"art" wrote in message
ups.com...

Dave wrote:
"art" wrote in message
ups.com...

Richard Clark wrote:
On 21 Sep 2006 19:09:38 -0700, "art" wrote:

Notwithstanding that the
upper half of the major lobe serves no usefull purpose to what the
antenna is required for there is a mass of radiation in many
directions
and levels that have no connection to the required purpose of the
antenna, thus we have a lot of wasted radiation that if we harness it
so that it is used for the antennas primary use the efficiency of the
antenna would increase immensly.

Hi Art,

The classic solution is to stack yagis vertically. This draws down
the higher radiation lobes and puts their gain in the forward
direction.

Well you are getting closer to the question at hand. You have now
doubled the
power input but only slightly gained directionality(2db) efficiency I
would also suspect that you have flattened the lower lobe only into a
pancake shape. But again I go back to the desirable radiation which can
be said in this case to be the lower half of the major lobes half power
envelope which for a directional radiated array is very small compared
to the total radiated field.True propagation can play games but the
ARRL
give the average arrival angles over a 11 year period so it is not a
hopeless task to get a ball park figure regarding usefull radiation
knowing where the target is
I suppose I could make a model and slice out the half power lobe
portion and compare the two volumes for myself, I just thought that it
had already been looked at
Oh well back to the drawing board
Art

what you are missing is the variability in that arrival angle. if you
are
interested in a specific path you must be able to receive all the
possible
arrival angles, which with yagi's requires mounting several of them at
different heights. for instance consider a path from w1 to western
europe
at the sunspot peak on 10m... it is not uncommon for the band to open at
a
very low angle, say where a single yagi at 120' is the best antenna, then
as
the day progresses the angle increases so much that the 120' antenna is
almost worthless but one at only 30' is working great. if you put
everything into getting that 10-12 degree angle you lose out by mid
morning
when the arrival angle is up to 30 degrees or more...


David that is not absolutely correct, we are talking about a single
point to point communication where the arrival angle is below 10
degrees. If the angle of arrival is above that then it is created by
unusual propagation or deflection of radiation path. For a given
distance one can say that the communication energy level is comensurate
with the number of skips taken where a point is reached when the number
of skips controls the amount of energy left at the communication
distance. Thus the east may hear the west coast talking to Europe where
they cannot hear the transmitting station because of the excessive
number of hops. Remember, I am talking about point to point
communication
which largely defined by the number of skips taken which is why dipole
to dipole transmissions are pushed aside for those desiring DX contacts
tho I am sure you are not advocating dipoles for DX.



but at the same time
that top antenna may be working great into siberia!

what you are looking for is not normally called 'efficiency', but
'directivity'. unfortunately horizontally polarized yagi's vertical
radiation pattern is very dependent on height

do you really mean "vertical: radiation pattern?

and the terrain so increasing
the directivity is seen mostly in the width of the pattern. and as noted
above, controlling the vertical pattern is normally done by changing the
antenna height, usually by stacking multiple antennas on the tower and
selecting them one at a time or in combinations to give the desired
vertical
coverage.

No... stacking is used purely to provide a vector to combat the earths
magnetic field
which affects all radiation directional patterns not only a vertical
pattern

There have been some experiments with variable phasing of stacked
yagis, but it is not a common capability in amateur installations.

Exactly since these methods provide a vectoir to counteract the
terrains magnetic field
unfortunately this requires extra power supply points where the desire
is for just one.
Art


you have some big misconceptions that i can't begin to address here.. but
just a couple of points for you to go study on.
1. the arrival angle is not a fixed value for a point to point circuit. the
angle changes with the height of the ionosphere and also with which layers
are supporting the path at the time. the angle can change minute by minute,
or it can be fairly constant for hours depending on the state of the
ionosphere. but it will not be constant for all time.

2. also, it is not like the pretty single ray that some people draw when
showing reflections off the ionosphere. the ionosphere is not a mirror, it
is a gradient in a layer of ionization. the signals that are 'reflected'
are actually refracted and do not arrive perfectly focused as they went up.
in addition the polarization is changed which affects the efficiency of the
path, this is very evident on 160m and 80m where the prefered polarization
can change hour by hour over night.

3. i have no idea where you are going with this idea of stacking is to
combat the earths magnetic field. the only effect the earths magnetic field
has is on the ionosphere, not on how your antenna works. it is well known
that changing the height of a yagi changes the vertical radiation pattern
and hence the arrival angles that it favors. stacking yagis at different
heights and selecting them separately or in combinations lets you adjust the
elevaion pattern to compensate for the changes in the arrival angle. in
most cases all the yagis in a stack are fed in phase so their signals
combine at the horizon, but there have been some experiments where the
phasing is changed to intentionally raise the pattern higher to cover
different arrival angles more efficiently.