On 3/8/2010 1:22 PM, MCH wrote:
That's just it. Everyone likes to throw names out such as 'alligator' or
'elephant', but few people realize it's all relative to the station
*using* the repeater, too. What repeater may be an alligator for one
person is an elephant to another depending on *their* equipment.
The repeater itself is neither an alligator or an elephant.
Joe M.
Absolutely. You can design a system that is nicely balanced for 5W HT's
with factory-stock rubber ducks, or nicely-balanced for mobile radios
with 50W transmitters and higher gain antennas, of course. The trick is
to actually *design* the desired coverage and then stick to "the plan"...
Or the other method... which is usually more practical...
Don't worry about transmitter power at all at first. Make the thing
receive just as darn well as ALL of the variables allow considering what
it can "see", and only then design the transmitter power to match,
including any additional desense (hopefully not!) by additional TX
power. (Which would likely drive you to do a filtering system re-design.)
In this discussion, method #2 might be called "Try to build an Elephant
for your desired coverage area -- you do have a desired coverage area
don't you? If you don't, how are you measuring your success? -- then use
more than average power for the TX to match. If your system truly ends
up behaving like a pachyderm, adding TX power after that can be done if
a) time, b) money, c) willpower, d) other needs permit.
At high mountain sites with tons of RF, it's just usually impossible to
"build the perfect Elephant" even with top-notch filtering, pre-amps,
antennas, feedline, etc... due to the overall "site noise floor". You
do the best you can and then "road test" it a bit to see if the TX is
covering the same places a high system can "hear". We've had more than
one occasion to think "the repeater sounds a little weak", drive to the
site, and find it's operating on exciter power alone... Height Above
Average Terrain trumps ALL in VHF+ repeater work, and we're quite
"spoiled" by our large granite "towers" 20 or so miles to the West.
The other thing we all *should* do, but don't: Part of that initial
design/trial process should be measuring a "baseline" performance level.
We quite literally have sites that everyone *remembers* performing
slightly better over 10 years ago, and the *assumption* is that the
freshly-measured noise-floor at those sites is higher than it was many
years ago... but in many cases, the technician working on the site 10+
years ago didn't a) write it down, b) put it where someone could find
it, c) care. (GRIN)
We have *partially* "fixed" this problem by making sure every site has a
"site book"... just a spiral wound notebook where all pertinent changes,
measurements, and other items are noted in the book. It's simple, it
works... the only downside is... the book is up at the site when you
might want to reference it. A better solution today would be something
digital stored off-site, but that no matter what... you *always* have
some data entry method available at every site. We have talked about
this Utopia now for a while, in hopes that we'll find the time to finish
getting IP/Internet access to all of our sites, and a PC at each, such
that no tech would ever have the excuse ("I couldn't put it in the
system" or, "I couldn't get data cell service at the site" or... well,
you get the idea.)
Seriously though -- measure, and then set a goal to try to increase the
performance. Often-times over many years, little problems sneak in (the
infamous GE MASTR II "whisker" problem comes to mind) that seriously
degrade the performance of a receiver, but without regular measurements
and historical data, just becomes "wow, that site seems worse than it
used to be", and even that is only noticed by the techs/people who
really pay close attention.
An example of it done wrong, is a local group who also has a number of
older members who "remember when the repeater worked better" up on a
particular mountain. The original techs may or may not be members of
that aging group, who can no longer get to the site to see for
themselves just how noisy it is.
A new gung-ho tech from somewhere else, moves into town -- gets asked by
the repeater group's vocal minority to "go fix the repeater" and strange
site shenannigans start happening, like split-antennas, etc... with no
idea what the "baseline" was, the new tech tries to "fix" the problem,
each time realizing (by proper measuring) that they made no significant
effect -- or worse, starts causing an interference problem in an
environment laden with PIM problems that haven't been hunted down yet,
and may never be -- eventually they throw their hands up in disgust
after multiple folks say, "That's been tried. It won't help."
And the cycle starts over again in a few years with a new tech. All
because nothing's been WRITTEN down, and kept AT THE REPEATER. Where
any curious/smart tech could find it, read it over a cup of coffee in
their truck, and know they were about to embark on the infamous chase of
wild water fowl.
Seen this type of thing happen. More than once. Even been suckered
into my own ill-advised changes, but at least I had measurements to say,
"Wow... look how dumb that was!", after making things worse...
Nate WY0X
