Subject: [NEXTEL-1] :Re: Calculate Path Loss- [email-masked]

From: [email-masked]
Date: Tue, 04 May 2004 18:29:57 -0400

-------- Original Message --------
From: Jim Lux <[email-masked]>
Subject: PROP-L:Re: Calculate Path Loss ([email-masked])

At 12:02 PM 5/4/2004 -0700, you wrote:

>    1. Re: Calculate Path Loss ([email-masked])
>
>--__--__--
>
>Message: 1
>To: Linda Hamilton <[email-masked]>
>S
>Dear Linda
>
>The answer to your question is "You can't". Of course you can compensate for
>cable-losses and such, but you cannot compensate for the antenna gains. The
>antenna power patterns and the environmental spread patterns form a product,
>which you cannot easily separate into individual factors. Only in a LOS
>situation with infinite K-factor does it make sense to use the classical
>pathloss formulas. Many people make this mistake, but that does not make it
>correct.
>
>A reference where there are some examples is section 10.1 in
>Blaunstein and Andersen, Multipath Phenomena in Cellular Networks, Artech
>House
>2002.
>
>Regards and good luck
>
>Jørgen Bach Andersen
>
>Quoting Linda Hamilton <[email-masked]>:
>
> > Hi!
> >
> > I conducted some indoor propagation measurements using
> > vector network anlyzer (VNA). The measurement setup
> > are as follow:
> >
> > Transmitter (TX) side:
> > Port1 of VNA(set at -10dBm)+1m cable(loss
> > -1.2dBm)+power amplifier(gain +30dB)+2m cable(loss
> > -2.5dBm)+TX antenna(gain 2dBi)
> >
> > Receiver (RX) side:
> > RX antenna(gain 2dBi)+20m cable(loss -31.6dBm))+low
> > noise amplifier(gain +32dB)+1m cable(loss
> > -1.2dBm)+Attenuator(set at 20-50dB depending on TX-RX
> > distance)+1m cable(loss -1.2dBm)+Port2 of VNA
> >
> > How do I remove all the effect of measurement system
> > from the recorded channel frequency response so that I
> > can find the path loss at each TX-RX separation
> > distance?
> >
> > Thanks.
> >
> > Linda
> >

One could sort of separate things out.  First, you can do the VNA cal
with
the measurement cables in, so the reference plane is the antenna
connector.
Bear in mind that the cable loss will vary somewhat as it's flexed, as
will
the loss/mismatch from connectors being tightened and loosened (e.g.,
SMA
at 13 GHz can have .25dB gain variations with 1/8th turn on the wrench).
Unless your cables and antennae have precision connectors (e.g. APC-7),
the
repeatability of the connector itself is not all that great (0.25-0.5
dB,
at a guess).

Now, you've got to sort out the antenna from the path.  As Jørgen
correctly
points out, the antenna is hardly isotropic, and you have a multipath
environment, so you've got a summation of a variety of paths, each
multiplied by the antenna gain and phase in the direction corresponding
to
that path.

However, all is not lost, depending on how much work you want to do,
because it basically involves separating out all the paths. Your VNA can
do
time gating (or, for that matter, you can look at the time domain
response
of your path), by which you can probably isolate the direct from
reflected
paths (the direct path being the first to arrive, no?).

The next step is to use multiple antennas, with different (known)
patterns,
and essentially you wind up with a multiple equations and multiple
unknowns
situation.  One can do a quick and dirty pattern modification by using
sheets of absorber in the far field (but not too far away).  For
instance,
a tube of absorber blocks the signals coming from the sides.   This is
hardly a precision quantative technique, but is useful for a "process of
elimination".  The absorber loss varies depending on angle of incidence,
among other things, but, if you suspect you've got a reflected path
that's
say, 5-6 dB down from the direct path, putting another 20 dB in the way
will make a noticeable difference.

You didn't mention what frequency you're working at, but, one thing that
might be useful is a standard gain antenna. For lower frequencies, you
can
use the NBS standard as described by Yang in IEEE Trans Ant Prop, Nov
1966,
page 792, or the antennas described by Cottony in NBS circular 598.  For
higher frequencies, a rectangular horn has well known and well
understood
properties.

You could also do a 3-antenna type measurement to characterize your
antennas accurately.  There's a fair amount of literature on making
measurements of this type. Most practically oriented antenna textbooks,
like Kraus, also discuss this in some detail. The April 1978 issue of
IEEE
Proceedings had some decent review articles covering the state of the
art
25 years ago, and, for low GHz frequencies, not much has changed, except
that the test equipment is a lot nicer to use these days, and near-field
ranges are much more popular.

James Lux, P.E.
Spacecraft Telecommunications Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
tel: (818)354-2075
fax: (818)393-6875

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