Marshall - The coverage maps should show you path loss in a number
that is in dB. What is the path loss at the outer edge of the 50 mile
circle?
I found the coverage circle to be a bit short here as well, it cut off
well before my stations capabilities gave out on some calculations I
made. I found the station to station calculations to be more
informative, so try one of those calculations with a distant station
you can work regularly.
The path loss is a characteristic of the path terrain, climate,
distance, and the scattering medium. It is independent of the station
capability.
Once you have the path loss, you need to calculate the path loss
capability of your station. Here is a short explanation on how to do
this. It helps to read the November 1983 QST article "Tropospheric
Scatter Propagation" by G3YGF. That is also reprinted in the ARRL
publication "Beyond Line of Sight"
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,
and ers is the effective receiver sensitivity of the receiving station.
SPLAT! calculates the Path Loss, pl, for the link being attempted. Or
you can get a rough idea from the plots in the Antenna Handbook or old
VHF Handbook.
The signal to noise ratio, snr, expected for the path can be calculated:
snr = plc - pl
The effective radiated power is given by:
eirp = (10*log(P)) + Gtrans - transTL
where P is the transmnitter power in Watts, Gtrans is the transmitting
antenna gain in dBi (decibels over
an isotropic radiator), and transTL is the transmitter stations
transmission 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 commonly
available noise figure, nf, by:
T = ((10^(nf/10))-1)*290
If you know the receiving noise figure, plug it in, for most
mainstream multi-mode VHF rigs using 5 dB will get you close.
B is the receiver bandwidth in Hz. 2500 Hz is a typical bandwidth for
SSB, 500 Hz is a good bandwidth to use for CW.
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.
If you don't know what the guy on the other end is using, you need to
make some assumptions as to these numbers, but 4 or 5 dB noise figure,
13 to 15 dBi for antenna gain and 1 dB feed line loss are reasonable
assumptions for a moderately serious VHF station.
I hope I haven't made any mistakes. I am sure that someone will
correct me if I did. :^)=
From the above calculations, it is easy to see why CW at narrow
bandwidth outperforms SSB and why SSB far out performs FM.
I have had SPLAT! give some unrealistic terrain maps from time to
time. Bll knows about this and is looking into what the problem is. It
doesn't seem to affect the path loss plots though, or the path loss
calculated, or the table of terrain data.
Holler if you have any more questions. Once you understand what is
going on you will find SPLAT! to be a valuable tool. - Duffey
--
KK6MC
James Duffey
Cedar Crest NM
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