If you're modeling it with EZNEC or a similar program, just include the
mast in the model as an additional wire. It's probably not practical to
include the entire mast at full segmentation, but I'd think that two or
three wavelengths would be enough to get an idea of how much effect it's
likely to be having. You could model it as several wires, each segmented
more coarsely as you get farther from the antenna. Add and remove the
mast wire(s) to see what its effect is.

But your asking about for an equivalent element length for the mast
makes it sound like you're using some sort of Yagi-specific program. If
you are, perhaps someone familiar with the particular program could
answer your question.

Roy Lewallen, W7EL

Jim - NN7K wrote:
Hi.all, and BTW, Happy Holidays! Am attempting to model
a "Cushcrunch" A-627013, yagi and have it all but written,
and modeled what I have of it-- It is a 70 cm, with 6 and 2
meter yagis, the 6 meter horizontally polarized, both the
2 and 70 cm are vertically polarized, on the same boom, ofset
by aprox 1 inch , and the beast is mounted on a mast, 6 inch
from the two meter reflector. This is the first time attempted
modeling a multi band antenna, especially with the mast in the
same plane as the yagi. My question is 2 fold: 1; Given it is
behind the reflector, just how much diff does THAT add to the
pattern of the antenna, and 2; If it does, just HOW LONG an
element would this represent?? The mast length? and for the
small portion of mast, ABOVE the mast clamp?? Just curious
as to the effect it has . Your thoughts , please. Jim NN7K