Thursday, December 18, 2008

A noise bridge for cheap antenna analysis

nb top.jpgA noise bridge is a very simple circuit comprising a wide band noise generator (a conducting zener with some amplification) and a bridge arrangement with "known" variable resistance and capacitance on one arm and a socket for the unknown device.

There's a bit of extra capacitance thrown in on the unknown side so that the variable capacitor nulls half way.

The circuit I built is very simple, it's written up in "The Antenna Experimenter's Guide" by Peter Dodd, G3LDO (second edition), and documented here where it was available as a kit known as the G3ZOM noise bridge.

Ralph Klimek has a nice write up of his implementation as well.

A receiver is used as a single frequency detector to enable the finding of the null.

nb interior.jpg

Noise bridges have a reputation for being a very simple and cheap way to do basic measurements on unknown impedance's such as antennas, particularly when you're shooting for 50R and no inductance or capacitance.

The noise source works right through HF and is still going strong at 30Mhz, I imagine it's quite useful at 6m as well.

Given that most of us have some sort of receiver to be the null detector this is a very easy instrument to build. The hardest component for me to find was a 220R potentiometer, in the end I ordered from RS Components yesterday and it arrived today.

All this is prompted by the fact that I'm trying to make an antenna for 1.8Mhz work and I've currently loaned out my MFJ antenna analyser. It's great that such a useful instrument can be so simple.

In use

Hooked up to my 160m inverted L which I'm sure needs inductance, the bridge appears to show that the antenna is neither inductive or capacitive and the impedance appears very low, but I don't get a convincing null perhaps due to all the broadcast band rf coming back down the line.


Azzy M said...

Ok So your bridge did not work.
Then why put it here?

Peter Marks said...

It works Azzy but on 160m it got rather swamped with AM broadcast band stations.