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Won't this just have the effect of transmitting white noise to all and sundry?
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I think the answer to your question Robin is yes. The purpose of a noise bridge is to generate white noise. Having built several (although not the particular design referenced in the article here) it is a simple device to operate and use. You insert the noise bridge between a receiver's antenna input and the antenna coupler. You then adjust the antenna coupler's inductance and capacitance for minimum noise. When minimum noise is achieved you have the antenna coupler tuned to whatever frequency your receiver is tuned.
The writer of the article briefly mentions it but there is another very handy use for a noise bridge. I have frequently connected my noise bridge to a dead receiver. The white noise is broad-banded and can be used for finding circuit problems such as with a signal tracer.
Commercially-produced noise bridges (and many homemade ones) are roughly calibrated so that, once you have tuned to minimum noise you can "tweak" the bridge's variable resistor to "fine tune" and derive the impedance of an antenna.Omega-T and Palomar are two very popular versions.