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Coax cable loss is of course very important, but we should not forget that the Model 404-UL's coax is only 12m (40 ft.) long.
The Model 404-UL was designed to be an ultra lightweight antenna for use with tall, lightweight fiberglass poles. This rules out the use of RG-213 immediately.
RG-58 could have been used but the improvement in coax loss would have been just 0.8 dB across the hf spectrum. This would have added more weight, requiring dropping the antenna's feedpoint one or two meters down the fiberglass pole to prevent pole breakage.
The fraction of a dB more cable loss is a fair trade-off for the performance advantage gained through lighter weight and higher installations.