Bob,
I cannot state that Radio Mobile is not as accurate as a commercial program.
However, the commercial programs use digital elevation models that have been
extensively refined with additional data points and calibrations that are not
provided by the NGS. It is this value-added aspect that makes these data
costly. In commercial broadcasting, when an error in computing coverage may
mean a difference of a million dollars in advertising value, accuracy counts.
I don't think that the potential advertisers are going to be swayed much by
predictions based upon a shareware program.
Nor will I embrace a simplistic assumption when I have the tools to verify my
calculations. One of my most valued tools is a BVS "Coyote" GPS-based mobile
signal measurement system. It creates a map of actual signal levels measured
during travels over the areas where coverage is desired and expected. There
have been very few instances where the predicted coverage determined by
ComStudy did not agree closely with the measured values. This is reason
enough to trust the commercial software.
But, let's consider Amateur Radio applications, since we are in it for the
pleasure and for Public Service, rather than for the money. The coordinating
bodies need to know, with some degree of accuracy, what the predicted signal
level will be at some distance from the transmitter. As I noted in a previous
posting, the free-space attenuation calculation is the easy part. But, to
determine diffraction and Fresnel losses to any reasonable degree of accuracy,
one must know the elevation profile of the path to a far greater accuracy than
the publicly-available NGS Digital Elevation Models will allow. A sharp-edged
ridge diffracts radio signals in a far different manner than a smooth-edged
ridge. Trees and brush on the ridge make a huge difference. Earth curvature
must be considered.
When calculating HAAT, the elevation of the radiation center of the antenna
must be known and considered in the equation. Many inexpensive or free
propagation programs (I dunno how Radio Mobile does this) simply assume that
the antenna is at the ground elevation. This, of course, will result in
inaccurate data. Who is going to verify its accuracy?
What about directional antennas? Can the propagation software accept the
antenna manufacturer's horizontal and vertical response patterns in the
industry-standard formats, and include their influences on the calculated
contour? Not all Ham repeaters use omnidirectional antennas; some use offset,
corner reflector, and Yagi antennas.
I guess it doesn't really make much difference how accurate a propagation
program is, since the Hams I know who have used Radio Mobile or similar
programs, simply put the computed coverage plot on the wall and proudly
proclaim that it represents the coverage of their particular station. I don't
think any of them check to see how accurate it is. If they're happy with it,
let them be happy.
73, Eric Lemmon WB6FLY
[EMAIL PROTECTED] wrote:
> >Radio Mobile should be used with the realization that an inexpensive (or
> >free) program cannot possibly duplicate the features and accuracy of
> >commercial propagation programs like ComStudy, TAP, CovLab, RPS, and
> >others which are in the $5,000 to $12,000 class.
>
> The question here is are the $5k-$12k programs really $5k to $12k better
> than Radio Mobile?
>
> > One of the key elements in determining coverage is a high-resolution
> digital elevation model, and this data is costly, perhaps $500 for a CD
> covering only a few states.
>
> I wasn't aware of any wide-area digital elevation data more accurate than
> the publicly available (free) SRTM data.
>
> Bob NO6B
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