What brought this to my attention was Moxon who advocated 2 element beams
(lightnes translates to greater heights) so I modeled a 20 meter beam on a 7
foot
boom and obtained 12dbi max for the band (ala 2:1) but one can get 13dbi
if you drop the impedance from 50 to 12 ohms.
This then raised the question would a transformer cancel the increased
gain. On the F/B
question it was of real interest to me to compare the first frontal lobe
gain to the second lobe at
the rear on the basis that propagation rules the second rear lobe as the
most important .
But all of this comes to a grinding halt if there is not a datum curve to
which one can
compare ones modeling when using more than two elements on such a short
boom..
I agree that one has to model to one's specific needs and as can be seen
from the ARRL curves
using short booms max gain varies tremendously.depending on curves chosen
none of which
are the cat's whiskers such as NEC based curve.
Art

"Ian White, G3SEK" wrote in message
...
wrote:
My ARRL books go back a decade or more
and the graph showing gain per boom length
has several curves based on different measurements
e.t.c. Has a graph been made based solely on NEC
program findings over say a perfect ground and at a uniform height?

There is a table of data calculated by VE7BQH for a large number of
different yagis, at:
http://www.ifwtech.co.uk/g3sek/diy-yagi/ve7bqh.htm
(These are not calculated by NEC, but by another very adequate program.)

You can draw your own curve from the data in the L(WL) and Gain columns.
Since all the data have been calculated by the same person, starting
from the antenna's mechanical dimensions, and using the same program,
this removes a lot of variables.

You will see a broad trend of increasing gain with boom length, but
there is still a lot of scatter due to variations between different
designs. Also bear in mind that gain isn't everything, and a long yagi
that has been maximized for gain may be a poor antenna in other ways,
such as sidelobe levels, ease of pointing, sensitivity to water on the
elements, ease of matching etc.

Drawing gain curves is a fun activity... but don't try to read too much
into them.


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