COMPONENTS OF AN EH ANTENNA
The following comments define the components of the EH Antenna. With reference to the drawing at right, note that there are two (2) cylinders. The size of the cylinders defines the amount of capacity between them, thus the instantaneous bandwidth for a given operating frequency. The spacing between the cylinders should be the same as the diameter.
The phasing coil provides a phase shift in the line between the tuning coil and the upper cylinder. This prevents the line having the same phase as the cylinder, thus preventing the line from radiating. This coil should be as large as the space allows. However, there will be a large voltage between cylinders, thus the phasing coil is best wound inside the upper cylinder. Because there is a large voltage developed across the tuning coil, the amount of the coil used as the phasing coil will reduce the amount of voltage on the line between the tuning coil and the phasing coil.
The inductance of the tuning coil (plus the phasing coil) will be in parallel with the capacity between the cylinders, thus causing the antenna system to be resonant at some frequency. It should also be noted that there will also be distributed capacity across the tuning coil and there is also capacity between the antenna and ground (where ground is defined as the other ‘plate” of the capacitor). That plate could be the coax, or the tower the antenna is mounted on. To maximize the bandwidth it is best to mount the antenna on an insulated pole such as plastic or fiberglass for at least two (2) antenna lengths below the antenna. The spurious capacitors will also affect the resonant frequency.
The line between the tuning coil and the phasing coil should be located outside the lower cylinder. If it is run inside the lower cylinder the radiation resistance of the antenna will be much lower compared to the value if it is located outside the cylinder. The spacing should be about one (1) cylinder diameter. The wire to the lower cylinder is not critical and can be located inside the lower cylinder.
The source capacitor reactance should be approximately 100 ohms at the operating frequency for 50 ohm feed line. The exact value is not critical.