Cecil, efficiency depends on what your objectives are
The yagi antennas objective is to obtain a radiation beam of gain
compared
to radiation else where. What is wanted is a radiation lobe that one
can use to direct communication. ..rThe yagi achieves the object of
producing a lobe which has a higher gain than other lobes that the yagi
produces.( A higher ratio) The yagi achieves its object by producing
this
main lobe but at what cost? If we look at pattern volume as reflecting
as energy applied to the yagi we must compare that volume with the
whole pattern volume. This means comparing the volume of the upper
lobes, the side lobes, the rear lobe and of course the vertical lobe to
the main lobe. Any cursury look at a three D radiation pattern will
immediately see that the main lobe is less than 50 % of the total
radiation pattern
Let us look at a common dipole with a reflector, the planar view of
radiation which ignores radiation outside the plane is a figure 8 where
the addition of a reflector does nothing to enhance increased forward
radiation so immidiately we can say that the forward lobe achieves what
is termed a major lobe plus other forward lobes outside of the main
lobe
where as the radiation to the rear achieves nothing that enhances the
forward main lobe. So just comparing the forward and the rear lobe we
have only achieved 50 per cent of our object
and this is not counting other losses. Now you may disagree with the
objective of a yagi beam and I understand that may be the case.
Hopefully the above answers your request to
define efficiency as I was with respect to the yagi antenna. I think
the above pretty much explains what I stated in the initial post tho it
appears that some read inbetween the lines to read what they wanted to
read as a diversionary tactic and there is not much anybody can do
about that. One really has to ask themselves the question that if an
antenna came on the market with only one main lobe would they buy it
Art.

Cecil Moore wrote:
art wrote:
When one looks at a.radiating array pattern one can see that the yagi
is very inefficient.

Please define "efficiency".
--
73, Cecil, http://www.w5dxp.com

art wrote:
. . .
Any cursury look at a three D radiation pattern will
immediately see that the main lobe is less than 50 % of the total
radiation pattern . . .

Out of curiosity, did you

a) not read
b) not understand, or
c) not believe

what I posted about the fraction of power in a Yagi's minor lobes?

A cursory look at a 3D pattern is probably one of the least reliable
ways to determine anything quantitative about an antenna pattern. By
choosing the scale (e.g., field strength, power density, linear dB,
ARRL-scale dB), you can make the relative sizes of the lobes just about
anything you'd like and lead the casual observer to the conclusion of
your choice(*). But why bother trying to divine a value from a 3D
pattern, when it's so simple to numerically show that the power in the
lobes is insignificant?

(*) One of the slides in the "Antenna Basics" talk I've given at many
hamfests shows several very different directional patterns, and I ask
the audience which one is the most desirable. After the votes are in, I
reveal that they're all the same antenna, just drawn to different common
and legitimate scales.

Roy Lewallen, W7EL

Roy Lewallen wrote:
art wrote:
. . .
Any cursury look at a three D radiation pattern will
immediately see that the main lobe is less than 50 % of the total
radiation pattern . . .

Out of curiosity, did you

a) not read
b) not understand, or snip

I did not understand your logic.
Front to back means nothing in terms of energy containment
The measerment is a ratio not a quantity
It also is a ratio only for a given plane and does not account for
anything outside that plane.
Have I stated anything wrong there?

Point out the error of my ways starting with a simple dipole. Does any
additional element to the array revert the radiation from the rear
direction so that it is additive to the forward directive radiation of
the main lobe? If so what percentage of the rearward radiation in the
rear hemisphere,,(ot a planar amount) ? The question is to you Roy to
answer for a one on one,It would help if you gave an actual percentage
instead of a "major fraction" which you stated before

And if you don't understand then just drop the thread as it has gone
on way to long with relatively little specifics with respect to the
original post
Art








c) not believe

what I posted about the fraction of power in a Yagi's minor lobes?

A cursory look at a 3D pattern is probably one of the least reliable
ways to determine anything quantitative about an antenna pattern. By
choosing the scale (e.g., field strength, power density, linear dB,
ARRL-scale dB), you can make the relative sizes of the lobes just about
anything you'd like and lead the casual observer to the conclusion of
your choice(*). But why bother trying to divine a value from a 3D
pattern, when it's so simple to numerically show that the power in the
lobes is insignificant?



I do not remember seeing any numerical answer ,was it specific and
relative to the
contained energy in the primary lobe because that is what I was looking
for ?
What was the actual percentage that you arrived at?



(*) One of the slides in the "Antenna Basics" talk I've given at many

Roy
the actual scale doesn't matter one iota on a given pattern with
respect to my question !
To compare different patterns one must have a common denominator such
as scale,
to do otherwise is an attempt to deceive. You also use the word
"desirable"and my connoctation of that is a major lobe and nothing else
whereas some may look for other characteristics. Did the group come to
a consensus as to what was desirable? Some may want to hear what is
said behind their backs!


erns, and I ask
the audience which one is the most desirable. After the votes are in, I
reveal that they're all the same antenna, just drawn to different common
and legitimate scales.

Roy Lewallen, W7EL

art wrote:

I did not understand your logic.
Front to back means nothing in terms of energy containment
The measerment is a ratio not a quantity
It also is a ratio only for a given plane and does not account for
anything outside that plane.
Have I stated anything wrong there?

I thought your interest was in the amount of power contained in lobes
other than the main lobe of a Yagi. My posting showed a simple way to
relate front to back ratio, which is commonly known for many Yagis, to
the fraction of power in secondary lobes, which is not.

Point out the error of my ways starting with a simple dipole. Does any
additional element to the array revert the radiation from the rear
direction so that it is additive to the forward directive radiation of
the main lobe?

Of course.

If so what percentage of the rearward radiation in the
rear hemisphere,,(ot a planar amount) ? The question is to you Roy to
answer for a one on one,It would help if you gave an actual percentage
instead of a "major fraction" which you stated before

In my earlier posting I apparently overestimated your ability to do what
I thought was a simple calculation. So I'll do it for you so you can
have a number.

Since there's no "typical" Yagi, I presented one which most people would
consider to be worse than average -- one having only a 6 dB front/back
ratio. I also assumed for a starting point that the shape of the rear
lobe (that is, beamwidth and height) is the same as the front lobe. The
first calculation is to determine just what the ratio is of the powers
in the front and rear lobe. The answer is 4:1. That is, the front lobe
contains four times the power of the rear lobe. Since you seem to be
interested in energy rather than power, simply consider the amount of
energy each radiates in some amount of time: each second or other unit
of time, the forward lobe radiates four times the energy of the rear lobe.

What this says is that if you manage to get all the reverse-lobe power
(or energy, if you prefer) into the front lobe, without any change in
the front lobe's shape, you'd increase the gain just about exactly 1 dB.
If you end up fattening it, you'll lose some or all of that gain.

So there are some numbers for you. You'll have to do a bit of estimating
if the rear lobe is fatter or skinnier than the front lobe, but now you
have a number to start from.

Or let's say that the front/back is 10 dB instead of 6, a more typical
number for a Yagi. With the same criterion of similar lobe shapes, the
power ratio for the front and rear lobes is 10:1. So if you got all that
rear lobe power or energy into the front lobe without changing its
shape, you'd gain a whopping 0.4 dB. If you had two equal rear lobes,
both 10 dB below the front lobe, and both of the same shape as the front
lobe, the power ratio of the front to all rear lobes would be 5:1, and
you'd be able to increase your forward gain by 0.8 dB if you got all
that rear power into the front lobe without changing its shape.

So there's your actual percentage -- around 25% for a very poor Yagi,
and around 10 - 20% for a fair-to-middlin' one. From which you could
gain about a dB by very hard work in getting all that rear lobe power
into the front lobe(*).

If you question any of the calculations, I'll be glad to show how I
converted ratios to dB and vice-versa, although you should be able to
find this in many publications, as well as on the web. Or you can
continue drawing your conclusions from cursory looks at 3D plots. Your
choice.

And if you don't understand then just drop the thread as it has gone
on way to long with relatively little specifics with respect to the
original post

I'm afraid I do understand, but it's a good idea anyway.

(*) Being an engineer, I didn't include placebo effect gain in the
calculations. After a lot of hard work squeezing every last bit of power
into that front lobe, the signals are going to *seem* a lot stronger,
and the reports sure to be better.

Roy Lewallen, W7EL

Roy Lewallen wrote:
art wrote:

I did not understand your logic.
Front to back means nothing in terms of energy containment
The measerment is a ratio not a quantity
It also is a ratio only for a given plane and does not account for
anything outside that plane.
Have I stated anything wrong there?

I thought your interest was in the amount of power contained in lobes
other than the main lobe of a Yagi. My posting showed a simple way to
relate front to back ratio, which is commonly known for many Yagis, to
the fraction of power in secondary lobes, which is not.

Point out the error of my ways starting with a simple dipole. Does any
additional element to the array revert the radiation from the rear
direction so that it is additive to the forward directive radiation of
the main lobe?

Of course.

If so what percentage of the rearward radiation in the
rear hemisphere,,(ot a planar amount) ? The question is to you Roy to
answer for a one on one,It would help if you gave an actual percentage
instead of a "major fraction" which you stated before

In my earlier posting I apparently overestimated your ability to do what
I thought was a simple calculation. So I'll do it for you so you can
have a number.

Since there's no "typical" Yagi, I presented one which most people would
consider to be worse than average -- one having only a 6 dB front/back
ratio. I also assumed for a starting point that the shape of the rear
lobe (that is, beamwidth and height) is the same as the front lobe. The
first calculation is to determine just what the ratio is of the powers
in the front and rear lobe. The answer is 4:1. That is, the front lobe
contains four times the power of the rear lobe.

I am not interested in front to back for what I am looking for but this
4:1 has my interest
What does it represent and how did you get it? The rear usually has
more than one lobe
and the reflector ndestructs or deflects the energy to 90 degrees of
impact. But that 4:1
figure where does it come from?
Art






Since you seem to be
interested in energy rather than power, simply consider the amount of
energy each radiates in some amount of time: each second or other unit
of time, the forward lobe radiates four times the energy of the rear lobe.

What this says is that if you manage to get all the reverse-lobe power
(or energy, if you prefer) into the front lobe, without any change in
the front lobe's shape, you'd increase the gain just about exactly 1 dB.
If you end up fattening it, you'll lose some or all of that gain.

So there are some numbers for you. You'll have to do a bit of estimating
if the rear lobe is fatter or skinnier than the front lobe, but now you
have a number to start from.

Or let's say that the front/back is 10 dB instead of 6, a more typical
number for a Yagi. With the same criterion of similar lobe shapes, the
power ratio for the front and rear lobes is 10:1. So if you got all that
rear lobe power or energy into the front lobe without changing its
shape, you'd gain a whopping 0.4 dB. If you had two equal rear lobes,
both 10 dB below the front lobe, and both of the same shape as the front
lobe, the power ratio of the front to all rear lobes would be 5:1, and
you'd be able to increase your forward gain by 0.8 dB if you got all
that rear power into the front lobe without changing its shape.

So there's your actual percentage -- around 25% for a very poor Yagi,
and around 10 - 20% for a fair-to-middlin' one. From which you could
gain about a dB by very hard work in getting all that rear lobe power
into the front lobe(*).

If you question any of the calculations, I'll be glad to show how I
converted ratios to dB and vice-versa, although you should be able to
find this in many publications, as well as on the web. Or you can
continue drawing your conclusions from cursory looks at 3D plots. Your
choice.

And if you don't understand then just drop the thread as it has gone
on way to long with relatively little specifics with respect to the
original post

I'm afraid I do understand, but it's a good idea anyway.

(*) Being an engineer, I didn't include placebo effect gain in the
calculations. After a lot of hard work squeezing every last bit of power
into that front lobe, the signals are going to *seem* a lot stronger,
and the reports sure to be better.

Roy Lewallen, W7EL

On 29 Sep 2006 05:47:14 -0700, "art" wrote:

one having only a 6 dB front/back ratio.

But that 4:1 figure where does it come from?

Hi Art,

Do you know how to work a calculator using logarithms?

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On 29 Sep 2006 05:47:14 -0700, "art" wrote:


one having only a 6 dB front/back ratio.


But that 4:1 figure where does it come from?


Hi Art,

Do you know how to work a calculator using logarithms?

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On 29 Sep 2006 05:47:14 -0700, "art" wrote:


one having only a 6 dB front/back ratio.


But that 4:1 figure where does it come from?


Hi Art,

Do you know how to work a calculator using logarithms?

73's
Richard Clark, KB7QHC

Which reminds me, I wanted to start a thread on calculators.

tom
K0TAR

art wrote:
Roy Lewallen wrote:

Since there's no "typical" Yagi, I presented one which most people would
consider to be worse than average -- one having only a 6 dB front/back
ratio. I also assumed for a starting point that the shape of the rear
lobe (that is, beamwidth and height) is the same as the front lobe. The
first calculation is to determine just what the ratio is of the powers
in the front and rear lobe. The answer is 4:1. That is, the front lobe
contains four times the power of the rear lobe.

I am not interested in front to back for what I am looking for but this
4:1 has my interest
What does it represent

I can't think how I can state it any more clearly than I did in the last
sentence of the text just above which you quoted.

and how did you get it?

When dealing with a power ratio, dB = 10 * log(ratio). Solving for ratio:

ratio = 10^(dB/10)

Here's where you'll probably need to get out that pocket calculator. dB
is 6 (see above text), so ratio = 10^(0.6) ~ 4.

Conversion between ratios and dB is a skill that anyone interested in
antennas should develop. I had assumed that it was part of the knowledge
required to pass a general class amateur exam, but apparently I was
mistaken. If the calculator operations are too complex for you, get a
chart of conversion factors which have already been calculated.

The rear usually has
more than one lobe

True. See the remainder of my previous posting for a discussion of this.

and the reflector ndestructs or deflects the energy to 90 degrees of
impact.

That's more nonsense. You'd develop a much better understanding of
antennas (or any physical system) by developing and learning to apply
some basic math skills than by dreaming up alternate explanations for
well-known physical phenomena.

I don't believe I can help you any more -- if indeed I've helped you at
all --, and think (or at least hope) that most other readers have
understood what I'm saying. So I'll bow out here.

Roy Lewallen, W7EL

Terman treats decibels on page 8 of the 1955 third edition of
"Electronic and Radio Engineering".

The value of the decibel is 10 log of the power ratio.

When the ratio of the powers is 4, the decibel value is 6. This 6 dB
value applies among other things to doubling the distance between a
transmitting and receiving antenna. Half as many volts will be induced
in the receiver and will result in half as many amps. This is 1/4 the
power induced at half the distance between antennas..This may be
expressed as a positive power ratio of 4 to 1, or 6 dB.

Best regards, Richard Harrison, KB5WZI