[lug] cheap 802.11b for linux...

[J. Wayde Allen]

On Thu, 21 Mar 2002, D. Stimits wrote:

> 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).

In the last couple of weeks I've learned that these are called access
points.

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

It is certainly possible to measure SWR at these frequencies, but the
question is really why you'd want to and whether or not there is much
meaning in this for 802.11b?  The standing wave ratio or SWR is a
relatively poor measure of the effect of impedance mismatch (no phase
information).  A more complete measurement would be the reflection
coefficient.  The problem is that 802.11b is a spread spectrum signal.  
You'd need a single frequency signal to generate a standing wave on the
line, so you couldn't just hook a meter in the line between your 802.11b
transmitter and the antenna.  You'd really need to look at the mismatch
characteristics of the antenna/transceiver combination across the signal
bandwidth.  Very doable, but I doubt you are going to find an inexpensive
SWR meter for the job.  The best tool would be a network analyzer.  The
last one of these I bought was from Hewlett Packard (now Agilent) and cost
$100,000.

> That is what you want from your antenna line.

No, you don't want this to be resonant.  You want to couple the energy in
the line to the free space impedance of typically 377 ohms.

> 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.

There is no phase information in an SWR measurement.  It is the ratio
between two E-field magnitudes.  This destroys the fact that the E-field
description is a complex number and simultaneously throws away phase
information.

> 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. 

In a spread spectrum system you need the antenna to be broadband, not
tuned as you imply.

> 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.

Not true.  Your dielectric loss would go away, but you'd still have ohmic
losses in both your inner and outer conductors.

> 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.

Not resonant frequency, but the impedance of the line.

- Wayde
  (wallen at lug.boulder.co.us)