Hi everybody! I want to mount a dipole FM antenna on top of a three
story building. My transmitter will be located in a room in the bottom
floor. Has anyone have experience in this?

Many people do, I'm sure.

I know that I will get some signal attenuation with that long of a
coaxial cable (100 meters). If my cable has a characteristic impedance
of 50 ohms (matching the output impedance of the transmitter), will it
adversely affect my transmission?

It may, depending on the type of cable you use. Thin-gauge coax cable
(e.g. RG-58) will cause significant attenuation over that long a
run, at those frequencies. Heavier-gauge cable would have lower loss.
I wouldn't use anything less than a high-quality RG-8-type cable (e.g.
LMR400), and would seriously consider using a hardline (e.g. cable-TV
aluminum-jacket).

I was reading my electromagnetics book and it mentions that depending
on transmission line length the cable might look like an inductor or
capacitor.

What other factors should I take into account

Umm, the possiblity that you might end up having your equipment seized,
and/or be hit with a really heavy fine, if you don't do all of the
paperwork properly?

my calculations for the
effect of the cable length on the system?

My current setup includes a 100w low power fm transmitter being fed by
a computer running some school audio programs. Thnx!

Are you intending to operate a legitimately-licensed educational LPFM
(low-power FM) station on behalf of your school? If so, whomever
filed your LPFM application to get an FCC construction permit ought to
have done a lot of the necessary design work and calculations at that
time.

If you haven't received an LPFM license from the FCC (new applications
haven't been accepted since 2003, apparently, and such licenses aren't
available to private individuals), I'd advise against attempting this
sort of installation. The FCC occasionally raids "pirate" FM
stations (those operating unlicensed), seizing the equipment and
citing the owners/operators. The financial penalties for operating an
unlicensed FM station could be really severe.

I don't think there's any chance at all that a "100w low power" FM
transmitter could reasonably be used in a way which would qualify it
under the FCC Part 15 limits for field strength (250 microvolts per
meter, measured at a distance of 3 meters from the antenna) if you
feed that much power into a dipole antenna.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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