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The engineer in this article recommends 10′ long radials for frequencies above 1000 KHz and 20′ radials for frequencies below 1000 KHz. The article has a sketch that shows a clever way to design the system.
“The ISS patented PowerPlane® Factory-Assembled Groundplane is composed of 62 copper ground radials (12-gauge) tied to an inner ring of (4-gauge) copper wire. Sixty-two radials extend outward from the inner ring in every direction, creating a circular pattern.” (See the drawing in the linked article.)