What frequency are you building for?
The higher the frequency, the smaller the tubing can be.
It's for 440 MHz
Depending on the radius of the bend, size makes a difference.
The tighter the bend, the smaller the tubing you should use.
Pack sand and use a spring bender on the tubing for best results.
Tubing material also makes a difference in the radius of the bend.
NAPA sells a corrosion resistant brake line that is a copper/nickel
alloy which is soft and easily bent.
Air conditioning tubing is also a softer alloy and bends well, too.
Aluminum is difficult to bend, unless it is alloyed for it.
However, aluminum holds up better than copper, unless,
as noted above, you use a corrosion resistant alloy.
If strength is needed, you may want to heat treat after bending
to increase the strength. But it does cost to have it done right.
If you don't need the bandwidth larger tubing has, use wire.
Aluminum wire is light and strong, and easily worked.
Another benefit of wire is, water doesn't get inside to freeze or corrode.
I was trying to bend 3/8 utility grade copper tubing. They also had a different grade, which might bend easier. I was also thinking of using 10gauge copper wire, that should be stiff enough to hold up.