confusion about wlan antennas

I've been messing about with my wireless card and I was using a 6db omni antenna mounted
outside the house (size-wise it's like half a broomstick). My Windows XP PC showed 6 AP's
in 'View Available Wireless Networks' , I got a couple of strong signals and the rest were
weak. For comparison I then took down this antenna and put up a 15db omni antenna
(bigger - 1.6meters long) in its place. What puzzles me is the 15db antenna picked up 12
AP's - which is double the previous antenna - but the signal of all AP's found was weak,
not as strong as when using the 6dB. Is this normal behaviour for high gain antennas? What
factors might cause this? poor wiring maybe? For the 15db antenna I use RG213 cable with
N-connectors and for the 6db one I used ordinary TV coax with TNC. Cable length for both
antennas is about 10 meters. Thanks for any pointers.

On Jul 8, 7:07*am, "tg" <t...@nospamevereverever.net> wrote:
> I've been messing about with my wireless card and I was using a 6db omni antenna mounted
> outside the house (size-wise it's like half a broomstick). My Windows XP PC showed 6 AP's
> in 'View Available Wireless Networks' , I got a couple of strong signals and the rest were
> weak. For comparison I then took down this antenna and put up a 15db omniantenna
> (bigger - 1.6meters long) in its place. What puzzles me is the 15db antenna picked up 12
> AP's - which is double the previous antenna - but the signal of all AP's found was weak,
> not as strong as when using the 6dB. Is this normal behaviour for high gain antennas? What
> factors might cause this? poor wiring maybe? For the 15db antenna I use RG213 cable with
> N-connectors and for the 6db one I used ordinary TV coax with TNC. Cable length for both
> antennas is about 10 meters. Thanks for any pointers.

My theory is: It is all related to the meaning of "antenna gain". Why
is it that one is said to have 6dB and the other 15dB? Think for a
second... antennas does not have power plugs, so they don´t add
power... how one can have more "energy" than the other?
Here lies is the answer for your question. What an antenna with bigger
gain does is concentrate the signal in a smaller area (or volume if
you will, as it is 3D), so the area that is serviced get more signal
(sacrificing other areas that will bet weaker signal).
The same happens with reception, the bigger the gain, the smaller is
the region it will do a good "listening". So an omni 15dB antenna only
gets good signal from APs antennas that are on the same height as it.
If the origin AP is slighter higher or lower (can be as little as few
metters) it will get a weak signal.
Your 15dB can hear far, but your neighbors APs (the first 6 ones) are
not so well aligned with it, so the signal gets weaker.

tg wrote:
> I've been messing about with my wireless card and I was using a 6db omni antenna mounted
> outside the house (size-wise it's like half a broomstick). My Windows XP PC showed 6 AP's
> in 'View Available Wireless Networks' , I got a couple of strong signals and the rest were
> weak. For comparison I then took down this antenna and put up a 15db omni antenna
> (bigger - 1.6meters long) in its place. What puzzles me is the 15db antenna picked up 12
> AP's - which is double the previous antenna - but the signal of all AP's found was weak,
> not as strong as when using the 6dB. Is this normal behaviour for high gain antennas? What
> factors might cause this? poor wiring maybe? For the 15db antenna I use RG213 cable with
> N-connectors and for the 6db one I used ordinary TV coax with TNC. Cable length for both
> antennas is about 10 meters. Thanks for any pointers.

I am not an RF guru, but a little reading around suggests to me that

a) you should definitely be using a 50 ohm coax like RG58 or your RG213
rather than standard 75 ohm TV drop cable, which has high frequency
losses which are just way too high, and

b) those cable runs are long - you can get signal attenuation well over
1 dB/meter at 2.4GHz, even with RG58 (I don't know about RG213, but it's
probably a little better).

I think the rule of thumb is to put the router as close to the antenna
as possible, then run CAT5 from the router to where you need it.

Compare these two articles, for instance:
1. http://en.wikipedia.org/wiki/RG-59
2. http://en.wikipedia.org/wiki/RG58