passive reflectors

[Chris Hill]

Hi Drake,

The main typo was about signal drop versus distance.

The correct statement is:

Every time you double the distance, you lose 6dB.

There are a few assumptions behind this, but it doesn't get any better than
that statement... only worse.

So whatever the signal strength is at 2m distance, it'll be 6dB weaker at
4m, 12dB weaker at 8m.


Something else worth mentioning;  all the PCMCIA wireless cards I've tested
so far, are directional.  My laptop works fine on my desk, with the wireless
card pointing towards the Access Point...  but it drops significantly
(roughly 10dB down) if I spin the laptop through 180 degrees.  On my old
laptop, the PCMCIA port was on the 'wrong' side, so I had to use an external
antenna.

Tim, can you put the access point up in the roof, to get OVER the granite
wall?  What about if you put it very close to the granite wall?


Regards,


Chris



----- Original Message -----
From: "Drake Diedrich" <dld at coyote.com.au>
To: <wireless at lists.samba.org>
Sent: Tuesday, June 11, 2002 8:44 AM
Subject: Re: passive reflectors


> On Mon, Jun 10, 2002 at 01:39:11PM +0100, Timothy Murphy wrote:
> >
> > but going through a very thick (1ft) granite wall.
> >
>
>    We have hollow concrete brick walls work, and we get very little signal
> through them.  To find out how little, and whether there's hope, try
putting
> your two transceivers within a few inches of the wall on either side, and
> check your signal strength.  If there's good signal right up against the
> wall, but it vanishes over the 8-10m, you might get enough through using a
> higher gain antenna (5-8dB omni-directional, say 1-2 feet tall).  If you
> know how the card's signal strength matches to dB, you could calculate it
> exactly, but this is seldom published.  All other things being equal,
every
> time you double the distance you lose 3dB, so if it works at 1m, to get to
8m
> you'll lose another 9dB.  If you get nothing even at this range there's
> probably no point even trying to use moderate gain antennas (on either
side)
> to boost your signal.  Two 8dBi antennas, compared to the two 2.2 dBi
> antennas already in the wavelan cards, would gain you 11.6 dB.  Each are
> about 2 feet tall (for omnis), or a bit smaller for patch antennas.
> If you stick to one 8dBi omni at the base station, and the built-in 2.2
dBi
> dipole on the wavelan card, you'll still have gained 5.8 dB over the
> original path, if you were already close (ie, it works at 2-3 meters, but
> you want 8m).
>
>    Our walls aren't quite solid though (hollow spaces), and are a bit less
> than 1 foot thick.  We get good signal though through the floors and
> ceilings, and so are ending up with many tall, skinny wavelan cells.
> It's possible that no signal is travelling through your walls at all, and
> that the signals you are getting are in fact bouncing off walls/metal
> cabinets/etc and going through doorways and ceiling spaces that are not
> blocked by granite.  If this is the case, a higher gain antenna might even
> hurt your transmission, if aimed right at the wall, but if you aim it
around
> you might stumble on the path that the weak signal was getting through.
>
>    I suppose drilling holes for regular cables through presumably historic
> (and hard) 1 foot granite isn't an option..
>
> 1 Bel = an order of magnitude more signal (10x).
> 10db (deciBels) = 1 Bel.  In other words: (1dB)^10 = 10x
> 1dB, when being listened to, is about the limit of human perception in
> signal strength difference.  3dB is about twice as much signal strength.
>
> dBi = dB above an isotropic antenna (theoretical completely
omnidirectional)
> dBd = dB above a 2.2 dBi dipole antenna (most basic antenna, what's
usually
>       in wavelan cards)
> dBm = db relative to 1mW real power.  So 1mW = 0dBm, 10mW = 10 dBm,
>       100mW=20dBm, 1W=30dBm, ...
>
> Receivers are often rated in minimum dBm (sensitivity), while wavelan
> transmitters are in either dBm or in mW.  You just subtract all the loses
> over your path from your transmit power, and if it's greater than your
> minimum receiver sensitivity, you should (in principle) get signal
through.
> Less than a 10 dBm margin though is pretty unlikely to actually work.
>
>
>    Another thing that might work: lower frequency transceivers.  The
> 2.4 GHz that 802.11[ bg] uses has a wavelength of 13cm.  Every 13 cm
through
> an object it tends to decay rapidly.  Moving to 900 mHz (many pre-standard
> wireless cards, mobile phones, pagers, cordless phones,
garage-door-openers,
> ... use this frequency) has a wavelength close to 1 foot, and might
> penetrate much better.  Granite though, as far as I know, is a pretty poor
> medium for carrying electromagnetic waves.
>
>    (if I made a typo or something in there, I'm sure some of the real
radio
>     people here will correct it).
>
> -Drake
>
>
>