Is it possible to build a rf or fm transmitter that would fit inside a pen and run on a small battery with a range of 100yards?? It only need to emit a frequency or a beep that i could use a receiver to find it.

 

There are many "spy" transmitters available at tons of web stores.

Trying to get 100yd range from one is a different issue, unless it is line of sight and you have a very directional antenna on a good receiver, and the band used is "clean" (no distant radio stations near frequency).

 

what frequency range? Do you have a receiver for that frequency range? What's the application?

 

frequency dosent matter, I do not have a receiver but im guessing that will be the easy part. I do not know much about electronics, but I would be willing to pay someone to build me one. It would also need to run on a very small battery. It will be used to track small animals in the woods.

 

You should type in obvious search terms into your favorite search engine. I typed in "radio tracking device for small animals" and found something suitable in the first few hits: http://www.jbgizmo.com/page4.html and http://www.jbgizmo.com/page32.htm (and here's one that uses an FM radio receiver). The schematic is there along with a parts list; I'd imagine any ham could construct the thing for you. That's half the battle, as you'll also need a receiver (a ham could help you with that too).