NN7KexNOSPAMk7zfg wrote:
Have a 6 meter yagi (the 7 El. M Squared), and modeled it - the pattern
gain,front/back ratio, E & H Degs., were way off from those claimed- at 50 MHz.
Upon reconsideration, changed freq to 49 MHz, and VIOLA, got the
expected gain, ect ! I then shortened the elements by 2/3 of the boom
diameter- then it seemed to meet the claimed gain, ect. at 50.5 MHz !
Changed that to
3/4 the boom dia. and then , got claimed gain, ect., where it should
be. My question is : It is my understanding that this is correct for
calculating (or building ) antennas , with elements THRU THE BOOM, but
this antenna has
the elements supported on TOP OF THE BOOM ! Am curious as to what is going
on (what am I missing)? any ideas ?


Adding a metallic boom anywhere close to an element will shorten its
effective electrical length. The same applies to various kinds of
metallic mounting plates and clamps.

Before you model a yagi, you have to change all the element lengths that
you input to the program - in order to input the correct electrical
length, you must *subtract* the boom effect from each physical element
length.

In the reverse direction, if you want to build a yagi from a model, you
must either use a non-conducting boom, or else make your choice of
metallic boom diameter and mounting method, and then *add* an
appropriate correction to each element length.

This is exactly what M-Squared will have done, to develop their yagi
from a model... if you ask nicely, they might tell you the correction
they used, or even supply the NEC file.

This boom correction is not a simple fraction of the boom diameter.
There are a variety of formulas, and the ones for most forms of VHF/UHF
yagi construction are summarized at:
http://www.ifwtech.co.uk/g3sek/diy-yagi/


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek