[NLRS] Path loss calculations

[Baker, Donn B]

Hi Mike, guys.
If I may toss out a couple of things...=20
>-----Original Message-----
>From: Mike A. King - KM0T [mailto:scsueepe@mtcnet.net]
>Sent: Friday, March 05, 2004 9:41 AM
>To: Northern Lights Radio Society
>Subject: Re: [NLRS] Path loss calculations
>
>
>Hi all,  Thanks Gary for the equations.
>
>The last few nights I have been working on a spread sheet=20

SNIP

>However, the changes to the total path loss from water vapor=20
>based on say 95% relative humidity from one chart to another at 24 ghz =
will=20
>not change things a whole lot.
>

*************
Don't forget that the "humidity" in the loss calculation is actually =
based on "absolute" humidity, and not "relative" humidity.  But, since =
relative humidity is much easier to measure that's what's used most =
often.  When you do that, you MUST reference it to a temperature for the =
numbers to "mean anything".  A relative humidity of 50% at 4*C (~40*F) =
indicates less absolute humidity than 50% RH at 30*C (86F*).  Relative =
humidity is a measure of how much water vapor is in a parcel of air vs. =
what COULD be there.  Absolute humidity is a measure of the actual =
number of water vapor molecules in a parcel.  The warmer the parcel of =
air, the higher the possible absolute humidity.  Quite often, you will =
see a value of 7.5 grains/m^3, which I think is the absoulute humidity =
at Standard Temperature and Pressure (0*C & 1013mb Hg).

All I found for Atmosphereic Absorption (oxygen and water vapor) was a =
graph.  However, I did find a reference to the source for the graph.  =
See:
NBS (NIST)
Technical Note 101=20
Transmission Loss Predictions for
Troposheric Communication Circuits
Volumes I & II
<http://its.bldrdoc.gov/pub/ntia-rpt/tn101/>

Its LARGE, ~15MB, and I haven't downloaded it (yet ?).  The Contents =
look interesting, though.
**************
SNIP

>I began to wonder if indeed I had been attempting a=20
>tropospheric scatter
>contact in lieu of what I thought might have been a true "Line=20
>of Sight"
>qso.  (this was based on Gene being at 1950' ASL and me being at 1450')
>
***********
I would be very surprised if there is a LOS path anywhere in our part of =
the mid-west over a 222 Km distance.  The rough rule of thumb for the =
distance to the RADIO HORIZON is :
Distance (in miles) =3D 1.4 * SQRT(height in feet)
This gives ~62 miles for 1950' and ~53 for 1450'.  Combined is 115 =
miles; the 222 Km path is ~137 miles. =20

Seems close, but rememebr that LOCAL TERRAIN is ~1400 ASL.  The formula =
is for height above ground, NOT ASL.  Therefore, the REAL horizon =
distances are:
1.4 * SQRT(1950-1400) =3D 1.4 * 22.5 =3D ~33 miles
1.4 * SQRT(1450-1400) =3D 1.4 * 7.1 =3D ~10 miles
or a combined distance of ~43 miles.  And this assumes a smooth earth =
(i.e., no hills, trees, buildings, etc.)
***********
SNIP

>
>The one last factor that I can attribute to this, and I cant=20
>find a way to put an numeric value on it is attenuation from HAZE.
>
>The day we did the QSO it was Hazy as all get out on both=20
>sides of the path. From what I have read that this is an additional =
attenuation=20
>factor at 24Ghz, so this would have to be added to free space, oxygen =
and=20
>water vapor (humidity) losses.
>
>The ARRL UHF/Microwave book talks about fog attenuation and=20
>haze is similar, but not the same thing I think.  If it could be =
modeled=20
>as "light fog", great, but the data in the UHF/Microwave manual is =
sketcy at=20
>best and hard to interpet where one needs to figure the numbers.
************
"Haze" can be things other than water (fog), such as smoke, dust, etc.  =
For the most part,  I think you be OK in assuming that it was water fog. =
 (Besides, if its smoke, dust, etc., its real hard to account for !)

Rain attentuation is highly dependent on rain-fall rate as well as =
frequency.  Emperically, Atten(rain) =3D a(f)* R^b(f), where:
R =3D rainfall rate in mm/hr,
a(f) =3D 4.21E-5*(f)^2.43 (for f =3D 8.5 to 25 GHz)
b(f) =3D 1.41*(f)^-0.0779 (also for f =3D 8.5 to 25 GHz)
(Other coefficients for 25-54 GHz, and 54-100GHz)
Below 50 GHz, b(f) is essentially unity, so attenuation is proportional =
to rain rate.

Typical values for R:
0.25 mm/hr =3D ~.01"/hr mist, drizzle
1.25 mm/hr =3D ~.05"/hr light rain
5 mm/hr =3D ~.2"/hr moderate rain
25 mm/hr =3D ~1"/hr heavy rain=20
50 mm/hr =3D ~2"/hr very heavy rain (thunderstorm-type rain)

The formula stuff is from Reference Data for Engineers, 7th Ed.  (The =
old "ITT Radio Handbook")  I may have more in one of my old Remote =
Sensing books, but I'm not even sure where they are, or if I still have =
them anymore.  I'll look over the weekend.
**************
>
>So, the question is IF that was the final nail in the coffin=20
>for my 24 ghz attempt, would it have attributed for a number db of =
S/N?,=20
>even more than 100% RH attenuation figures?  Good question. =20
>More information required to be sure.
>

SNIP
>73
>
>Mike- KM0T
>
>