I'd wonder if anybody has any ideas about the legality of such a system. Sounds like either a jammer or an eavesdropper, to me. Both of which may not be legal, if you care.

Fellow subscribers,

I am well aware of the legalities of such a device and did not post my question here for such responses. I was educated in mechanical engineering therefore only have a basic knowledge of electronics. I have searched for the existence of such devices and have attempted to gain further knowledge on the subject but this self education has only taken me so far. Therefore I have turned to this forum for assistance in furthering my project. Please keep responses on topic. Thank you.

Thank you for the clarification. This will help me stay on topic in the future.

Your attempt, " to gain further knowledge on the subject" is admirable.

^_Bullshit!

I don't know what country you are in but you can forget it if you plan on building a transmitter working in any of the the cellphone frequency bands as this is illegal.

It is also highly likely that you would need a licence and test report to operate a transmitter even if you were operating in one of the unrestricted frequency bands. Refer to 47 CFR Part 15 (Radio device regulations) and the FCC or equivalent local radio spectrum management division for more information.

http://www.fcc.gov/oet/info/rules/

Radio frequency device approval is a very detailed and confusing area and it is by far the simplest and best option to buy an already approved complete radio frequency (RF) device or module that can be incorporated in a product or design.

eg - http://www.google.co.nz/search?hl=en&source=hp&q=radio+module&meta=&aq=f&oq=

What is the application?

There are probably twenty five signal generators on the market that would do what you are asking for. Older ones are available on the used market at bargain rates.