On Tue, 20 Sep 2011 07:16:40 -0500, dave wrote:

Theory and practice are quite different.

One day, you're going to eat those words, when you have to decide
whether to follow theory or practice. When I find that they're
different, it's usually because I'm doing something wrong. Also, if
you understand the theory, you can probably figure out the practice
(what to do). However, if you know the practice (i.e. seat of the
pants engineering), you're highly likely to fumble somewhere.

The tower owner should have an inventory of every transmit and every
receive frequency, plus all the standard I.F., plus nearby external high
powered sources. The owner should have cleared each frequency before it
went on the air, and should not add a tenant if doing so would create a
harmful spur to existing users. This is site management 101.

You almost made me spill my hot chocolate. You're correct. Site
managers should do all that. The problem is that all but one of the
site managers that I know of are business types, not engineers. They
hire engineers, tower jockeys, construction crews, and generally run
the business. It's not unusual for me to get a call or email with "I
just signed on to have [insert name] company put their radios in the
building. I'll let you know if anyone complains". This translates to
"Don't burn any billable hours doing calculations until AFTER someone
experiences interference. In short, I get paid to clean up the mess,
not to do the planning. If I want to enforce any engineering
standards, it's also done post mortem. At best, I would get an email
asking where in the building and tower I would guess the new radios
should be installed, usually without telling me the frequencies or
equipment. Interrogating the prospective new customer is something I
try to do, but often they contract out the repeater service to a comm
shop, which claims that they don't know anything because they're
afraid I might steal the customer. I don't wanna talk about
licensing, HAAT calcs, and coordination. Hopefully, your operation is
a bit closer to theory than practice.

I don't care how the WL people run their data streams. Cellular folks
don't like high mountains (except for backhaul).

Generally true. The CDMA crowd doesn't like high mountains for the
same reason they don't like CDMA operation in airplanes. The noise
floor is much higher up high and there are not enough channels
available to handle all the potential users if in a metro area.
However, they do like medium high mountain tops with fairly well
controlled coverage areas. They also like to share site ownership and
management with public agencies to reduce costs.

I know they use very
advanced techniques to hear signals below the noise floor; keeping that
noise floor as low as possible is of paramount importance when you are
looking at 100 mW devices in people's pockets 5 miles away.

100mw is about the maximum that a cell phone can belch. Power control
will usually keep that down to about 30-50mw.

FWIW, Tek has a real nice analyzer that will reverse engineer TDMA spurs.
make time-lapse spectrum analysis, and can even write on a map for you.

Well, the 20+ year old P25 radios are finally being forced into
service by FCC edict, along with various incompatible TDMA
implementations. Meanwhile, cellular is heading towards various CDMA
spread spectrum technologies (CDMA200, WCDMA, LTE, etc), which makes
TDMA look kinda dated. Anyway, I can't afford much in the way of
expensive test equipment and usually borrow or rent what I need. I
haven't actually seen a spur, mix, intermod, or noise on a spectrum
analyzer for many years as the receiver sensitivities are well below
the analyzer noise floor. Same problem with PIM (passive intermod).
It takes quite a bit of power to produce PIM making it almost
impossible to measure PIM while the xmitters are in operation. Trying
to see PIM on a spectrum analyzer is futile.



--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
# http://802.11junk.com
# http://www.LearnByDestroying.com AE6KS