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James -

As LC said, if you're planning to use LMR400 or any other 0.405" OD coax, do NOT wrap it around anything with a radius less than 4" (which means a coil form has to have a DIAMETER of at least 8"). Smaller than that, in warm (hot) sunshine with a few hundred watts and the internal geometry of the coax will change. The center conductor (not the "core") will migrate right through the dielectric, right up against the inside of the braid. This, as you'd expect, is unwanted.

For RG8X and other 0.24" coax, it's the same basic formula: 10 times the OD (which would give you the RADIUS of 2.4". Then multiplying by 2 to get the equivalent diameter gives you 4.8", rounding up = 5").

[QUOTE="Beetle, post: 155653, member: 1066"] James - As LC said, if you're planning to use LMR400 or any other 0.405" OD coax, do NOT wrap it around anything with a radius less than 4" (which means a coil form has to have a DIAMETER of at least 8"). Smaller than that, in warm (hot) sunshine with a few hundred watts and the internal geometry of the coax will change. The center conductor (not the "core") will migrate right through the dielectric, right up against the inside of the braid. This, as you'd expect, is unwanted. For RG8X and other 0.24" coax, it's the same basic formula: 10 times the OD (which would give you the RADIUS of 2.4". Then multiplying by 2 to get the equivalent diameter gives you 4.8", rounding up = 5"). [/QUOTE]

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