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

ORIGINAL MESSAGE:

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


Well this is where I am comming from, I am presently building an
antenna for this winter where I will be communicating with the U.K.
Thus my major lobe needs to be robust between about 10 degtrees and 4
degrees to ensnare most of the communication.

------------ REPLY FOLLOWS ------------

A parabolic dish about 500 feet across will fit your needs just fine.

Once again, proving how a little knowledge is a dangerous thing. As if
more proof were needed. :-)

Bill, W6WRT

art KB9MZ wrote:

Thus my major lobe needs to be robust between about 10 degtrees and 4
degrees to ensnare most of the communication.

I don't recall you stated which band but I'll assume 20 meters.
Assuming flat terrain (for your Indiana QTH), a single Yagi at 120'
(your maximum possible) would cover these angles best. 140' (or 2
wavelengths high if it is not 20m) would be the optimum heightl to
center your main lobe at 7 degrees.

Two stacked Yagis at 60' and 120' would be better than a single one at
120' or 140' for 4-10 degrees, but obviously this means more work and
expense.

If your terrain is not relatively flat, and if you are lucky to be on a
hill with a gentle slope in the direction of England, the optimum
height will be much less. However you would need to model this using
HFTA in the most recent Antenna Handbook or YT in older editions.

73, Bill W4ZV

Bill,
None of that is really a problem to me.I have multi points to which I
can feed for different patterns depending on the time of the day as
well as option of tilting the array. If I don't get my elmer I will
assume he is not on the air and yes it is twenty meters and I am
located in the couintryside that I understand is the highest between
Chicago and New Orleans and since this is the bread basket of the U.S
the ground loam is excellent. I use 7/8 andrews plus a long length
underground and the tower is hinged in two places
as well as another one for array tilting. I have power gain over a yagi
but I was just curious as to how much more radiation energy was
available to ensnare which Is why I referred to array volume since gain
is really dependant on half power beam width required or one can live
with I once had a 80 foot yagi and with the looseness in my prop pitch
rotor combined with the narrow beam one was never sure if one was
really taking advantage oif gain available. Pretty simple question for
those who know the answer after all you start off with 3 db gain in two
different directions thereffore it would seem to me that a yagi was
only 50% efficient but people are quarrelling about every thing except
the posed question to excuse them selves from real thought. Reg would
have come up with the solution after a bottle of wine after noticing
the english provided by the cue
Regards
Art

wrote:
art KB9MZ wrote:

Thus my major lobe needs to be robust between about 10 degtrees and 4
degrees to ensnare most of the communication.

I don't recall you stated which band but I'll assume 20 meters.
Assuming flat terrain (for your Indiana QTH), a single Yagi at 120'
(your maximum possible) would cover these angles best. 140' (or 2
wavelengths high if it is not 20m) would be the optimum heightl to
center your main lobe at 7 degrees.

Two stacked Yagis at 60' and 120' would be better than a single one at
120' or 140' for 4-10 degrees, but obviously this means more work and
expense.

If your terrain is not relatively flat, and if you are lucky to be on a
hill with a gentle slope in the direction of England, the optimum
height will be much less. However you would need to model this using
HFTA in the most recent Antenna Handbook or YT in older editions.

73, Bill W4ZV

Hi Art,

A few points below, but first just remember that the TOA of any
horizontally polarized antenna is primarily a function of ground
reflections which vary according to height above ground. Previously
you mentioned that your antenna was designed for a TOA of 10 degrees.
That cannot be true except for a specific height above ground. Whether
Yagi, Quad, Log, Rhombic or any non-vertically stacked antenna.
Something like a Sterba curtain is different because it has multiple
elements stacked vertically which CAN be steered by phasing.

art wrote:

yes it is twenty meters and I am
located in the couintryside that I understand is the highest between
Chicago and New Orleans and since this is the bread basket of the U.S
the ground loam is excellent.

1. Absolute height above sea level means nothing. What is important
to TOA is your relative height above the terrain within a mile or two
of your tower. I operated from Colorado for ~30 years and always got a
chuckle from the guys who said, "My antenna is over 1 mile high". In
fact what is important for determining TOA is not height above sea
level but height above surrounding terrain.

2. Ground conductivity has minimal effect on horizontally polarized
ground reflections. You may be thinking of vertically polarized
antennas like verticals where it has a huge effect. HFTA does have
conductivity as an input parameter but it has minimal effect, at least
in my case (average ground versus salt water).

I use 7/8 andrews plus a long length
underground and the tower is hinged in two places
as well as another one for array tilting.

Phsical tilting has minimal effect on the ground reflections for the
angles of interest (4-10 degrees) because the vertical lobe is not very
narrow (typically a Yagi has ~50 degree 3 dB vertical beamwidth). In
other words tilting has a far secondary effect on TOA versus changing
the antenna height. You can prove this to yourself by modeling with a
program like EZNEC (HFTA does not allow tilting because it is
physically impractical and has little effect).

Pretty simple question for
those who know the answer after all you start off with 3 db gain in two
different directions thereffore it would seem to me that a yagi was
only 50% efficient

I believe you're referring to a dipole which has 2+ dB gain over
isotropic in the two directions broadside to the element. If we add ~6
dB from ground reflection gain, we get 8 dB gain over isotropic, but
this is ONLY for a specific TOA which is determined by the antenna's
height above ground. Of course a good Yagi will typically have 25-30
dB Front to Rear, so its backward lobe has very little of the total
energy (far less than 50%).

Bottom line to all of this is that your antenna's height above ground
has the primary influence on TOA. The only other way to "steer" the
vertical lobe is to mount your antenna on a motorized tower (unless you
go to vertically stacked elements and phasing).

Put your single antenna at 120' and the vertical pattern will be
centered on about 9 degrees (assuming flat terrain).

73 & GL!

Bill W4ZV

P.S. Here are some results using HFTA for my 10 meter 3-stack:

http://users.vnet.net/btippett/terrain_&_toas.htm

Tom Ring wrote:
art wrote:

When one looks at a.radiating array pattern one can see that the yagi
is very inefficient. Does anybody know of the relative volume
Art


Art

Yagis, when made of almost anything but something like nichrome wire,
are very efficient. Aluminum element yagis run in the high 90's of
percent efficiency when properly designed.

Hmmmmm I would question your logic on that figure. since I am not aware
of the normal ratio between actual resistance versus radiation
resistance which would point to
the manufacture of radiation energy relative to the total energy input
however my question relates to the efficient radiation to the
requirement at hand
Art




tom
K0TAR

On 21 Sep 2006 19:19:52 -0700, "art" wrote:


Tom Ring wrote:
art wrote:

When one looks at a.radiating array pattern one can see that the yagi
is very inefficient. Does anybody know of the relative volume
Art


Art

Yagis, when made of almost anything but something like nichrome wire,
are very efficient. Aluminum element yagis run in the high 90's of
percent efficiency when properly designed.

Hmmmmm I would question your logic on that figure. since I am not aware
of the normal ratio between actual resistance versus radiation
resistance which would point to
the manufacture of radiation energy relative to the total energy input
however my question relates to the efficient radiation to the
requirement at hand
Art


Art, from Wikipedia and me, some definition that might help you
express your problem in conventional terms:

Directivity is a property of the radiation pattern produced by an
antenna. It is defined as the ratio of the power radiated in a given
direction to the average of the power radiated in all directions.

Gain is the product of the efficiency of the antenna and the
directivity.

Efficiency is the ratio of total power radiated to power into the
antenna.

Efficiency of practical Yagis is very high as Tom has told you. Loaded
/ trapped Yagis are not so efficient due partly to losses in the
loading coils / traps. This is the same issue that commonly arises
with shortened antennas.

It is quite wrong to say in general "that the yagi is very
inefficient".

You seem to be talking HF, the azimuth beamwidth of most practical HF
Yagis is so large that you are unlikely to notice much difference in
gain within 3 deg of boresight, so on a fixed heading you would expect
to cover the 10 deg to 4 deg target area comfortably with little
variation in gain.

If you want to maximise the transmitted signal for that specific path,
you should minimise losses (eg avoid lossy traps and coils, feed
system losses etc), increase directivity (more elements, better
design), pay attention to the desired path elevation (eg mounting
height of the antenna).

Owen
--

When one looks at a.radiating array pattern one can see that the yagi
is very inefficient. Does anybody know of the relative volume
contained in the main radiation lobe versus the total volume of the
entire pattern? I know there are a lot of different type antenna gains
and
arrangement but I am trying to determine in an informal way the
efficiency ratio and compare it to what would appear to be a very
efficient
antenna such as a dish.

I think the parameter that you are searching for is GAIN !!!!

An antenna only has gain by compressing power more into one direction more
than another. It is a case of robbing Peter to pay Paul, the more power you
have in the main lobe the less you have in other directions.

73
Jeff

It occurs to me that by art's definition, all antennas are
"inefficient". When you're talking with someone, only a teeny, tiny
fraction of the radiated power is going precisely in the right direction
to be collected by his antenna, so the remainder is wasted. Shucks, I'd
be amazed if the "efficiency" of the best HF antenna is better than
0.001% by this criterion.

Roy Lewallen, W7EL

The other point I note is that he wants enhanced HF radiation between
10 and 4 degrees elevation and apparently beamed to a specific point
on the globe... The cubic size and the towers and the arrays that it
will take to accomplish this are not efficient in time, money, and
effort... He is chasing a unicorn...
As has been pointed out already, the percentage of time that the major
portion of the arriving HF EM wave is below 10 degrees can be
enumerated on the fingers of one hand... Besides, who is going to have
the array on the other end with comparable response?

denny / k8do