Low Power Antenna Design
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Contents |
Introduction
A good antenna design is required to realize good range performance and be the right type for the application. It must be matched and tuned to the transmitter and receiver. To get the best results, a designer should have an idea about how the antenna works, and what the important design considerations are.
Basic Terminology
Wavelength - Important for determination of antenna length, this is the distance that the radio wave travels during one complete cycle of the wave. This length is inversely proportional to the frequency and may be calculated by: wavelength in cm = 30,000 / frequency in MHz.
Ground-plane - A solid conductive area that is an important part of RF design techniques. These are usually used in transmitter and receiver circuits. An example is where most of the traces will be routed on the topside of the board, and the bottom will be a mostly solid copper area. The ground-plane helps to reduce stray reactants and radiation. Of course, the antenna line needs to run away from the ground-plane.
dB (decibel) - A logarithmic scale used to show power gain or loss in an RF circuit. +3 dB is twice the power, while -3 dB is one half. It takes 6 dB to double or halve the radiating distance, due to the inverse square law.
The Basic Antenna
An antenna is a conducting wire that carries an alternating current. This current generates an electromagnetic field around the wire which varies with the current through the wire. If another wire is placed nearby, of similar dimensions, the electromagnetic field will induce an electric current similar to the original at a reduction. If the wire is relatively long, in terms of wavelength, it will radiate much of that field over long distances.
The simplest antenna is the “whip” which is a quarter wavelength wire that stands above a ground-plane. This design is most commonly found within vehicles for broadcast radio, CB and amateur radio and was discovered by a gentleman by the name of Guglilmo Marconi in the 1890's.
Universally, antennas require at least two connection points as with any other electronic component. The whip antenna has a connection to ground with the completion of the circuit being formed though the electromagnetic field that exists between the whip and the ground-plane. Another consideration when designing an antenna is the size of the ground-plane. In this case, it should be at least a quarter wavelength radial spread around the base of the whip. To be made smaller would affect the performance and would require a longer whip to compensate.
Antenna Characteristics
Gain - Antenna gain can be thought of as how strongly the antenna radiates compared to a reference isotropic antenna where an isotropic antenna radiates equally in all directions. An antenna that radiates poorly has low “gain”. A dipole is similar to a whip, but the ground-plane is replaced with another quarter-wave wire. Overall performance is about the same. An antenna that is 6 dB less than a dipole is -6 dB. This antenna would offer one half the range, or distance, of the dipole. Compact antennas are often less efficient than a dipole, and therefore, tend to have negative gain.
Radiation Pattern - Antennas radiate in concentric circles centred on the antenna itself. For the whip antenna, the radiation pattern can be described as omni-directional with a ‘null’ point at the end of the whip. This means that the signal will be at a minimum at the end.
A vertical whip will therefore be ideal for communication in any direction except straight up. When designing an antenna, it is important to know the radiation pattern for the particular antenna being designed so as not to have a ‘null’ point in the desired direction of propagation.
Polarization - Orientation or polarization of the antenna is almost important. If two communicating antennas are inversely polarized, the signal reception at each antenna would suffer greatly. Signals may also become either linearly or vertically polarized upon leaving the antenna and reflecting of metal objects and the ground.
Impedance - Another important consideration is how well a transmitter can transfer power into an antenna. If the antenna tuning circuit on a transmitter (or receiver) is designed for a 50 ohm load, the antenna should, of course, have an impedance near 50 ohms for best results. A whip over a flat ground-plane has an impedance near 35 ohms, which is close enough. The impedance changes if the whip is mistuned or bent down, or if a hand or other object is placed close to it. The impedance becomes lower as the antenna is bent closer to ground. When the whip is tilted 45 degrees, the impedance is less than 20 ohms. When the whip is bent horizontal to one-tenth of a wavelength above ground, the impedance approaches 10 ohms. The resulting impedance mismatch, a 5:1 ratio (VSWR) will contribute an additional loss of 2.6 dB.
Types of Antenna
- Printed Circuit Whip, or “Stub”
- The Short Whip
- The Short PCB Stub
- The Spiral
- The Helical (Coil)
- “Chip” Antenna
- The Loop
- Semi-Loop
- Modified Dipole Antenna
- The Slot
- The Patch
References
- Miron, Douglas B., Small Antenna Design, Communication Engineering Series, Newnes, 2006
- Smith, K., Antennas for Low Power Applications, RFM Corp
- Milligan, Thomas A., Modern Antenna Design, WileyBlackwell; 2nd Edition edition, 2005
