Hi Ari

I always wonder whether broadcast at the most common IF frequencies
would be a viable alternative? (eg 455khz for AM) Legality issues aside
of course... This would make frequency selection a little easier to
engineer. You wouldnt have to divide your power budget into many
operating frequencies as well. Problem is that you'd need a lot higher
ERP especially since car mounted radios tend to be better shielded than
portables. (You'll need to experiment some)

Since part of your engineering exercise will be to determine what ERP
you want from the system you are proposing, the size of the antenna only
needs to be considered in line with the transmitter output power.
Obviously there will be a useful compromise somewhere that is cost
effective. You might for example want a 100 watt TX for each frequency
with a fairly lossy/terminated and loaded antenna. Your first port of
call will be to determine what the likely highest strength of an AM/FM
broadcast signal you will need to override, add maybe 20dB to that and
work out your needed ERP. You will find lots of texts about the
inefficiencies of MF antennas mounted on motor vehicles so that will
give you some numbers to work with. Remember that making the antenna
wideband will also introduce substantial loss. Note that you cant design
the system to always work as you'll have situations where the radio
stations transmitting antenna is right next to then freeqy where the
event occurred! Decide on some statistical coverage percentage thing etc
etc...

I worked for a concern in Sydney Australia that put in tunnel
rebroadcast systems in the major underground roadworks in Sydney,
Melbourne and Perth. Being in a tunnel of course there werent really any
licensing issues. The system consisted of not only broadcast band (AM &
FM) receivers but spot VHF/UHF FM receivers and transmitters for police,
fire etc vehicles. Each broadcast frequency had its own transmitter. It
was thought to be a better idea than using a wide band TX covering all
frequencies. AM broadcast was coupled to a very long wire and the rest
used a terminated leaky coax run.

Cheers Bob W5/VK2YQA

Ari Silversteinn wrote:
I am learning AM broadcast basics in particular the issues regarding the
geo-targeted broadcasting of alert messages from moving vehicles. Would
most appreciate any assistance.