Most precisely it is a form of half waves in phase. Putting half wave
radiators in a line like that ( also called co-linear because they are on a
common "co" line "linear") gives gain by compressing the radiation into the
volume ( a disk or flying saucer- like shape ) perpendicular to the "line"
of the elements.



It is a common type used in cellular base stations. Each section is
approximately a half wave long. It is approximate because of the velocity
factor of the coax and other factors. Reversing the center and shield for
each section is the way the sections are all brought into the same phase.



I believe the ideal spacing (for gain) of half waves in phase is 1/2
wavelength. However the "reversed coax" construction is easier. It does
not need some type coaxial phasing lines to feed all the separated elements
and do it in phase.



The use of the term "sector" (by the author of the web site referenced) for
each of the coax sections may be a misuse of the term "sector" which comes
from the antennas common use in the cellular field with an added corner
reflector that makes it have a 120 or 60 degree-wide pattern and thus used
to cover only one Sector of a sectorized cellular site. Wi-Fi base stations
can also have 3 or 6 sectors and use this type of sector antennas (with the
reflector).



Hope this helps understand.



73, Steve, K9DCI




"KG0WX" wrote in message
oups.com...

http://wireless.gumph.org/content/4/...s-antenna.html

The author calls it a sector antenna but I'm sure I've seen it in ham
applications but with another name.

Also, can anyone quote me a dbi gain figure ) for this?

Thanks for the help!

Ken KG0WX

Minor addition below ************
"Steve Nosko" wrote in message
...
Most precisely it is a form of half waves in phase. Putting half wave
radiators in a line like that ( also called co-linear because they are on
a
common "co" line "linear") gives gain by compressing the radiation into
the
volume ( a disk or flying saucer- like shape ) perpendicular to the "line"
of the elements.



It is a common type used in cellular base stations. Each section is
approximately a half wave long. It is approximate because of the velocity
factor of the coax and other factors. Reversing the center and shield for
each section is the way the sections are all brought into the same phase.



I believe the ideal spacing (for gain) of half waves in phase is 1/2
wavelength. However the "reversed coax" construction is easier. It does
not need some type coaxial phasing lines to feed all the separated
elements
and do it in phase.



The use of the term "sector" (by the author of the web site referenced)
for
each of the coax sections may be a misuse of the term "sector" which comes
from the antennas common use in the cellular field with an added corner
reflector that makes it have a 120 or 60 degree-wide pattern and thus used
to cover only one Sector of a sectorized cellular site. Wi-Fi base
stations
can also have 3 or 6 sectors and use this type of sector antennas (with
the
reflector).

************
He also says it has "two sectors" when it actually is three half waves in
phase. His "whip" is one half wave.



I also forgot that due to the coax loss factor, the gain is not the ideal,
for close half waves in phase, either. It IS very easy to make, however.



I also wonder if he has the dimensions incorrect with the, as he calls it,
"whip" shorter than the coax sections. I haven't designed these, but it
seems the "whip" should be a free space half wave long and the coax sections
a "coax" half wave long.



Another consideration is the fact that these are not full half wave
*radiators*, but shortened approx to 66%, or 0.33 lambda (when using this
coax). I don’t know how much effect the shorter element (vs. having full
half wave elements at this tip-to-tip spacing) affects the gain compared to
precisely getting 180 degrees of phase shift (or getting non-180degrees to
get closer to half wave radiators) , not to mention the feed impedance. I
believe commercial antennas like this use air line to get closer to true
half wave radiating elements.





Hope this helps understand.



73, Steve, K9DCI











Hope this helps understand.



73, Steve, K9DCI




"KG0WX" wrote in message
oups.com...

http://wireless.gumph.org/content/4/...s-antenna.html

The author calls it a sector antenna but I'm sure I've seen it in ham
applications but with another name.

Also, can anyone quote me a dbi gain figure ) for this?

Thanks for the help!

Ken KG0WX

Steve Nosko wrote:
Another consideration is the fact that these are not full half wave
*radiators*, but shortened approx to 66%, or 0.33 lambda (when using this
coax). I

Then that's a big problem.

Unless the outside of the cable has the same Vf as the inside, the
sections won't have the correct phasing.

Phelps Dodge did that two ways.

First, when they used solid dielectric coax, they filled the antenna
with a very thick wax that gave the correct Vf on the outside of the
shield.

Second, when they used air insulated coax inside, they suspended the
coax in air with only an occasional piece of foam for spacing.

There is more to this antenna than just throwing some .66vF sections in
series! Every coaxial collinear I have built and measured from articles
in Ham mags has had virtually no useful gain at zero degrees angle over
a dipole.

73 Tom

wrote in message
oups.com...

Steve Nosko wrote:
Another consideration is the fact that these are not full half wave
*radiators*, but shortened approx to 66%, or 0.33 lambda (when using
this
coax). I

Then that's a big problem.

Unless the outside of the cable has the same Vf as the inside, the
sections won't have the correct phasing.

If I sounded like I was saying it was a "good" antenna I'll clarify.

As I haven't actually measured any, I may be talking out of you-know-where,
but will anyway...

To pick at a point.
I believe the phasing comes out correct. That is, you should be able to
get 180 degrees change in the coax sections. However, because the resulting
outside radiating element is shorter than a half wave (and they are touching
end-to-end ), the element spacing is closer that you get with even full half
waves stacked end-to-end and therefore the lobe compression will be less.

Also, as the phase departs from 180, due to measurement errors, you get
further degradation and then the coax loss means unequal power in the
elements, so things go down hill fast. I would suspect that simply putting
a 1/4 antenna outside the box would give same-or-better performance --- and
save the possiblilty of cutting up your fingers with the Exacto.

In spite of the bad science, with RF being RF, it *could* work out that
just having some lesser power extricating itself from the upper quarter wave
element may provide the poor bloke with improved reception. What do they
have inside the computers or cards anyway? --- it most likely isn't a clean,
in the open, well defined antenna, but some compromise design. Good enough
is always good enough even if good enough *is* 20 dB down.
Another source of false science is the location/orientation of this
antenna and the one it is talking to. This construction (assuming he mounts
it upright the way we might think) is assuming that the WiFi antenns is
horizontally in line with his and the polarization where he is, is vertical.
You have an RF sea to swim in.

73, Steve, K9DCI



Phelps Dodge did that two ways.

First, when they used solid dielectric coax, they filled the antenna
with a very thick wax that gave the correct Vf on the outside of the
shield.

Second, when they used air insulated coax inside, they suspended the
coax in air with only an occasional piece of foam for spacing.

There is more to this antenna than just throwing some .66vF sections in
series! Every coaxial collinear I have built and measured from articles
in Ham mags has had virtually no useful gain at zero degrees angle over
a dipole.

73 Tom

Well, folks I finally decided to test my new antennas. First, I decided
to shorten the antennas to 4 elements instead of 8 so I could mount
the antennas directly on the back of my thinkpad's screen. The 8 el
jobs would have made life harder, requiring connectors, mounting
plates, etc. All that for just 3db more gain. Nah...

I went on a "war walk" this morning, scanning my neighborhood for any
WiFi access points. I've got a neighbor down the street who has one
and with my old antennas (2 inverted V's), I could pickup his network
from about 5 houses away. With the new antennas, I picked up his system
from twice the distance. Nice! As I understand it, a 4 el coaxial
colinear
antenna will have about 6dbd gain so getting double the range seems
right. I was curious if my setup was directional so I got to the fringe
coverage range and turned around 360 degrees but saw no drop in the
signals. It seems like microwaves go right through my screen (and
me!).

Now to construct a pair of 8 el antennas for my WRT54G router.....

Disclaimer: I am NOT trying to "hack" free internet access from the
neighborhood WiFi 802.11b/g systems - that's not what wardriving is
about......

Ken KG0WX

KG0WX wrote:
. . .
Disclaimer: I am NOT trying to "hack" free internet access from the
neighborhood WiFi 802.11b/g systems - that's not what wardriving is
about......

What is it about?

Roy Lewallen, W7EL

It's about radio, internet, technology....

Kinda similar to ham radio.... I mean - why use 20 meters when the
telephone is right there in the shack?

Get it? :-)

Ken

"Roy Lewallen" wrote in message
...
KG0WX wrote:
. . .
Disclaimer: I am NOT trying to "hack" free internet access from the
neighborhood WiFi 802.11b/g systems - that's not what wardriving is
about......

What is it about?

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
KG0WX wrote:
. . .
Disclaimer: I am NOT trying to "hack" free internet access from the
neighborhood WiFi 802.11b/g systems - that's not what wardriving is
about......

What is it about?

Roy Lewallen, W7EL

Oh, there's a certain element of fun in "seeing what works." Try to find a
ham who's never tried
anything goofy -- it can't be done!

At that frequency, you can do a great deal with a cone of metal mesh (a
minor purchase at the
hardware store). Put the small end around the antenna and the open end
pointed in the direction
of the desired signal.

My entry into that band was not ham or Wi-Fi, but MDS television. There
is a TV band around 2300 MHz. (I don't really want to go out in my cold
garage and look
it up.)

I bought a downconverter for that band (which was used for porno movies
in my area at the time) but I didn't have enough signal. So, I started
goofing with mesh cones.
I got big time improvement with very little effort.

If the aim is just experimentation, then a three-foot metal mesh cone will
actually be an appealing
feature of the setup.

KD6VKW