there are programs that will calculate it, but usually not very accurately.
it is best to measure it and either adjust it to match the feedline or add a
matching network to get it to match the feedline. how closely it must match
the feedline depends on what it is being used for, which you haven't said.

"yhan" wrote in message
oups.com...
Greetings,

I built my antenna with the help of this newsgroup and other articles
from the net. thanks to all. Now, the only problem is i am a little bit
confuse about impedance. (but still studying for it). how will i know
the impedance of my antenna so that i can buy a transformer or balun
that will match the ... feedline(is it the right term?). Is there a
standard formula for calculating impedance of a home-made antenna?

thanks in advance,

ian