100MHz GROUND PLANE ANTENNA

Here you can find plans for a simple ground plane antenna that works well in the FM Band (88-108MHz). It's built of a small plastic disk (around 5cm in diameter, not critical) and stiff copper wire (preferably enamel coated) some 3mm thick. It's omnidirectional, therefore it works equally well in all directions. This makes it suitable for most applications.

Please excuse the cheesy graphics, it's the best I could do in 5 minutes with Paint Shop Pro. As you can see, there are four wires used. The three lower ones are all 50cm long, symmetrically placed (one at every 120° horizontally) and bent at 45° vertically. Connect them together to earth ground. The longer 75cm wire should be connected to the transmitter output and must have no electrical connection whatsoever to ground. That's why a plastic disk is used to hold everything together.

It's a good idea to fasten a BNC connector at the bottom of the plastic disk so that you can connect a coaxial cable more easily. Use the central pin to carry the signal and the metal case for ground. The coaxial cable should be 50 Ohm and at least 10m long. The transmitter and the antenna must not "see" each other.

Erect the antenna by attaching a long plastic pipe to the central insulating disk. Make sure that neither wire touches anything. There should be no other antennas or power lines in the vicinity.

To achieve best results with FM transmitters, you must first understand some basic principles. VHF radio signals travel in a straight line and do not reflect from the ionosphere like HF. Therefore, their range is limited to the line of sight. Far more important than power is the height of the antenna. This is the factor that actually determines the maximum range. Mount the antenna as high as you can and just don't worry if your transmitter has the output power a Watt or two lower than you'd like.