I'm scaling down a successful 6M Yagi design to work on 10M, but getting confused about element diameter corrections. When building any antenna - especially a Yagi - the diameter of the elements has an effect on how long the element should be. For example, consider two antennas that are identical except for the diameter of their elements. Given two elements that are the same length, the larger-diameter element will be electrically shorter (e.g. higher frequency) than a smaller-diameter element. This means that if an antenna designed with, say, 1/8" diameter elements is built using 1/4" diameter elements, it will be slightly high in frequency.

My 6M design used 3/8" elements but I want to use 1/4" rod for 10M. How much should I compensate the element lengths?

Going from 3/8" to 1/4" diameter means smaller elements, so you'll need to lengthen them slightly compared to a direct frequency scale. Start with about 2-3% longer than the calculated scaling factor suggests, then tune with an antenna analyzer. Use an SWR meter or analyzer to fine-tune performance.

I've done similar scaling projects and the diameter effect is real but not huge. Use tools like MMANA-GAL or EZNEC. Model both versions in software first - it'll save you hours of cut-and-try work in the field.

Just finished a similar project scaling my 6M beam down to 10M. The element diameter correction was about 1.5% for that size change. The accuracy of NEC-2 engine can't be compared to the much more expensive NEC-4 engine that's used in EZNEC Pro/4. That's not to say EZNEC+ is not useable – it's just not quite as accurate. In practice, I found EZNEC+ gave excellent results provided you took the slight frequency upwards 'bump' into account.