On 7 Feb 2005 21:27:32 -0800, wrote:


I couldn't find anything there that was similar
to this:

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

You can lead a horse to water, still can't get it damp.

You'd have to look at the data sheet. All the commercial people
are aware of this and it's accepted practice.

What you missed is a 2 or 4bay dipole is a really nice antenna
that can offer gain and pattern control. The usual use is a 4 bay
vertically oriented with each of the 4 dipoles spaced 90 degrees
around the mast for 5.6db omnidirectional gain. Now, if you want a
directional pattern, such as cartioid then put all four on one side,
also expect slightly higher gain as well. The commercial version are
expensive but are known for their durability but, the good news is
they can be built using copper pipe and will give the same
perfomance. I might add, the gain numbers I gave are not theory,
they are real numbers from proven designs.

Ok, here are some links one how to build it..
http://www.repeater-builder.com/rbti...seddipole.html
some good info here, the metal used is less desirable than
CU pipe. The general design is proven.

http://www.w4dex.com/kc4fwc/ant.htm
Info on making a phasing harness, needed for a 4bay but not
rocket science.

http://dipole.w4zt.com/ look at the plumbers dipole page.
This can be built as a 1/2/4/8 dipole array. The limit is 4
based mostly on size though I've seen one 8 dipole array
and it's long(nearly 45ft!) but very effective. I might add if the
mast pipe is made with 1" copper it can also serve as the supporing
mast. This is a prefered design and offers good all grounded
construction (lightining avoidance and static noise reduction).

The two dipole array with both on one side of the mast is the same
gain as the super jpole with two differences. the 4 bay will be longer
but offers a real gain increase and slightly better pattern control.
The gain increase for this type antenna is predictable, being 3DB
for each time you double the elements. The single being 0 DBd,
2bays 3Dbd, and 4bays 6Dbd . Thereal world the practical antennas
built as omnidirectional are really 0, 2.8 ,5.6dbd due to small but
measureable losses in the cables. If the elements are lined up
on one side the gain is higher (sme claim 9db) with a cartiod
pattern. It's all copper and no required insulators and can be built
more robustly. The pattern is predictable, less is left to chance.
This type of antenna also works well against the side of a metal
building (less tuning difficulties) though you will get a more
directional pattern from the building shielding the opposing
direction. These designs will you get away from theory and use
practical designs.

A repeater group I work with locally used the DBproducts 4bay and
found it the best antenna they've put on the tower to date. It wasn't
cheap and it was heavy. Experience at that site was anything less
robust would barely stand a year before the SWR went to unacceptable.
I've built the 4bay for UHF and it's a solid performer. The all
copper design weathers very well, is very cheap to build
and performs just as well as the commercial versions which are welded
up from aluminum.

I'm sure your getting results from the Jpole but, I can be certain
from using them myself that your results are part luck and can easily
be attributed to the added height (the 70+inches can really help) and
placement more than the presumed gain. If you compare a yagi or
dipole to a J-pole the radiating element must be the same height.
Since the J-pole is really an end fed dipole it has a 38 inch
advantage. For a yagi the centerpoint is the boom.. In direct
comparisons the yagi you built would have to be minimally 3ft higher
to compete on fair ground.

J-poles in general do not like to be near (less than 1/2wave) metalic
structures as it detunes them. Yagi behavour is also sensitive to
metalic structures close to them. The Yo model is not complete
enough for those cases.


Allison
KB1GMX