I'm working on a project at my school to try and make a transmitter that works on the same 457 kHz frequency as an avalanche beacon as a training aid. Before someone tells me to google it, I already have and come to find out the specifications for the transmitter:
Transmit frequency: 458±.1 kHz
Modulation: A1A (carrier on/off)(some form of pulse modulation, I can't tell which, let alone find any pulse circuits)
Period: .9±.4 seconds
Active minimum: 70 ms
Passive minimum: 200 ms
The problem that I am running into is the fact that I cannot find a circuit that is for a pulse device that has a set period like an avalanche beacon does, therefore does not take input.
Thanks for any help,
-Fenris
Most of your questions can be answered by looking to amateur radio texts, and FCC rules. A1A is a continuous wave transmission with the signal being turned on and off. There is no "pulse modulation." The thing of it is, transmitters in that band and service, I believe have to be type accepted. That means you can't just craft one for training purposes and allow it to transmit. Lest you think, and just who will catch me? I'd bet the first ones are the owners of avelanche beacons. On the other hand, if you are in some sort of engineering program you should have a faraday cage that you can set the project up in and it won't radiate out into the world...
Now that I've been a killjoy, I'll answer the question. There is no "circuit that is for a pulse device that has a set period like an avalanche beacon does". You would use either a 555 and some flip-flops or a PIC and program the interval or the emission pattern and use the output to key and unkey the transmitter. I would use a PIC, maybe even the Parallax Basic Stamp. More expensive, but way easier to program. You could use something like an 68HC series programmale microcontroller, but that is overkill (but cheaper if you have the programmer.) Most of what you need to know you can gleen from a copy of the ARRL Amateur Handbook. You can often find a copy in your local library and some used book stores. you don't need to worry too much about what year as they all have the basic information about emission types and keying methods. The other infor source would be the Parallax web site (or Nuts and Volts magazine.)
Good luck on your project and stay out of trouble!
GH
I have already found out about 555 circuits late last night when I hassled my roommate about this project again.
I don't think that I need to worry too much about FCC for creating an avalanche trainer because of the broadcast strength and my location. An avalanche beacon is picked up only up to about 20-40 meters away by another beacon, and this will not be used in avalanche terrain while a group other than my own is around.
Thanks,
-Fenris
oh yeah, I was a paper engineer. we were given a sheet of paper and told to make more from scratch. sigh....
Anyway, what it sounds like you are trying to do is similar to the amateur radio practice of "foxhunting." Foxhunting involves placing a low power transmitter of known frequency in an unknown place, and having a group of folks with radio direction finder gear locate it. I've never played, but it sounds like fun. Back in the day when people still made things, you'd build the transmitter and the receivers. There appears to be plenty of Goggle-able info on foxhunting. One such site I found quickly was:
http://www.ardf-r2.org/en/equipment.shtml
There are some schematics there, although admittedly, they are operating on a little shorter wavelength than you want. You're down about 660m, and the little transmitter I saw was at 80m. The principles should be the same. If you were up into VHF or something, then circuit design is more fiddley and so are the construction techniques. Using a 555 should actually work, and if I remember correctly, the ubiquitous Forrest M. Mims had some circuits using them for RF. IIRC, the 555 is good to a couple of MHz.
All you really need is an oscillator and a driver, though at the wattage you need (teeny-tiny) maybe it can be done all in one stage. Here's a scan out of one of my circuit books. Really simple crystal controlled oscillator.
I may have some other info, if you'd need it. I'm no EE, but I play one on TV.
Mike