[lug] cheap 802.11b for linux...

[D. Stimits]

Rob Nagler wrote:
> 
> > RG-58 at 2.4 Ghz is just a leaky sieve.  Very little of the very little
> > power the cards put out will make it to the antenna.  However with the
> > attic mount thing, since received signals should do okay, it may still
> > just work and it's a lot cheaper than running heliax or similar.  It's

It would be interesting if one of the 802.11b's had an ethernet plug on
it, and could be used independently of the laptop. Having the
transceiver portion mounted directly at the antenna and running ethernet
to it could have some advantages (though need for weather-proofing is a
problem).

> 
> I definitely had loss.  The big problem is I don't know how to measure
> it.  I was walking around my neighborhood with a laptop and a number
> of antennas (which everybody thought was very interesting ;-), and it
> seemed to me, the cable length had nothing to do with the signal
> quality.  I was getting very good quality transfer speeds (500KBps)
> using 10 feet of RG 58.  As I walked hundreds of feet away, the signal
> remained good as long as it was line of site.  I don't have the data
> right here.

I'm curious if there are any SWR meters for low-powered 802.11b lines.

> 
> Perhaps I don't understand radios well enough to understand the
> effects of loss in the cable vs. distance.

If not interested in trivia, please skip this next paragraph, it could
be painful to those who hate irrelevant details that nobody needs to
know.

WARNING, BORING TRIVIA: Consider that there are ways to store electrical
energy. Batteries are chemical storage. Magnetic coils, when they build
up a magnetic field, store energy; they release energy when the field
collapses. Capacitors build up a static field, and also release energy
as the static field collapses. Now if there is physical resistance in a
magnetic coil, some of the power during buildup or collapse of the field
gets turned into heat...the longer the wire the more resistance. On a
capacitor, sometimes the space between plates has a special material in
it (the dialectric), and it can provide more ability to store static
charge, but it also has some heat loss also (a more efficient dielectric
can either store more charge for a given amount of heat, or store the
same charge for less heat). The entire cable length is a capacitor and
also an inductor. The interesting part comes in the fact that voltage
and current buildups between capacitors and inductors are out of phase
with each other. If you have the right combination of capacitance and
inductance, they complement each other, and have a net result of no
apparent capacitance or inductance...that is the resonant frequency.
That is what you want from your antenna line. The antenna itself is just
the opposite: Since energy cannot be created or destroyed, if you
arrange it such that signal reflecting through the antenna perfectly
cancels itself out, the energy has no place to go and must radiate. The
standing wave ratio (SWR) of forward and reflected power is perfectly in
phase in the transmission line, and perfectly out of phase in the
antenna. If you have a meter (SWR meter) that can measure the phase
differences, you can use it to tune the antenna and make sure the line
going to it does not radiate. The goal is to have the wire radiate
nothing, and the antenna radiate everything...if you succeed at that,
then your only losses are dielectric heat and wire physical resistance
heat. If your antenna and line are not tuned correctly, then the line
itself acts as an antenna, but has poor radiation patterns during
transmit; during reception, some of the received signal would get
re-radiated and never make its way to the receiver (plus the line would
be receiving energy from wavelengths you are not interested in, and
basically require the receiver to separate out more noise). So loss
versus distance really depends not just on length, but also on how well
tuned it is...if it is perfectly tuned, then it is simple to figure out
how well it will do based soley on the wire resistance and dielectric
efficiency. Doubling length would double heat lost to resistance, and
heat lost to dielectric heating. The antenna lines at WWV, and the power
supply capacitors, are rather interesting (I grew up with someone who's
dad ran WWV back then). The lines are literally gas pipes, filled with
low pressure cooled nitrogen, and a huge center conductor sits in the
middle suspended only at intervals (at least this was how it was long
ago). An antenna line with a pure vacuum instead of a dielectric would
in theory not generate any heat at all, but then the diameter ends up
larger for the same capacitance. The larger and fatter wires could use
the same conductor size, but be more efficient just because they use a
better dielectric. The tough part is that as you go up in frequency, to
shorter wavelengths, smaller and smaller physical distortions of the
line begin to cause more profound defects to the desired resonant
frequency, so the nick in the copper, or the slight kink from stepping
on the cable, will end up hurting more. Even if you put out the effort
to tune the line and the antenna correctly, the same effort will be less
perfect on shorter wavelengths (2.4 GHz is slightly shorter wavelength
than a microwave oven at 2.3 GHz, which was chosen because it is the
resonant frequency of water molecules). Then there is spread spectrum
technology...

D. Stimits, "boring trivia", stimits at idcomm.com


> 
> There are also ways to increase the output of the Linksys by twiddling
> some registers.  (Jonathan, did you ever do this, or did you send the
> link to me so I could blow up my access point first?  ;-)
> 
> The cost thing is less important to than the line-of-sight thing is.
> I had notions of creating a business (probably non-profit) out of this
> until I actually tried it.  There is at least one company trying to do
> this, but I believe it just won't work with the existing technology.
> My guess is if anybody made a one amp transmitter, it would be the way
> to go.  Again, my knowledge of radios is almost zero.
> 
> Rob
> 
> _______________________________________________
> Web Page:  http://lug.boulder.co.us
> Mailing List: http://lists.lug.boulder.co.us/mailman/listinfo/lug