For the antenna, you can either use 1/4 wavelength or 1/2 wavelength. Do you know about wavelengths? hopefully you do or this will make no sense.
If you're transmitting @ 100MHZ 1/4 wavelength is 75cm and 1/2 wavelength is 150cm.I'm not sure what the difference is in using 1/4 as opposed to 1/2.
One thing that I dont yet grasp because i haven't found any suitable calculations/explanations is impedance matching (gotta get one of them aarl handbooks). This applies very much to antennas.
OTHER TIPS:
1. DO NOT assemble it on a breadboard
2. When testing it, make sure you have your radio 10 feet from the transmitter. The transmitter can overload the radio.
3. Make sure you have your transistors in the right position.
4. Test your battery before you plug it into the transmitter. a low battery can cause all sorts of weird strange things.
5. When you tune the variable capacitor, a screwdriver will mess up the frequency. Just when you think you hear the transmitter come over the radio as you're tuning the capacitor, when you take the screwdriver off, the frequency will change. A way to get past this is take a plastic knife, and shave it down so that it fits in the variable cap. it helps a lot :lol:
The reason that the screwdriver does this is because it becomes part of the capacitor and changes the capacitance.
6. Use this calculator to determine your base frequency i dunno if chemelec has a base frequency calcultor. if he does, use his, its probably more accurate.