 RACES ATV PATH PROFILE
CALCULATIONS
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Premise |
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To calculate the path profiles for a 2.411 GHz wideband
televison path from Loma Ridge to several ATV repeater sites at various distances
with results and commentary posted on this page for the pupose of easy reference
on the progress of the project. ..and also to play with HTML. Please email any information that would be helpful. |
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Disclaimer (weaseling) |
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Since I have little or no experience with ATV, I can't guarantee the results from this analysis to track reality. I do know a bit about propagation, though, so some of this may have value... Just don't bet the farm on it. |
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Background |
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On a sunny Saturday morning in March Jack Barth and a county radio tech were contemplating the cancellation of an antenna party on the roof of the EOC. The ATV system was brought up and they realized that the paths to Signal Hill and Mount Disappointment were smack dab across the L.A. basin and its inversions ('member smog?). A rough free-space path calculation followed and everything looked OK. However, after doing a little (very little!) research, the startling fact was found that FMATV seems to require 50 db S/N at the detector for snow-free video recovery. Uh-oh! No fade margin. Thus: this page. |
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Assumptions |
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Actually, there shouldn't be any assumptions. However, since I don't
have a full picture of the stations involved yet, this is the best we can
do. As each parm is confirmed, these assupmtions will disappear. Again, please email any corrections to obviously bogus information
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All Rx Noise Figures = 1db
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Repeater antenna gain = 20db
 per Jim Carter
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Repeater output power = 10 watts
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Receiver bandwidth = 11MHz
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Repeater site feedline loss = 6db
 per Jim Carter
Path loss calculations are for free space only. More elaborate propagation
models are available, but this analysis is what I could come up with in the time available. Typically, one can expect unobstructed path loss to be from +6 to -40 db of that for free space depending on conditions (ducting, fog, inversions, rain, etc.).
The literature available here ('96 ARRL Handbook) impies that FMATV requires a signal to noise ratio of over 50 db to render snow-free video. Full chroma recovery equires a S/N of 30 db.
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The Profile Diagram |
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Shows the various parameters considered in calculating the path. 
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Profile Calculations |
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The Longley-Rice path prediction Fortran source code has been located, downloaded and successfully compiled and installed. The Longley-Rice propagation algorithms allow one to predict the path loss over a wide variety of terrain and conditions. As soon as topological data can be gathered, moderately realistic path loss information will appear here.
- Table for Lomas to Mt. Disappointment - Shows the effect of using 80' of Superflex from antenna to the transmitter.
- Table same as above, but with the transmitter mounted at the antenna (no loss). Note the slightly better value in the excess signal column, but still nowhere near enough to avoid even a 20db fade.
- Table For Lomas to Signal Hill (24mi). Note that the shorter path still doesn't afford any margin for fade.
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Comments/Conclusions |
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So far it looks like the paths are at or below 0db fade margin (figuring a potential fade of 40db). In other words, if there's an inversion, the signal will probably show serious degradation. It all depends what's tolerable. Please note that none of the paths checked using free space path loss seem to have a comfortable fade margin. This is assuming, of course, that the 50 db S/N ratio is necessary for perfect video recovery. |
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