Wire Antennas by Don Johnson, N4DJ. Part 1 of 3

Wire Antennas by Don Johnson, N4DJ

Part 1

Wire Antennas at first glance can appear to be very simple. In some respects they are just that. In other respects they can be quite involved. There is probably an infinite number of possibilities for a wire antenna. For example, there is the feed point location. It can be in the center, at either end or anywhere in between. Next is the length of the wire. It can be anywhere from very short to very long. It can be resonant or non resonant. We usually talk about the length of the wire in terms of wavelength. For example it may be 1/2 wavelength or one wavelength. It may also be measured in degrees, 180 degrees being 1/2 wavelength and 360 degrees being one wavelength. 

The length is the major characteristic that determines the radiation pattern or directivity. The length and feed point together with distance above ground are the major items that determine the impedance of the antenna.

Almost any piece of wire can be used as an antenna, but there must be some way to supply power to the wire. The problem becomes one of matching the impedance of a feed line to the impedance of the antenna at the point the feed line is attached. Since the most common coax feed line has an impedance of fifty ohms, I will use this number in most of the examples.

In the most simple case, if there is a point on the wire that exhibits an impedance of or near fifty ohms, we can connect the feed line there. If a wire is close to a half wavelength in length, the center of that wire will have an impedance in the neighborhood of 50 ohms. Actually, the impedance of a half wave wire in free space is 73 ohms at the center. If that wire is close to the earth, the impedance will vary around 73 ohms for heights of half wave and above. For heights less than half wave the impedance will decrease as the antenna height decreases. For all practical purposes, unless the antenna is very close to the ground, the center point of a half wave wire or antenna will be a good match to 50 ohm coax. 

In dealing with impedances we must consider both resistance and reactance. In the case of 50 ohm coax cable we need to connect the coax to a load that is not only 50 ohms impedance, but the 50 ohms needs to be resistive and not reactive. With the half wave wire, we find that the reactance at the center (if you feed it at the center) is close to zero when the resistance is near 50.  In fact regardless of where you feed a resonant wire the impedance (at the feed point only) is mostly resistive as long as you are close enough to the resonant frequency.  As you move away from the feed point of a resonant wire, the impedance increases. In other words the ratio of voltage to current increases.  Both the resistive and reactive components increase. A similar thing happens when a half wave wire at one frequency is operated "off frequency". If the wire is fed at the center, but is no longer a half wave long the impedance will quickly become reactive and no longer present a 50 ohm resistive load to the coax. This simple antenna is called a half wave dipole. It is one of the simplest and best antennas if not operated too far off its design frequency. It is also a major building block for more complicated antennas.