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 ? Just curious-- Jim NN7K

--
To reply, remove the NOSPAM

Adding a boom of any diameter to a model as a single wire or
series-connected single wires won't have any effect on results in EZNEC
or similar programs, if it's placed exactly symmetrically with respect
to the elements (the usual case). This is because of the simplified
model of a wire and its current used by these programs. So the model has
to be modified in order to account for the real-life effect of a boom
(or clamps).

According to Lawson, in _Yagi Antenna Design_ (p. 7-11), when an element
goes through a boom, the element should be lengthened by about 0.7 times
the diameter of the boom in order to maintain optimum length. This means
that the boom effectively shortens the element, and the model element
should be made shorter by that amount. This is from an NBS report where
measurements were made of actual Yagis. The report is available at
http://www.boulder.nist.gov/timefreq...al/pdf/451.pdf.

Lawson also says that when an element is on the boom but not through it,
"the element length should be increased by only six percent of the boom
diameter; this small correction rapidly disappears as the element is
spaced away from the boom (even by a small amount)." (p. 7-12)

Leeson details modeling requirements for a number of different clamping
methods in Chapter 9 of _Physical Design of Yagi Antennas_. You haven't
mentioned what sort of clamping system is used, and perhaps it's the
presence of the clamp that makes the difference. I recommend taking a
look at Leeson's analysis. Both Lawson's and Leeson's books are
available from the ARRL.

Roy Lewallen, W7EL

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 ? Just curious-- Jim NN7K

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