ARRL Antenna Handbook, 18th Edition, Chapter 17 page 2.

"Vertical-beam [antenna pattern] downtilt is generally produced by
feeding the elements of a collinear array slightly out of phase with
each other." [NOTE: Collinear not Yagi]

See "L. Barrett, "Repeater Antenna Beam Tilting", Ham Radio, May 1983,
pps 29-35. Correction issued Ham Radio, July 1983, p 80."

All of above from ARRL Antenna Handbook.

Deacon Dave, W1MCE
+ + +

VE8AE Andrew wrote:

I was looking at a 2.4 GHz antenna that had 20 1/2 waves in phase.
Then there was the same antenna, almost, except it said it had a 3
degree downtilt to it. How would the antenna be made to give it this 3
degree downltilt? It stated it also had 20 1/2 waves to it also and
was same length overall.
Andrew VE8AE