An antenna’s length defines its resonant frequency. For an antenna like a dipole, the length of each arm will be ¼ of the wavelength of the resonant frequency. The trick with the ¼ wavelength number is that the two arms equal ½ of the total wavelength. This results in maximum signal strength, as an individual cycle consists of one entire wavelength of two halves — one positive and one negative. Half of a wavelength captures the maximum amount of signal moving in the same direction; in other words an entire positive- or negative- going half-cycle can be utilized. If the dipole consisted of two arms half a wavelength long each, one arm would pick up a positive-going half-cycle, and the other arm a negative-going half-cycle. These two opposite cycles cancel out, leaving little signal.
As described above, the basic arm antennas, such as the dipole and the monopole should ideally have an arm length equal to ¼ of the wavelength. Some antennas, however, are a little different. The dipole and monopole have a relatively narrow bandwidth and a clearly defined resonant spot. Other antennas are designed to be broadband, and work differently.
A log-periodic antenna, for instance, is actually a series of properly spaced and sized parallel dipoles. Each dipole is set to a different resonant frequency. Collectively the two dipoles work together to create a whole. The overall bandwidth of the log-periodic antenna, then, is set by the length of the arms of the smallest dipole and the length of the arms of the longest dipole.
Some antennas, such as the spiral antenna, use a different scheme to set bandwidth. The spiral antenna is a very broadband design. By nature of its design, the low frequency of a spiral antenna is set by its circumference — the bigger the circumference, the longer the length of the arms.
Low-frequency performance is often the limiting factor for any antenna, as the antenna simply must be long for efficient low-frequency reception.
Making Electrically Small Antennas Work
As can be seen above, the length of an antenna sets its resonant frequency, and is especially important at low frequencies. But what if there isn’t enough room to make the antenna long enough? There are ways around this.
One common trick is to artificially make the antenna longer by making the arm zigzag such that the overall length is more correct. This is not as efficient as a true, full-sized antenna. The zigzags, as they are going in opposite directions, tend to produce counteracting current movements in the antenna, reducing signal strength. However, this type of antenna may very well work better than a simple, too-short straight arm. For example, the impedance will match better.
For too-small antennas there are ways to resonate the antenna at a lower frequency with the aid of external circuitry in order to bring up the signal strength. This can increase overall efficiency, but reduces the bandwidth by making the antenna’s “sweet spot” more critical. For an interesting application of this scheme, see our articles on “Antennas as Part of a Receiver.”
In the final tally, a shorter, artificially lengthened antenna will not be able to perform as well as its full-sized counterpart. That said, the results may still be sufficient for the application. In the end, the antenna length usually ends up being determined by size constraints as opposed to resonant frequency.
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