Tutorial on Antenna DesignPart 2
In part two, the focus is on antenna operation and determining antenna length.
By: Engineering Staff, Linx Technologies
Note: In part one (see Tutorial on Antenna DesignPart 1) of this tutorial, we focused on transmitter/receiver antennas and transmission lines. Now, let's look at how an antenna works.
Contents
How antennas work
Determining antenna length
Determining length
How antennas work
The electric and magnetic fields radiated from an antenna form an EM field. The field is responsible for the propagation and reception of RF energy. To understand an antenna's function properly, an in-depth review of voltage, current, and magnetic theory would be required. Since this is not in keeping with the brief and basic nature of this article, a simplistic overview will have to suffice.
Assume for a moment that a coaxial transmission line was stripped and the shield and center conductor were bent at right angles to the line as illustrated. Presto, a basic antenna called a half-wave dipole has just been formed.
An engineer may wonder how two pieces of wire that were originally intended to contain RF energy are now able to radiate it efficiently into free space. Since the lines are now separated with the ends open, a difference in voltage between the two points now exists, allowing an electric field, called an (E) field, to exist.
In addition to the (E) field, a magnetic field generated by current also exists, which is referred to as the (H) field.
When RF energy is introduced onto the antenna element, the (E) and (H) fields alternately build up, reach a peak, and collapse (Figure 2). Together these fields make up EM waves that are able to radiate into and be received from space.
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Determining antenna length
An antenna can be considered as a complex resistance capacitance (RLC) network. At some frequencies, it will appear like an inductive reactance, at others, like a capacitive reactance. At a specific frequency, both the reactances will be equal in magnitude, but opposite in influence and will cancel each other. At this specific frequency, the impedance is purely resistive and the antenna is said to be resonant.
Maximum antenna efficiency is always obtained when the antenna is at resonance. When an antenna is not of the proper length, the source will see something other than the pure resistance, which is present at the resonant point. If the antenna is too short, capacitive reactance is present. If it is too long, inductive reactance will be present.
The indicator of resonance is the minimum point in the VSWR curve. In Figure 3, an engineer can see that antenna (A) is resonant at too low a frequency, indicating the antenna is excessively long, while antenna (C) is resonant at too high a frequency, indicating the antenna is too short. Antenna (B), however, is as the three bears would say, "just right."
Obviously, it is critical that an antenna is the length necessary to assure resonance at a desired frequency, but the real question for the engineer is how to determine that length.
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Determining length
Every frequency has a certain physical length that it occupies in space. That length is aptly referred to as the wavelength and is determined by two factors: first, the frequency itself, and second, the speed of propagation.
In free space, a frequency's wavelength can be found using the following formula:
L= 984/FMHz ft
However, since an antenna has a dielectric constant greater than that of free space, the velocity of a wave on the antenna is slower. This, along with several other factors, has led antenna designers to accept the following formula as accurate for all practical purposes in determining the physical length of a full-wave antenna:
L= 936/FMHz ft
While this formula is excellent for getting the antenna's length in the ballpark, always bear in mind that the true issue is antenna resonance. Depending on physical factors such as antenna diameter, nearby conductors, and other limitations, it may be necessary to add or cut the antenna slightly to reach resonance.
An antenna does not have to be the physical length of a full wave in order to operate. Indeed, most of the time for size and impedance considerations, the antenna will be some fraction of a full wavelength.
A half-wave antenna is the shortest resonant length of an antenna. However, shorter wavelengths can be resonant on harmonics. For example, a common antenna used in today's wireless systems is the quarterwave antenna, which is defined as one quarter of a wavelength.
In order to operate effectively, the quarterwave must radiate against a ground-plane. The ground-plane can be a metal case or ground area on a printed circuit board (PCB) of at least equivalent area to the antenna's surface. The ground-plane acts as a counterpoise that forms the other quarterwave element, in essence forming an effective half-wave dipole.
The three-part series will conclude next week. The last installment will focus on issues such as gain, polarization, and multipath effects.
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About the Author:
Engineering Staff, Linx Technologies, 1089 Medford Center, Bldg. 137, Medford, OR 97504. Tel: 800-736-6677; Fax: 541-471-6251.