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