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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. |
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