Technical Radio Broadcasting Questions
 

I am the director of an NGO building out independent radio networks in
Afghanistan. The combination of geography and security problems, as
you can imagine, are ferocious. As a result, we need to examine a lot
of different technical options and I would like to ask the group some
questions:

1) How can AM be made more one directional than another? We might want
to put strong AM signals in towns near Afghanistan's borders but
cannot antagonise neighbours by having large signal spillage into
their territory. What are cost implications of channelling an AM
signal in this way?

2) How far can an STL wireless transmission reliably broadcast? I
understand most STLs are wire, but there are also wireless STLs using
the 940-950 Khz band. Could we use this wireless STL to spread signal
to repeater networks across mountains where FM would be difficult?

3) How far could a big FM signal reach, assuming little constraint on
power?

4) In FM repeater networks, how many links down a chain can a signal
reliably be put through? Are repeaters normally only a one stage
phenomenon, used like spokes in a wheel, or can you set up a chain up
and down a long valley for example?

5) Does anyone know about renting space on satellites? How much it
would cost to have a 24 hour audio channel beaming down to receivers?
How much a single uplink mechanism might cost, and receiver dishes?

Many Thanks

John

On 20 Nov 2003 06:16:51 GMT, umarc wrote:

I don't do repeaters, but there used to be a network of public radio
stations in New England that relayed "Morning Pro Musica" from one
station to another. The farthest station was 3 or 4 stations down
the chain and sounded pretty grungy, according to what I was told.
But this was 20 years agio, and technology has come a ways since.

I hosted "Morning Pro Musica" with Bill Cavness in the days of of air
networking. Until microwave came in the signal from WGBH, Boston to
WFCR, Amherst, MA was pretty bad. It was known as the Eastern
Educational Radio Network.

Just imagine the quality from stations taking WFCR off the air.

Rich

950 mHz Aural STLs can go a lot farther than 40 miles.

On 20 Nov 2003 06:16:51 GMT, umarc wrote:

(John) writes:

1) How can AM be made more one directional than another?

The usual method is to put up two or more towers approximately
1/4 wavelength tall and vary the phase and amplitude of the signal
fed to each to produce the desired directional pattern.

2) How far can an STL wireless transmission reliably broadcast?

Up to about 40 miles depending on the height of the antennas, their
gain, and the power used. The terrain must be favorable.

3) How far could a big FM signal reach, assuming little constraint on
power?

The reach will depend on the antenna height and the surrounding
terrain. A California FM where I used to work ran 18,500 watts from
the top of a 3,000-foot mountain and had regular listeners 100 miles
away.

4) In FM repeater networks, how many links down a chain can a signal
reliably be put through? Are repeaters normally only a one stage
phenomenon, used like spokes in a wheel, or can you set up a chain up
and down a long valley for example?

I don't do repeaters, but there used to be a network of public radio
stations in New England that relayed "Morning Pro Musica" from one
station to another. The farthest station was 3 or 4 stations down
the chain and sounded pretty grungy, according to what I was told.
But this was 20 years agio, and technology has come a ways since.

5) Does anyone know about renting space on satellites?

Yes; we're doing that right now.

How much it
would cost to have a 24 hour audio channel beaming down to receivers?

We're paying $2,200 a month to lease channel space on Galaxy 4R from
National Public Radio. We use the channel 24 hours a day to feed
a dozen or so affiliates from Maine to California.

How much a single uplink mechanism might cost, and receiver dishes?

The uplink cost us about $50,000 to set up. The receive dish I set up
at one affiliate cost about $10,000, including the dish, the receiver,
and installation.

But of course we're not in Afghanistan.


umar

"David" wrote in message
...
950 mHz Aural STLs can go a lot farther than 40 miles.

With US power limits, you can get pretty dicey even at 35 miles and up; I
have worked on a 50 mile one in Puerto Rico, and we had to put up two
antennas about 60 feet apart on the receiving tower and use diversity
reception to overcome variances in the arrival of the signal. A classic is
740-AM in LA, with studios in LA and transmitter on Santa Catalina Island.
The over-the water propagation varies so that a single receiving antenna
will loose the signal often. This was written up in the 70's in one of the
engineering magazines.