The ground plane ought to be 1/4 wavelength minimum in each direction, which (at 146 MHz) is about 20.2 inches. The longest dimension (the diagonal) of a 3x3 plane is over 25 inches, big enough.
You mention 30 feet of coax. It wasn't the same 30-foot piece of coax, was it, the piece with the bad connections? Linking RG8X to RG58 shouldn't be a problem; neither is exactly 50 ohms (they're both about 52 ohms) but that's way close enough.
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-- Carl
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