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