I need to build a mains powered FM transmitter, for relative short distances, about 100 - 200 meter. Going to use them in a kindergarten and they have to be very audio sensible, and at the same time to work
7/24... Frequency - about 100MHz.. I'm aware of the safetly rules, they will be placed near the ceiling; I've seen several devices as "surveillance or spy devices", and I mean exactly the same devices, but for other purposes. The devices - as I've
understood are powered by a capacitor (250V), a diode bridge, a regulator and an electr. cap. for filtering. One of the most important things - they use no ANTENA, but how this can
be made is "misterious" for me. They claim they use the MAINS wires as antena.... I'll be very thankful for any help, idea or circuit :) TIA, Regards, Nina
Such devices use an ingenious, but not mysterious, bit of "trickery".
You basically have a mundane modulated oscillator powered as you say from a pair of wires that have 60 Hz power on them. The trick comes in when you want to get the signal output not into the open air where anyone can get at it, but to a specific intended receiver, and the trick is named "filtering".
The FM output is coupled back on to one of the wires through a capacitor (or more complex circuitry for "zap" protection) that has low reactance at the modulated frequency, but high at 60 Hz so that
60 Hz power doesn't get back into the transmitter. As far as the device is concerned, the incoming power line and "antenna wire" are separate because its rectifier etc "looks" at the power line through a filter that easily passes 60 Hz but blocks everything else, while the modulator output "looks" at the AC line through a filter that blocks 60 Hz but easily passes the modulated signal.
The receiver uses the reverse of this trick; it's an FM receiver whose power supply "looks" at the AC line through a filter (rectifier etc) that easily passes 60 Hz but blocks everything else and whose FM input "looks" at the same wire(s) through a filter that blocks 60 Hz but easily passes the modulated signal; as far as it can tell, the power and signal come in on separate wires.
BTW, this works best on a given branch of the power line; often people try to use them on branches fed by separate circuit breakers and run into trouble because the breakers or GFCIs between separate branches may block or dump RF to ground. YMMV.
Thank you very much Mr. Fergerson, I'm not sure I've got the point: What I'm looking for is a WIRELESS transmitter, and I think you ment some kind of "Mains Intercom"... I think so because the receiver's trick... Please, take a look here : h**p://
This is an example of more or less what I need, just using it as a monitoring device, a wireless baby-sitter :)
Well it seems that instead of getting some electronical help...I'm getting something else.... LOL I don't buy the devices, as they are expensive, and may get problems ordering them (custom)/ The range is up to 100m because there are walls that may do some kind of "screeneng". The devices are not for me, just been asked to build. They are suposed to monitor the kids when sleeping. Now, maybe I'll have a professional answer? Have I passed the test? :D Nina
Well, maybe you will, maybe you won't. This is usenet. You may get a right or wrong answer or even one that is deliberately misleading.
You're looking for a professional answer, so that you can take an amateur approach.
It's like asking how to build your own locks for the doors. Or how to print your own textbooks for the kids. It's a pretty sure bet that buying a professionally designed and built system will be less expensive and have less problems than you trying to reinvent the wheel.
If you've got kindergarden kids sleeping in a building with someone monitoring them
100-200 meters away, in another building, then I suggest that "how to build" is the least of your worries.
Giving that link with your first post would have been helpful. You said before:
Why do you consider this mysterious? It's done as I wrote before, by coupling the output of the transmitter to the power line through a capacitor that won't pass the power frequency. As far as the transmitter is concerned, pretty much any decent conductor makes as good an antenna as any other as long as it's more than a few wavelengths long. And carrier-current systems, which is what I described, do tend to radiate a bit.
From reading your posts, it appears that somebody wants you to covertly provide them with something that they could buy off the shelf, except doing that would leave a paper trail leading back to them. If I were in your shoes I'd be very suspicious.
Besides, from reading the specs, are you sure you could tune the thing as tightly as the commercial unit and get it to stay there without constant tinkering? This isn't a matter of soldering parts to a board and expecting it to work perfectly right off the bat. Narrowband FM transmitter/receivers can be cranky beasts.
I notice prices aren't listed; have you contacted them? If you have, have you been offered at least as much to build something that will do the same thing?