In general, the element material doesn't make much difference in the electrical
performance of a yagi (unless it is steel or other magnetic material). The elementdiameter
, however, is important. In most cases you can take a yagi that
is designed for, say, 1/8" elements, and adjust it to use 1/4" elements. So if
you find a yagi design that you like using someother material (such as 8mm aluminum
tubing) it can be adjusted to get similar results using 1/4" copper (by lengthening
the elements slightly in this case.)
Where the difference in element materials is important is the mechanical construction:
copper is softer and heavier than aluminum. That means that the same length of yagi
will need a stronger boom and/or additional bracing when built using copper tubing.
The elements will sag more (if horizontal), and will tend to bend more over time due
to wind, birds sitting on them, etc.
So you don't see designs specifically calling for 1/4" copper tubing because it isn't
necessarily a good choice mechanically (and because aluminum is cheaper), but that
doesn't mean you can't use it to build yagis. I used to have a spreadsheet around
that I wrote to scale yagis for changes in element diameter and frequency: if you
need it I'll look around and see if I can dig it up.
My recommendation would be to start with something around 6 elements, perhaps
a boom length between 4 and 7 feet. That is practical to build without worrying too
much about the mechanics, and it should give a noticeable improvement in
performance. That will give you some idea of how much more gain you need, and
you can choose further designs based in that.
The WA5VJD "Cheap Yagi" designs are great for homebrew:http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf
Here are some other links that might give you some ideas:http://www.g0ksc.co.uk/http://commfaculty.fullerton.edu/woverbeck/quagi.htmhttp://www.k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf.htmlhttp://www.mydarc.de/dk7zb/http://www.yu7ef.com/
(he includes dimensions for 1/4" elements)http://www.cebik.com/content/a10/vhf/2mowa1.html
GM0SEK has some good general information in his Long Yagi Workshop:http://www.ifwtech.co.uk/g3sek/diy-yagi/index.htm
And you can model your own yagi designs online:http://fermi.la.asu.edu/w9cf/yagipub/index.html