On 5 Dec 2004 18:28:44 -0800,
(E-Mail Removed) (Joshua
Anderson) wrote:
>It needs to be omni, I need to catch as much area as possible, 360
>degrees if possible.
Aw, I was kinda hopeing for a big 6 ft dia dish on the roof. Oh well.
I'm philosophically opposed to using outdoor omnis (as I stare at the
8dbi omni plugged into my access point). My problems with omnis are:
1. They spray RF in directions where there are no users. They are a
major source of interference when used outdoors.
2. With sufficiently high gain to be useful, they have a very narrow
vertical radiation pattern that easily causes roof or tower top
antennas to shoot over everyone's heads.
Ideally, a 120 sector antenna should be used outdoors. If you need
360 degree coverage, you use 3 sector antennas, and 3 access points.
If high enough, use some downtilt as the vertical radiation angle is
still fairly narrow.
http://www.tiltek.com/products/anten...series=TA-2400
For example, the 14dbi gain will give a 7 degree vertical radiation
angle.
Of course, omni's are much cheaper than sectors and multiple radios.
(Remember, you asked for the best, not the most optimum or cost
effective antenna).
>If I cannot get access directly under the antenna thats fine I will
>use another AP for that area, what I really am looking for is extended
>coverage in the wider vacinity.
Well, some numbers here would be nice.
1. What kind of range were you expecting? Figure on 6dB of gain
equals double the range.
2. To what type of client radio? Outdoor antennas? EtherAnt?
Indoors? Laptops? PDA's?
3. Any vertical changes in elevation? This is critical with only a 5
to 7 degree vertical beamwidth.
4. How many clients? How much traffic? If larger, perhaps 3 radios
would not be such a bad idea.
Basically, I can't suggest a specific gain and beamwidth configuration
without some clue as to the topography, technology, and use. Once the
required gain and beamwidth are determined, the rest is just picking a
suitable manufacturer and reputable supplier.
>Roof wise it will be about the standard house size, with the antenna
>mounted at the highest point obviously.
One, two, or three story? Hillside (like mine)? Any vertical changes
in altitude to the client radios?
>I assume a dish is no good for omni, I was looking at the Dlink 15dbi
>antenna:
>http://www.dlink.com/products/?sec=&pid=289
>This looks ok to me, price is reasonable too, is this something I
>could use?
$190/ea is not my idea of reasonable. 15dBi gain, 5 degree vertical
beamwidth, and no downtilt. Overpriced.
I recognize the antenna but I don't recall the manufacturer. I'll see
if I can remember in the morning (after I recover from hauling most of
my magazine pile down 50 stairs and overfilling my truck. Recycling
hurts).
Dig through:
http://www.fab-corp.com/I1.htm
$120 for 15.4dBi, ?? vertical beamwidth, 3 degree downtilt. Made by
Comet (NCG):
http://www.cometantenna.com/products...=15&childID=24
but apparently discontinued. My guess(tm) is that the shipping and
handling costs of these antennas killed them. They're one piece
construction and over 6 ft long. Awkward is an understatement.
I've had problems with high gain (over 8dBi) omnis on 2.4Ghz in that
the group delay and vswr at the band edges are awful. I can usually
make them work on mid-band Channel 6, but 1 and 11 are problematic. I
was asked to recommend a fix for a mountain top installation with one
of these 15dBi antennas on Channel 11. I just moved it to Channel 10
and all the data errors went away. Eventually, it was replaced with a
mess of sector antennas, and most of the close in coverage issues went
away.
I would only use a 15dBi omni if I were absolutely stuck with using an
omni, were on flat ground, and needed maximum penetration in all
directions. Also, remember that a high gain antenna also hears as
well as transmits. With very high gain, you're going to pickup much
more interference (other users, wireless TIVO, microwave ovens, etc)
than with a lower gain antenna. If you're in an area of high
concentration of wireless contraptions, a very high gain omni is not a
great idea. I would consider 12dBi about right.
One more problem and another reason I detest high gain omnis. With
vertical colinear antennas, most of the radiation comes from the
BOTTOM of the antenna. The guts are very roughly alternating 1/2 wave
sections, with a 1/4 wave top section, and a sleeve matching section
at the bottom. Depending upon the design, only every other half wave
section radiates. Half of the RF spews from the first half wave
section. 1/4th comes from the next radiating section. 1/8th from the
next. 1/16th from the next and so on. By the time we get to the top
section, there's very little RF left. The antenna may be 6ft tall,
but bulk of the signal comes from the bottom.
While I'm bashing omnis, there's also the problem of proximity to big
pieces of metal. As the gain goes up, the antenna has to be farther
away from nearby tower structures. Such an antenna will work well on
top of a pole, but will need to be fairly well isolated if mounted on
a tower yardarm. (This is not a problem with sector antennas). Yes,
you can mount the omni upside down. I've done that now 4 times and it
works nicely but attacts far too much questions.
One more and I'll shut up. High gain colinear antennas have a nasty
tendency to create uptilt in the pattern. With only 7 degrees of
vertical beamwidth, that's a big problem. Let's pretend you end up
with 3.5 degrees of uptilt. That means that *ALL* your signal is
going above the horizon. That's great for talking to distant
airplanes, but not very useful for talking to ground based stations.
You could also build your own from a kit:
http://www.aerialix.com/support.html
http://www.guerrilla.net/reference/a...r_omni_lowpwr/
I built a similar antenna that worked nicely until I got cheap and
used a too small ABS plastic pipe filled with polyurathane fence post
compound. The resonant frequency went from about 2430 to to a useless
2380. Oops. It still works on Channel 1 but nothing above that.
Forget the fence post compound idea.
--
Jeff Liebermann
(E-Mail Removed)
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558
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