For more accurate coverage/path analysis I prefer Radio Mobile, and
it's still freeware:
http://www.cplus.org/rmw/english1.html
p.s. - here's a giggle, I downloaded the DEM's for the entire
northeast quarter of the U.S. and converted them to DTEDs back in the
days of 56k modems!
Only took a few months! Worth every hour though.
-73, Ron WZ1V
At 09:46 PM 3/12/2008, James Duffey wrote:
>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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