{LONG} [NLRS] Interesting little 10GHz science project

[Baker, Donn B]

Hi Bruce,
I'm jealous... stuck in here today!

Actually, it was neither... it was scattering.  We tend to be very =
sloppy in termanology, and should be more careful. =20

"Refraction" is a re-direction due to differences in the "refractive =
index" of the medium.  Not applicable here.  (Besides, you can't get 180 =
degree "refraction.") The angles involved run to several degrees.

"Reflection" is a re-direction due the presence of an (RF) opaque object =
(or a MARKED change in the refractive index of the medium).  The =
buildings downtown are a good example. The re-direction may be anywhere =
from 180 degrees to +/- 90 (well, not quite 90, say 80+) degrees.  You =
can't "reflect" forward ( between 0 and 90 degrees from the original =
path).

"Scattering" involves particles that are 1) smaller than the wavelength =
of the signal (Rayleigh scattering) and 2) particles in size which are =
near the wavelength of the signal (Mie scattering). =20

Depending on the rainfall rate, raindrops vary from 0.5mm to a maximum =
of about 6mm in size.  National Weather Service radars show the return =
from the rain, and that can be correlated as shown here:
                               "Clear air mode"       "Precipitation =
mode"
Precip       Droplet size    Radar color    dBZ level  Radar color   dBZ =
level
Mist         ~ 0.5mm D       LT GRN to GRN  -16 to -4     ---           =
---
Drizzle      ~ 1.2mm         YEL/ORG/RED     0 to 12   BLU to LT GRN  5 =
to 15
Rain         ~ 2-3mm         RED to PINK     12 to 28  GRN to DK GRN  20 =
to 35
Moderat rain ~ 3-4mm            ----            ----   YEL - ORG      40 =
to 50
Heavy rain   ~ 6mm              ----            ----   RED - DK PINK  50 =
to 65
Hail           ??               ----            ----   LT PINK to WHT 70 =
- 80

Tom Williams, WA1MBA, has a really good paper on rain scatter on ehis =
web site.  URL is:
<http://www.wa1mba.org/10grain.htm>  and for a .pdf version:
<http://www.qsl.net/pe1cqq/rain-mba.pdf>

MOST of the stuff we see is Rayleigh scattering: backscatter or forward =
scatter.  (Note from Tom's paper that there is little chance of success =
for paths that include a 90 degree angle IF YOU RUN HORIZONTAL =
POLARIZATION -- which we do!).

Long winded, but the table is useful, I think.

Bill, K0AWU, noted this afternoon that "...35dbZ is workable"  That's in =
the moderate rain or 0.25" per hour range.  We get lots of that, =
although not necessarily from tall storm towers (i.e., no long range =
Qs).

73 Donn
WA2VOI/0

-----Original Message-----
From: Bruce Richardson [mailto:w9fz@w9fz.com]
Sent: Wednesday, May 05, 2004 12:24 PM
To: nlrs
Subject: [NLRS] Interesting little 10GHz science project just now...

SNIP

So set me straight--is this refraction or reflection?  Is backscatter
an ok term?