On Thu, 01 Sep 2011 09:35:37 -0500, amdx wrote:

Ya, I have my doubts about the 12 dbi gain figure.

Everyone lies about gain, but that's ok because few people can measure
the gain (and get reproducible results).

I have built a few
biquads, (with your feed point correction), it has amazed me how easy
it is to make them work (at 2.4 Ghz), I always thought the short
wavelength would cause many dimensional errors. Yes, they work, what
does "work" mean :-)

For a given physical size antenna, high gain antennas imply narrow
bandwidth and critical construction. On the other foot, low gain
antennas, such as the biquad, is fairly broadband, and therefore not
particularly critical to construct. What's fun is to attach the
antenna to a reflection coefficient bridge or directional coupler,
http://pe2er.nl/wifiswr/
http://www.qsl.net/n9zia/rlb/texscan.png
http://www.qsl.net/n9zia/rlb/
sweep generator, and oscilloscope to look at the VSWR curve. Then try
moving things around. On my crowded workbench, location of the
antenna relative to the highly reflective test equipment make a huge
difference. The changes do not really have a big effect on antenna
operation, but they certainly present a different picture as compared
to the nice clean curves on the data sheets.

Do you believe the 20 dbi figure?
http://www.ebay.com/itm/20-DBi-2-4GH...25749399603295

I guess you missed my previous rant on the subject. I bought two of
those yagi antennas (for $6/ea incidentally) just to see what was
wrong with them.
http://802.11junk.com/jeffl/crud/wi-fi-yagi-that-sucks.jpg
Notice the really crude dipole driven element hiding under the
plastic. That isn't going to work. No balun, no matching, exposed
coax wires, and offset from the center line too much. Some tests
showed that it has more gain to the side than towards the front. I
also suspect that the rather large size boom diameter was not
considered in calculating the element lengths. A piece of total junk,
but at the price, probably sells well (which is why I like the antenna
biz).

The above yagi is apparently a cost reduced clone of a similar yagi
that does have a proper feed and a more realistic gain claim. I think
(not sure) that this is the one:
http://www.ebay.com/itm/250847584296
I have no idea if this one works any better.

This is a more reputable source:
http://www.comtelcoantennas.com/yagi_1_8_2_4_ghz.htm
Note the radically higher prices and the 14dBi gain for a 16 element
yagi:
http://www.comtelcoantennas.com/PDF%20Datasheets/Y422416.pdf

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558