What large antenna works best for a crystal radio?

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I've had a similar problem with my electric radios. When I first setup my antenna/ground wires (antenna was an outdoor longwire, ground was a lot like yours) I found that poor ground connection quickly turned the ground wire into a pretty effective antenna. I suspected a problem with grounding, so I poured warm salt water over the ground rod, soaking the earth real good. With that done, my ground rod worked-well, long enough for me to determine that was the problem.