"Brenda Ann" wrote in message
...


Large numbers of stations in metro areas are already sharing antenna
systems.

When AMs share antenna systems, most are non-directional. A number of cases
exist where a directional AM share a single tower with an non-D station,
like 1390 and 1680 in Chicago. 1390 is directional, but the X Band staiton
just uses one tower. The efficiency of doing this vs. cost is good.

Not too many shared directionals exist, as the system has to be able to
create the appropriate pattern for each station, and that is a relationship
of phase and current and wavelength spacing of the towers.

When directionals share, it is in cases lik KTLK and KTNQ in LA, where the
towers could be made to work for two fairly similar systems. This cost over
$1 million to do, just for the tuning units and rejection networks... not
including towers and transmitters and buildings. It took the best DA guy in
the US, Ron Rackley, over a month to tune it.

The only case where share DAs by two directinal stations makes sense is
where there simply is no land available. The cost and maintenance is very
high, and it is not done to save money.

Most have done this either in order to decrease the number of physical
plants or to simply save on leasing of or purchase of land for antenna
systems. The towers themselves are only truly narrowband at resonance.

Actually, this has little to do with the tower. While a wider face tower is
easier to broadband, most of the bandwidth at a particular frequency has to
do with the Q of the tuning circuits in the system. In earlier times, higher
Q systems were designed as they were chaper, easy to adjust, etc.

When two stations tune to a tower, each has a tuning system (ATU or antenna
tuning unit) that matches the line impedance to the tower impedance. Then,
there are rejection networks to let each station feed into the tower, but
not allowing the RF from one to get back into the trnsmitter of the other.

As long as the system is broadband, the signal will be broadband.

When two (or even more) transmitters share a system, at most one of them
will be at resonance. As far as the matching hardware, it can be
engineered to be as broad as it needs to be.

Very few towers are a perfect 50 ohms with no reactance. So all require
matching. What changes, based on tower height, is the efficiency of the
radiator and the angle of radiation (each of these being about the same
thing) but I have seen 1/8 wave towers tuned for great bandwidth with high Q
circuits. In fact, I had an 8th waver at 570 that I diplexed a station at
805 into, and we had both sounding great because we made the bandwidth at
each frequency very good at up to plus and minus 15 kHz, and the filter nets
were very specific to each frequency against the other.