wrote:

Must be one horridly poor yagi. Most 3 eleemnt designs are better
than 8dBi and with a little effort an be 1 db better than that.



8 dBi WITH a 180 degree pattern?
And WITH A F/B ratio that doesn't exceed 11 dB
or so?

I don't think so.



On 6 Feb 2005 14:10:37 -0800, wrote:

How did you come to this conclusion? What
sort of field strength equipment did you use?

Simple testing with a stable RF power source and a sensitive field
strength meter in a very large yard. It's not hard to determine a
1Db
difference and a 3db one is easy.


What brand and model field strength meter
do you have? what sort of receive antenna did
you use? How did you do the conversion from
uV/meter to ERP?



The other is a colinear design the second element cannot contribute
3db unless the first didn''t radiate any energy. Thats rarely the
case.


????? How can the first not radiate any
engergy?


To get the gain you claim try a 8 or more segment colinear and
note that would be about 18-20ft tall for a 2m antenna.


According to most of what i have read,
every doubling of the sections give approximately
3 dB additional gain.



Again, i would be the first to say
that reality doesn't fit the math model,
sometimes not at all. But is this case,
you can really hear the difference.

Usually the math model is reality if its a real model. Often it's
not.
manyy of the simple shorthand models are far for accurate or
complete.


I used a program called Yagi Optimizer.

It should be close, it's not like
we are modeling active devices like FETs
and such!



Also in some locations there are third order factors not considered.
For a real model (NEC based) the stacked Jpole is 2.3-2.4dbi better
than a plain J and ends up at about 7.4Dbi. [In real terms that's
only about 2.4 DB better than a vertical dipole which a basic Jpole
is.]

Incorrect. A stacked J-pole cannot
be 2.4 dBi better than a plain J. It can only
be 2.4 dB better.


Anyways, You think a plain J-pole is 5 dBi?

Incorrect.



The
Jpole in this case is easier to evaluate because if the SWR is poor
it's definately not working well and a lot of things have to be
correct or the SWR will be poor.

For example, it may work better because its nearly 60inches taller
and
at a given location that could be the majik. Never ignore height as
a
significant factor at VHF.


Obviously. They are at about the same height,
considering the additional height of the Super J.



They work well to the designs limit if built carefully. I like them
for the ease of assembly of a rugged design that's reproduceable.


I would say that at VHF, a Super J is a
bit heavy and big, but the additional ERP
is worth it.


Slick

wrote:
On 6 Feb 2005 05:42:42 -0800, wrote:


Here is the theoretical H-plane plot of our Yagi:

http://www.drslick.org/Temp1/yagiplot.jpg

And a photo:

http://www.drslick.org/Temp1/yagi929.jpg

After looking at the picture and the plot I fired up yo
and the only way I could get that low a gain was to
seriously mess up the antenna. The end effect was
a 2 element yagi witha spurious useless element.

This is further confirmed by the picture. It appears
the rightmost element is the fed element (very strange for a three
element design) and the proportions suggest a reflector close to the
driven and longer. The third element (the leftmost) would be
spurious
as it's behind the reflector and far away.

Please explain that yagi, it's strange.


It was an unusual use of the YO
program, in that we used two reflectors
and NO directors, to get the shape we
wanted. We actually couldn't use the
optimizer, because it would actually
try to narrow the beam too much.
Remember that you must keep the
pattern about 180 degrees, and the F/B
ratio should be no more than about 11 dB.

If you could come up with something with
a higher overall dBi, i'd like to know,
but i doubt it.


Slick

On 6 Feb 2005 18:10:11 -0800, wrote:


snip


It was an unusual use of the YO
program, in that we used two reflectors
and NO directors, to get the shape we
wanted. We actually couldn't use the
optimizer, because it would actually
try to narrow the beam too much.
Remember that you must keep the
pattern about 180 degrees, and the F/B
ratio should be no more than about 11 dB.


Slick

Slick,
Could you explain the use of two reflectors? I've heard that if you
are building a two element yagi it's best to use a radiator and
reflector but haven't really learned anything about using more than
one reflector. Have seen antennas that use 3 reflectors, but they
were mounted perpendicular to the plane of the elements and there were
several director elements - if memory serves me it was Cushcraft that
did that. Haven't seen them added behind the first reflector and am
curious as to what characteristic of the antenna's performance it is
meant to enhance.

Thanks,
Howard
Howard

You can get just about any "theoretical" result if your theory is
adequately flawed. That's obviously the case here.

Roy Lewallen, W7EL

On 6 Feb 2005 18:10:11 -0800, wrote:

Remember that you must keep the
pattern about 180 degrees, and the F/B
ratio should be no more than about 11 dB.

If you could come up with something with
a higher overall dBi, i'd like to know,
but i doubt it.

Want a list...

MOXON ( two)
Batwing
Asymetric array of dipoles
Phased array of vertical monopoles
Dipole against a plane relector
2 element Jpole yagi


Allison

Howard wrote:
On 6 Feb 2005 18:10:11 -0800, wrote:


snip


It was an unusual use of the YO
program, in that we used two reflectors
and NO directors, to get the shape we
wanted. We actually couldn't use the
optimizer, because it would actually
try to narrow the beam too much.
Remember that you must keep the
pattern about 180 degrees, and the F/B
ratio should be no more than about 11 dB.


Slick

Slick,
Could you explain the use of two reflectors? I've heard that if you
are building a two element yagi it's best to use a radiator and
reflector but haven't really learned anything about using more than
one reflector. Have seen antennas that use 3 reflectors, but they
were mounted perpendicular to the plane of the elements and there
were
several director elements - if memory serves me it was Cushcraft that
did that. Haven't seen them added behind the first reflector and am
curious as to what characteristic of the antenna's performance it is
meant to enhance.


For this particular design, it was
required in that we needed something with a
180 degree pattern, and not to much F/B
ratio. I can't tell you much more other
than we had to manually tweak the variables
to get a decent result...we couldn't use the
program's optimizer, it would have made the
forward lobe too small.

So what ends up happening in the program,
is that you end up using two directors, but
in the H-plane the lobe goes to the rear (see
link). So in essence, you flip the whole
deal around, and your two directors are
actually reflectors. Interesting, eh?



Slick

wrote:
On 6 Feb 2005 18:10:11 -0800, wrote:

Remember that you must keep the
pattern about 180 degrees, and the F/B
ratio should be no more than about 11 dB.

If you could come up with something with
a higher overall dBi, i'd like to know,
but i doubt it.

Want a list...

MOXON ( two)
Batwing
Asymetric array of dipoles
Phased array of vertical monopoles
Dipole against a plane relector
2 element Jpole yagi



Remember that you must keep the
pattern about 180 degrees, and the F/B
ratio should be no more than about 11 dB.

I really doubt if you will
supply a link...


Slick

Roy Lewallen wrote:
You can get just about any "theoretical" result if your theory is
adequately flawed. That's obviously the case here.


Yagi optimizer programs
with genetic algorithms are well
understood and are quite accurate.

Come listen for yourself...


Slick

On 6 Feb 2005 21:22:19 -0800, wrote:

Come listen for yourself...

Possibly one of the worst methods of absolute measurement.

However, your claim ignores that volume is invariant to signal
strength by design at the limiting stage in any FM receiver.

73's
Richard Clark, KB7QHC

You are lucky to see the gain you have, considering the lack
of decoupling...Or at least in that pix....Also, thats a
pretty lame yagi, so like you say, not really a fair test...
My 3 el vertical yagi would eat that j pole for lunch, being
it has about 9.4 dbi gain...Or about 7.3 dbd....
The J poles are ok I suppose, and are good for people with
copper and torches...But they would be even better with some
additional decoupling...I bet they still have trouble matching
the old isopole that was made by AEA...That antenna was well
decoupled. Looked like a ballistic missile, but it was hard
to beat as far as performance. It was generally the benchmark
most other dual 5/8 designs were judged by.
:/ MK