The Noise Bridge

There is another instrument that you may use for resonance tests. It is known as a noise bridge. It is connected between your receiver and the feed line to the antenna. Your antenna will be purely resistive at resonance, so the null in the noise from the bridge will be the deepest at the frequency of antenna resonance.

The reactance controls on the noise bridge will be at zero when this null occurs. The bridge will provide a resistance reading when fully nulled, and this will indicate not only antenna resonance, but allow you to read the feed-point impedance (resistive) of the antenna. In order for this instrument to be accurate, you must use an electrical half wavelength of coaxial feeder between the antenna and the bridge.

The instrument generates white noise, and this is heard in the receiver output (speaker of phones). The bridge controls are adjusted for minimum noise to indicate antenna resonance. You will need to tune your receiver to various frequencies in the ham band of interest in order to locate the resonant frequency of the antenna.

The noise bridge is readjusted for a null at each of these frequencies until you find a frequency that yields a deep null with the reactance controls at zero. This may seem complicated now, but if you study the operating booklet for your noise bridge, things will fall into place easily.

source : Novice Antenna Notebook by DeMaw, Doug