Could I make a transmitter with just a microcontroller

[tackyattack]

Would this work? If I had a microcontroller output a 400mhz signal through one of its I/O then to a loop of wire. Would it transmit a RF signal at 400mhz?

 

[waht]

Yes, it would transmit that and integer multiplies of that frequency, since the output from IO is more like a square wave than a pure sine wave. In either case, you'd have to follow up with an output low pass filter to attenuate the higher harmonics. But I don't know of a microcontoller that could do that straight out of the IO pin.

Such signals are easier generated by a transistor oscillator, a simple one transistor oscillator could get you on UHF in no time. But there are a host of other problems to consider.

 

[Jiggy-Ninja]

Another issue is the amount of power you're likely to get from just a micro controller. I believe most micro controllers can only put out 20 mA max, with a square wave that works out to about 50 mW. However, it's very unlikely for a uC output to be impedance matched to the antenna, so the amount of actual transmitted power is probably much lower even than that.

 

[tackyattack]

How could I increase the RF power?

 

[HGE]

There are uC's with integrated rf receiver/transmitter, like rfPIC12F675F

http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en010385

 

[MATLABdude]

There are also very few (hobbyist) microcontrollers that operate fast enough for you to output a 400 MHz modulated signal. As HGE suggests, find one with the RF built-in--in addition to Microchip, I believe TI has some as well.

EDIT: Welcome, HGE!