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?

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?

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 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!

"MRW" wrote:
My current setup includes a 100w low power fm transmitter being fed by
a computer running some school audio programs.

What call letters did the FCC issue to that FM station?
--
73, Cecil, W5DXP

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
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

I forgot to mention that the radio station is not based under FCC
jurisdictions. It is used for a none government / non-profit
organization in another country.

In article .com,
MRW wrote:

I forgot to mention that the radio station is not based under FCC
jurisdictions. It is used for a none government / non-profit
organization in another country.

Ah, OK. That clarifies things quite a bit. Just make sure that you
cover your bases with the other government's authorities... some of
them may be as humorless as the FCC.

First thing I'd do is suggest that you take a look at coaxial-cable
attenuation figures. http://users.erols.com/rfc/attenrat.htm has a nice
table which shows the attenuation in dB per 100' of cable, at various
frequencies. For a first approximation I'd suggest using the "100
MHz" column.

As you'll see, 100 feet of RG-58 coax has an attenuation of 4.5 - 5 dB
at 100 MHz. That means that almost 3/4 of the power is lost as heat
in the cable - only about a third of the power makes it up to the
antenna. If you *really* need a 100-meter run of cable, triple that
loss... only a single-digit percentage of your power will reach the
antenna.

Good RG-8-size cable is better. LMR-400 loses 1.2 dB per 100', which
means that you'd get almost half of your transmitter power up to an
antenna 100 meters away. LMR-600 is even better, at 0.8 dB per 100' -
you'd have more than half of your power left. Some of the really
fancy/thick/expensive hardline cables are even better - LDF5-50A
would have just a hair more than 1 dB of total loss in 100 meters,
which would really be negligible.

This suggests two things:

- It'd pay big rewards to see if you can route the cable as directly as
possible. I can't see any good reason to run 100 meters of cable
to go up a "three story" building!

- Don't use cheap, thin cable... don't use anything less than RG-8.

As I mentioned before, aluminum-jacketed cable-TV "hardline" may be a
choice to consider. It's often available in useful lengths, either
for free or for scrap value, from cable-TV operators - they buy and
install it in large quantities and often don't have a use for the
"cut-off" pieces at the end of the spool (and such "cut-off" remnants
may be 100' or more in length).

This sort of cable has a 75-ohm characteristic impedance. It might
actually provide a better match to a dipole antenna than 50-ohm cable
would (depending on the antenna type and mounting arrangement). The
modest SWR at the transmitter might or might not be an issue... if it
is, a simple two-component (one-inductor, one-capacitor) "L" matching
network can allow the transmitter to "see" a 50-ohm load even though
the antenna and coax are both 75-ohm.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

"MRW" wrote in message
ps.com...
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?

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?

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

========================================
That the cable will look like an inductance or capacitor is a load of
nonsense. Change the books you've been reading. Don't read the
comics.

For minimum loss in the cable the input impedance of the antenna
should be approximately the same as the cable impedance.

If the cable loss is 3dB then half of the transmitter power will be
lost in the cable. Check the cable manufacturer's specification for
actual loss per unit of length.

Very approximately, if the diameter of the coaxial cable is doubled
then the loss in the cable, in dB, will be halved.

On the other hand, if 3/4 of the Tx power is lost in the cable, then
depending on the distance to the customers' receivers, it might not
matter very much. Everything depends on the required service area of
the transmitting station. There's not much point in buying expensive
cable to radiate 100 watts if 25 watts with cheap nasty stuff is good
enough.

Think in terms of your bank balance.
----
Reg, G4FGQ

First, at 100 watts do you have a license?

MRW wrote:

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?

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?

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 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!

Dear MRW:

The university where I teach licensed a 100 watt FM station last year. The
transmitter was placed near the base of the tower that supported the
transmitter's antenna.

The studio was some 1000 feet away from the transmitter. A cable
containing multiple shielded pairs was run from the transmitter to the
studio. Every low power FM broadcast transmitter that I looked at had
provisions to make easy remote control.

My suggestion is to mount the transmitter near the antenna and run
control cables down to the studio. A short run of coax is likely to have
acceptably low loss.

Good luck. 73 Mac

--
J. Mc Laughlin; Michigan U.S.A.
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