Re: How effective is an earth for reducing noise on AM?
I guess 'how effective is an earth at removing noise' depends on _how_ the noise is getting into the receiver; if it's mains-borne then just adding another earth [your DNO's grounding-schema also being relative for safety reasons] won't necessarily make much difference.
With my Eddystone 840A bedside radio the thing I did find made ab improvement was fitting a common-mode choke to the [2-core] mains cable _and_ using an earth. The common-mode choke stops noise coming via the mains; if you just add an earth without 'choking' the supply then noise on the mains may actually flow through the earth-impedance of your new earth and possibly make noise worse!
[My common-mode choke is a small ferrite item scavenged from a switched-mode computer power-supply, mounted in a diecast box and installed about 10 feet from the radio]
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