Re: 24+ Dbi antenna, does it work?

BANGBANGMAX <BANGBANGMAX.2ze6a4@no-mx.wirelessforums.org> hath wroth:

>I live in an appartement(8th floor) and there's a big neighborhood at
>about 600meters of my windows. I know there are some wifi signals in
>this neighborhood and i wanted to buy this antenna to receive some of
>those signals... my question is really simple... does it really work?
>will i be able to have a strong connection with that?
>'eBay.ca: Wifi 802.11 g Router And Strong Yagi N Type Antenna (item
>150176202865 end time 02-Nov-07 21:12:10 EDT)'
>(http://tinyurl.com/2puc3y)

I don't believe that thing has 24dBi gain. The picture is
intentionally distorted to prevent the average user from calculating
the length of the antenna. However, I can make a guess by counting
the number of elements (17) by approximately 1/4 wave at 2.4GHz
(3.13cm) to yield an overall length of about 55 cm. No way is an
antenna that short going to have 24dBi of gain. 14dBi is more
realistic. For example, see:
<http://www.mfjenterprises.com/products.php?prodid=MFJ-1800>
which is roughly the same length and number of elements but only
advertises 15dBi gain. They lie.

I just noticed that the antenna on eBay does not have a reflector
element behind the driven element. That will work, but not very well.

Incidentally, to get 3dB more gain out of a yagi, you need to *DOUBLE*
the length. Assuming the MFJ antenna is the advertised 15dBi gain and
is about 45 cm long, a 24dBi antenna would be approximately 360 cm
(141 inches) long.

>And do you think it's going to help if i also buy this:
>'eBay.ca: Indoor WiFi WLAN Wireless LAN Booster Amplifier Router (item
>270181444548 end time 03-Nov-07 11:00:00 EDT)'
>(http://tinyurl.com/37275h)

No. Amplifiers cause other problems. You end up with an improved
transmit range, but no improvement in receive range. Everyone can
hear you, but you can't hear anyone's replies. That's not a problem
with antennas, where the gain is (usually) symmetrical between
transmit and receive.

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

On Thu, 01 Nov 2007 19:14:36 -0700, Jeff Liebermann <jeffl@cruzio.com>
wrote:

>I don't believe that thing has 24dBi gain.

Just for fun, I decided to run a simple simulation of a mythical 24dBi
yagi. See:
<http://www.k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf_quick.html>
The result was 4.2meters (14ft) long monster antenna with 101 elements
Yikes. No wonder nobody sells a real 24dBi yagi.

The beamwidth worked out to about 7.5" degrees in both axes which is
slightly wider than a dish with the same gain.

The parameters I used were:
Freq: 2400Mhz
Gain: 22dBd (same as 24dBi)
Reflector: Per DL6WU
Director: Per DL6WU
Metal Boom (bonded)
Boom Dia: 5mm
Driven: 1mm dia
Parasitic: 1mm dia

Drivel: I actually did build an ultra long 2meter (146MHz) antenna
that was about 25 ft long for a hidden transmitter hung. It was two
pieces of twisted polypropelene rope, with 1/2 wave (38") pieces of
electrical wires suspended between the two ropes. The ropes were then
hung between a tree and a telephone pole. It worked great. I suppose
someone could buld something similar, with 101 elements, and actually
build a 14ft long 2.4GHz rope ladder yagi.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

"Jeff Liebermann" <jeffl@cruzio.com> wrote in message
news:4d1li3daup9kd2kp6djo0fd0in0b5h29ut@4ax.com...
> BANGBANGMAX <BANGBANGMAX.2ze6a4@no-mx.wirelessforums.org> hath wroth:
>
> >I live in an appartement(8th floor) and there's a big neighborhood at
> >about 600meters of my windows. I know there are some wifi signals in
> >this neighborhood and i wanted to buy this antenna to receive some of
> >those signals... my question is really simple... does it really work?
> >will i be able to have a strong connection with that?
> >'eBay.ca: Wifi 802.11 g Router And Strong Yagi N Type Antenna (item
> >150176202865 end time 02-Nov-07 21:12:10 EDT)'
> >(http://tinyurl.com/2puc3y)
>
> I don't believe that thing has 24dBi gain. The picture is
> intentionally distorted to prevent the average user from calculating
> the length of the antenna. However, I can make a guess by counting
> the number of elements (17) by approximately 1/4 wave at 2.4GHz
> (3.13cm) to yield an overall length of about 55 cm. No way is an
> antenna that short going to have 24dBi of gain. 14dBi is more
> realistic. For example, see:
> <http://www.mfjenterprises.com/products.php?prodid=MFJ-1800>
> which is roughly the same length and number of elements but only
> advertises 15dBi gain.

looking at the MFJ antenna -
it would be interesting to measure the length
of a couple of the elements,
and the inter-spacing of the elements...

and then compare that to the photo of the eBay antenna
to maybe get an idea of what freq that are using ??

physics is physics - the wavelengths don't change on eBay -