The upper beam is a hygain A3 20-15-10 3 element beam.
The lower one is the 17meter and 12 Meter 3 element Yagi made with Coaxial traps. Coaxial Trap design details may be found in past issues of ARRL’s QST.
The traps mechanically are made around plastic water coupling fittings. The trap is isolated by the plastic coupling. The coaxial trap, depending on the frequency passes (17meter) or rejects (12m) the signal fed into the beam element end or tips.
The construction of the reflector, driven element and director are the same length to start with.
The size of the tubes (except for insuring adequate mechanical strength) is unimportant. What is important is that the mechanical size will allow adjustment to resonate on 17m and 12m meter bands. The spacing between elements is important to insure proper impedance of the antenna (the spaceing is the same spacing as the hygain A3S).
Tuneup starts with making sure that the driven element is tuned to the center of the band on midband 17meters first and then 12 meters next. The other elements are made longer (longer by 5 percent reflector) or shorter (5 percent shorter director) based on the length of the driven element after tuneup.
When the antenna is raised the tuned frequency will be higher and a slight readjustment of the lengths of the elements will be needed.