Vertical & Groundplane Antennas

These antennas are what is known as “image plane” antennas. The image is the missing half of the antenna that makes it appear as if it were a dipole whose center is at ground, or at the junction of the ground plane wires. To a large extent, they behave as if they were ½ of a dipole, with an impedance of approximately 36 W.

Vertical antennas are even more sensitive to the quality of the ground around them then are dipoles. However, if the conductivity of the ground is high (as happens near the shore), outstanding low angle propagation is possible.

A ground plane antenna is a vertical antenna that has a simulated ground plane at its base. This ground plane can take many forms, but the most common is four equally spaced radials. The ground plane can also take the form of a metallic sheet, like the roof of a tin building, or an automobile.

Ground plane antenna patterns are affected by the height over earth, but for VHF use usually an antenna will be a significant fraction of a wavelength off the ground. The higher the antenna, the more lobes – but the lowest lobe continues to get stronger. This makes the antenna useful for terrestrial communication.

One of the things that can be done to improve the impedance match of the ground plane antenna is to tilt the radials down at approximately 45° as shown on the right. This should yield a very close match to 50 W cable.

Discone Antenna

Another vertical antenna often seen in VHF and UHF operations is the discone (Disc + Cone). The discone is similar to half of a bicone (a wideband vertical dipole where the elements appear as a pair of cones, mounted point-to-point), where the disc acts as an equivalent ground plane termination. At microwave frequencies both the disc and skirt are constructed of sheet stock, whereas at UHF frequencies the disc may be solid with a skeleton skirt. At lower frequencies, the skeleton radial approach is the norm.

Even with the horizontal disc, the discone is a vertically polarized antenna (like a bicone with a similar orientation). The discone acts as a high pass filter, having a high SWR at low frequencies, followed by a long region of more or less constant SWR over a 6:1 or 8:1 frequency range. This makes the antenna well suited to cover a number of VHF/UHF ham bands as well as the intervening public service bands. The antenna has an omnidirectional azimuth pattern similar to that of a vertical dipole.

A six-meter version would have L = 5 ft, while a two meter version would be more like L = 2 feet. Mounting is usually done with a pole that supports the skirt wires and provides the connection to the shield of the coax feedline.