passive reflectors

[Drake Diedrich]

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