[BC] STL Options

[Phil Alexander]

On 23 Feb 2006 at 10:25, Kent Winrich, K9EZ wrote:

> You know that is one thing I have not looked into...the inversion 
> layers.  I would think they would form at different levels, but 
> **typically** where do they form?
> 
> I have one station that has major fresenel issues in the summer 
> time.  I need to look into this further.

Theory says they come all the way down to the ground causing
widespread ground fog in the process. Systems more or less
aligned to the typical frontal approach seem to fair better
than others. IOW if the warm fronts approach from the west or
southwest as is typical in the Midwest, and your shot is upward
in a westernly or southwesternly direction with Tx low (< 100 ft)
and Rx higher (perhaps between 200 and 300 ft) they seem to get
along better. I have some experience with a 25 mile single hop
using Harris digital (2 W Tx) and a pair of 6 ft Mark Grids that 
has held up during some inversions that bounced a high located
Class A FM back into the sky. 

For some reason an Rx of 250 or 275 ft seems to work most of 
the time although I've never figured that one out. Inversions 
are strange things and it is good to have some weather background 
to understand them. They seem to form easily on a more or less 
flat plain. They do not need a bowl like the LA basin although 
that makes them form more often and last longer. 

There is a good reason for using large fade margins and designing 
STL's for at least 99.9999% system reliability. I know one local 
group that lasted all of three months with the available T-1 
"reliablility" before reverting to STL. 

One thing that most computer people without broadcast experience 
don't seem to grasp is the difference in "acceptable" levels of 
reliability between the broadcast and computer worlds. 99.9% 
sounds good until you do the math and find the system unplanned 
downtime probablility is about 45 minutes per year. The odds of 
hitting drive time are about one in six, or statistically about 
7.5 minutes per year average during drive time. If one of those
outages happens to coincide with a sweep ...  At the very least,
it won't be fun conversation. I think it was Burt Weiner that 
said something about being master of his own destiny in this 
regard, and there is a good reason in the numbers for that.

With good equipment, obtaining STL reliability > 99.9999%, or
about 30 seconds unplanned downtime per year is not difficult,
and taking it one more order of magnitude to around 3 seconds
is not impossible with the available technology without getting
into redundant systems etc. The comparison to T-1 in most areas
is rather clear.


Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037





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