Jim - For long distances on 432 MHz you will be relying on
tropospheric scatter, often abbreviated troposcatter. For want of a
lengthy description, troposcatter is scattering of our radio signal
from roughly wavelength sized inhomogeneities in air density or water
density in that part of the atmosphere below 30,000 ft or so. Some
refer to the inhomegenities as blobs. That is probably an accurate
description.
One of the classic articles on troposcatter is "Tropospheric Scatter
Propagation", by G3YGF in November 1983 QST. If you are an ARRL
member, you can download a PDF of this file from their web site.
You can calculate, at least to first order, how well your station will
perform over particular paths rather simply.
I use the on line version of SPLAT!:
< http://splat.ecok.edu/ >
which will calculate path loss between two points. You need to know
the coordinates of your station and the ones of the station you are
trying to work, plus the antenna height. There is a database of
stations on that web site which you can use and you can register your
own location and that of stations you are trying to work.
SPLAT! is also available as a standalone application. SPLAT! is not
the only software that will do this, RadioMobile will as well. THese
are good programs, but if you use the standalone version, you will
need to download the topographic information between you and the
station you want to work. This data is available on the web, but can
be tedious to download. The online version has all this online so that
you don't need to download the topographic data.
Then you need to calculate the capability, or path loss capability of
your station and the station you are trying to work. G3YGF shows how
to do this. In brief:
The path loss capability, plc is given by:
plc = eirp + ers
where eirp is the effective radiated power of the transmitting station
in decibels above 1 watt referred to an isotropic
radiating source. SO 10 Watts would be 10 dbW eirp, 20 Watts would be
13 dBW, 100 wWtts would be 20 dBW and so on. and ers is the effective
receiver sensitivity of the receiving station.
The effective radiated power is given by:
eirp = (10*log(P)) + Gtrans - transTL
where P is the transmnitter power in Watts, G is the transmitting
antenna gain in dBi (decibels over
isotropic radiator), and TL is the transmitter stations transmission
line loss in dB.
The effective receiver sensitivity is given by:
ers = -10*log(k*T*B) + Grecv - recvTL - threshold
the first term is the receiver noise floor where k is Boltzmann's
constant,
k= 1.38*10^-23
T is the effective receiver noise temperature in degrees Kelvin
(absolute temperature), which can be calculated from the
more coommonly available noise figure, nf, by:
T= ((10^(nf/10))-1)*290
and B is the receiver bandwidth in Hz.
Grecv is the receiving stations antenna gain in dBi
recvTL is the receiving stations transmission line loss in dB
and Threshold is the detector threshold for the mode being used. For
CW and SSB it is 0, while for FM it is 10dB
depending on the modulation index.
Then you can calculate the expected signal to noise ratio over your
path from:
snr = plc - pl
where pl is the path loss that you get from SPLAT! or Radiomobile.
That path loss is a 50% number, half the time the path is worse than
this, half the time it is better. The variation is usually +/- 7dB or
so.
There are two path loss capabilities involved, one from from your
station to the station you are trying to work and one from his station
to yours. If you don't know what is at his end, assume that he is
running a typical modest station on 432 MHz; 100 Watts out to 1 dB
loss feed line, 2 dB noise figure and an antenna gain of 18 dBi up 35
ft.
You can do excursions on this to see how much adding an amplifier will
help. One thing to note is that going to CW will help a lot, due to
the reduction in bandwidth. I have written all of this path loss
capability in a MathPad application for easy calculation. It runs on a
Mac, but if you want a copy, I can send it to you. MathPad itself is a
free application. - Duffey
--
KK6MC
James Duffey
Cedar Crest NM
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