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.

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

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.
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.] That gain (7.4 DBI) is easy to beat with a 3 element beams on 2m
(typical is 8dbi) though a simpler half square is 9.4dbi. Having
built many antennas and examples of the three mentioned I find the
models are accurate. However, if you cannot measure and test the
antenna effectively you cannot evaluate if it's working as predicted
or if its working at all. At VHF execution is everything and subbing
1/4 inch tube for a modeled 1/2 inch one will give error, sometimes
significant. I'd add that at VHF the feed section is critical and if
not done well the rest of the antenna is a waste of metal. 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.

If you care to read www.cebik.com/jp1.html all four sections. It's
an indepth treatize and explanation of J-poles and varients. Based
on my experience he's hit the nail accurately.

Oh, and I use two Jpoles (arrow OSJ146/440 and a Copper cactus
for 2m/440) and both match the model for performance in the real
world. Obviously I'm neither pro Jpole designs nor am I against them.
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.


Allison

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