On 6/21/2011 3:20 AM, Helmut Wabnig wrote:
What influence has a metallic boom on a Yagi antenna?
Normally the design is made without taking the boom into
consideration.

http://yfrog.com/janextgen14p

The voltage curves on the parasitic elements have their maximum
in the center. When using a non-conductive beam the situation is not
affected.

The current is maximum at the center, not the voltage.

With a metallic boom and with the parasitic elements fastened
to the metal would the oscillation pattern be changed?

At HF the effect is normally minimal depending on the mounting method
used. Using insulated through the boom mounting it starts to be
noticeable at 2m and by 70cm it definitely needs to be compensated for.
1 and 1/4 inch booms using Delrin through the boom mounts at 432 need
about 7mm added.

The previous statements assume you don't use insanely large booms and
mounting pieces.

tom
K0TAR