Hi guys,

I'm onto a new project. Before I start going down some dead-ends, I was hoping you might give me some guidance.

Let me describe the need first. There is a central receiver. Cost on this is not price-sensitive. It will be 2 miles (stretch goal of 5 miles but not req'd) from the transmitters. It receives the signals, distinguishes them, and logs error messages. I'm not too worried about this part.

The individual transmitters are the key to the design. They will be elevated, picture maybe something on a roof. Doing the inputs and controls isn't an issue, I can handle that. The challenge is the transmitter circuit. I need to keep costs within reason, maybe $20-$50. My thoughts are to have it transmit status every couple of minutes with a unique code so the receiver can constantly know if it's failed. It will be just a few bits of data, so I'm not sending video or anything like that. And I can't have a dish for the transmitter antenna, rather it needs to stay fairly small.

I picture something like a glorified car alarm system. I need to use open frequencies and not fight the FCC. I need the range without using a monster transmitter. Surely there are some chips designed to do all of this? I'm not looking for a packaged transmitter board, I think those are outside my price target.

Any suggestions?

Thanks
Tony

 

Is this pure digital, or some audio? Look into FMS, it has the range your after. Not sure what the regs are for digital on it though, probably a no go.

 

I need to transmit just a few bytes each time. Basically a serial number code to identify the source, and a couple of bytes of information. I'm not transmitting voice, video, or anything else with substantial data.

I did a quick search for FMS. I found mostly small walkie talkies. Any recommended sites that get into circuit design, recommended chips, etc?

Thanks
Tony

 

To do what you want to do is going to require a license. If this is a commercial project something other than a HAM license. If it is not a commercial project then a Amateur Radio license (AKA Ham) would cover it, especially since you want to build it from scratch. This is not a technical issue, it can be done easily, but you are going to need decent power levels, which means FCC permissions.

Part of the reason I mentioned FRS (I had the wrong initials) is the lack of a license requirement. It has some channels that require a specialty license, similar to what used to be required for a CB, that is very easy to acquire.

http://en.wikipedia.org/wiki/Walkie-talkie#Personal_use

http://en.wikipedia.org/wiki/Family_Radio_Service

 

Bill,

I'm well aware of this. Years ago I did EMI testing for FCC/CE certification so I know FCC all too well. But I did testing, I didn't do RF design. Rather I worked on reducing emissions...

But now I'm looking for some guidance on the design for intentional radiators!

Tony

 

Are you wanting antennas or circuitry? The FRS can have every feature you listed, including the ability to only answer codes set by the transmitter, squelch on signals that don't have the ID codes, and the ranges you listed as desirable (2 miles to 5 miles). This was explained in the links to the articles.

Reading the original post I was under the impression you wanted to design the whole thing from scratch, or close enough. Given the commercial units already have the features...

The traditional antenna on FRS is a short stub. I don't know what the frequencies are, but this puts them way up there, approaching 1Ghz.

I also get the feeling you want a slightly directional slant to it, but not too directional to cover a wide area.

If it is help with antennas we need more info, such as frequencies. If it is help with the circuitry we still need more info. Basically we need more information what you're trying to do. Under what permissions you are using with the FCC would be nice too. I have no clue as to what your technical background is. My Associates Degree was in RF and Communications, followed by a nice stint with Rockwell, Collins Radio Division.

Speaking for myself I would be much more comfortable if their was a license on your part somewhere to make this legal. For the miles of range you are wanting to cover 10mw won't cover it. You could use 27Mhz CB or FRS frequencies, but both of those need licensed transmitters (the design, not the user). Hams are allowed to build transmitters with power, but the license is personal there.

Bill
formerly KB5BNW

 

If digital on FRS is a No-No, what about using DTMF to send data codes?

 

I don't know that it is, they already do it on the high dollar versions according to the Wikipedia articles. I have a couple of cheap unit in reserve, it is handy to have decent long range walkie talkies now and again.

Mine also have a phone type input for earphone and mic, so interfacing to them is covered.

 

Hello,

In order to transmit the digital signal, you can modulate it in the audio spectrum using Frequency Shift Keying.
Tutorial 17 - Frequency Modulation (FM) , FSK, MSK and more

This PDF is one of the links from thsi page:
http://www.complextoreal.com/tutorial.htm

That way you are transmitting an audio signal.

Bertus

PS keep in mind that the FCC can be hars when you violate the rules.