RF .... real of topic

[sakis]

making this post cause really dont know where else to post it

i apologize to the forum for a real off topic but since in the forum exists so many electronics engineers may be i can get some help out of them ....

the problem

in a specific area there is a device that has an RS232 out the traffic presented there is only TX and the volume of information is very very small ...just a few bytes

this information needs to be transfered with a wireless transmitter a few kilometers away

there has to be received and also repeated in a RS232 port .....

the basic problem is that in this specific area a frequency of no more than 27 mhz is to be used

so
how to interface and broadcast from RS232 to CB freq or arround 27 mhz ???
how to receive and then interface from 27 mhz back to RS 232 ???

any help is very welcome and feel free to email me privatelly

[DRC]

FSK

[wakibaki]

You may have some problems here.

Radio spectrum allocation is controlled by law internationally. You need to ask a local radio amateur for advice. Perhaps one will chip in...

Until you can define the frequency and power and antenna you are permitted to use, the question is difficult to answer. The modulation type will almost certainly be specified too.

w

Given that the bitrate is low, if the installations are in line-of-sight you might be able to use a high-power visible-light LED with on-off modulation, depending on the % uptime required and the atmospheric conditions.

[Iain McNeill]

you have to use the bands you can.

do you have line-of-sight or is it hilly terrain/metalframe construction?

A microcontroller (PIC) can modulate a radio quite easily and directional antennae provide extra gain.

But why not IP?

[sakis]

generally about RF i really know zilts .....

27MHZ frequency is a forgoten band in my country ....in the past it was used mostly by truckers and not for any safety issues but mostly to keep company to the drivers .....

not any more ( at least in athens area ) all truckers now have GSM mobiles or 144/440 amatuer radio devices ....

so 27 MHZ iz more or less quiet but i can go bit higher or lower than that and still be ok with it .....

FSK i dont know what it is ....checked the wiki is all more or less understandable but dont now where to find or how to construct the device required .....

ip wifi and/or any frecuency above 30mhz is out of the question ( the paricular device might be easilly effected and not work propelry if transmiters of even low power exist next to )

[phase_accurate]

FSK is a two-state FM modulation. Slower modems use this.

Talking of modems: It should be possible to use old analog modems for this purpose. And there are chips by TI that allow an easy implementation of 300/1200 bps modems. I have to find out the type again. I might even have some of these.

You could then transmit this signal by either AM, FM or SSB. SSB would be thew most elegant version and it would represent the FSK Signal shifted up in frequency.

Maybe you could even directly modulate your carrier in frequency by the means of pulling an x-tal oscillator.

Regards

Charles

[sakis]

i found data of ic AM 7911 its actually ic that used in modems but in the particular application seems that can do th trick ///

AM 7911 is obsolete but id like to take a look or listen to any sugestion for something newer and even more simple if possible

[nonoise]

You could also try CML Microcircuits
http://www.cmlmicro.com/

[phase_accurate]

As far as I understand he can't use higher frequencies than 27MHz.
If it were otherwise there would be quite a good choice of complete RF modems, working at 433 Mhz or above, at interesting prices.

Regards

Charles

[jerluwoo]

Most radio controlled toys are manufactured to work at 27mhz.