On Fri, 8 Feb 2008 11:39:52 -0800 (PST), seaweedsteve
<seaweedsteve@gmail.com> wrote:

>On Feb 7, 10:11 pm, John Navas <spamfilt...@navasgroup.com> wrote:
>> I would go with panel or dish, not yagi.

>Just out of curiosity John, why is that?

Hint. How long do you thing a 2.4GHz yagi would be in order to equal
the gain of a 24dBi dish antenna? (Answer is at bottom of this
message).

>I was just looking at a 2.4 Ghz Yagi, which are commonly used for
>cellular, and wondering why panels and dishes are more common in WiFi.

Well, cellular is at 800/1900Mhz in the USA. That makes for a
somewhat larger antenna. A dish for 900Mhz would just be too large.
Many cellular yagis also end up in peoples car trunks for long range
use when in remote areas. A dish just wouldn't fit.

There are two problems with a yagi that makes it a bad fit for Wi-Fi.

The typical yagi has lots of side lobes, which causes lots of
reflections, and potentially picks up interference from the side and
back. Panels and dishes have much less side lobe junk.

The construction of the yagi is fairly critical and complex when
compared to the typical panel and dish. In fact, the dish can be
fairly sloppy, and still work. Same with a corner reflector. The
panel or patch antennas lend themselves to mass production. They're
usually nothing more than a plated circuit board inside. Once the
dimensions are determined, they can be cranked out automagically, with
little variations from unit to unit. I can't say the same for
mechanical contrivances like a yagi.

Yagi's are also susceptible to icing, bird perching, and being used as
a foot peg when climbing a tower. The traditional end mounting causes
problems in routing the coax cable so that it does not detune the
yagi.






Answer:
A 15dBi yagi is about 18" long. For example:
<http://www.mfjenterprises.com/Product.php?productid=MFJ-1800>
The antenna gain increase 3dB for every doubling in the length of the
yagi. So, doing it the hard way:
Gain Length
(dBi) (inches)
15 18
18 36
21 72
24 144
So, to equal the gain of a 24dBi 2.4GHz dish, you need a 12 ft long
yagi. I don't think that would sell too well.



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# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
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