rig's flea power down to milliwatts. It makes use of your ordinary shop multi-meter. If your DMM or analog voltmeter's accuracy is good, your results will be too. It works because power is proportional to the square of the voltage across a resistor, assuming the  resistance stays the same.

Although it can't measure a rig's output power while operating into an antenna, it will tell you how much power your QRP rig puts out into 50-ohm resistive loads like a well-matched coaxial feedline. The best thing is that it's accurate!

Click here for continued description.