Many antenna design applications require an antenna to support more than one frequency. Most RF communications operate at multiple frequencies, and it is periodically desirable for a device to operate in several different bands — all with the same antenna.
Broadband vs. Multiband
One thing to keep in mind is the distinction between broadband antennas and multiband antennas.
Most multiband devices only need good performance at specific, relatively narrow frequency ranges as opposed to covering a large spread of frequency ranges.
The Low Frequency Factor
The biggest limiting factor with any multiband antenna is the lowest frequency to be used.
Just as for any other antenna, the lower the frequency, the larger the antenna must be to perform well. The definition of “performing well” is, of course, dependent on the user’s minimum requirements. Shrinking the antenna will cause the performance to suffer.
Broadening the Bandwidth by Using Resistance
Generally speaking, many standard antennas (such as the dipole) do not make very good multiband antennas.
The dipole is sized for a specific frequency, and the gain and/or pattern will tend to suffer outside of a surprisingly narrow frequency range centered around the resonant frequency of the antenna.
As mentioned in a previous article, the bandwidth can be increased by adding resistance to the antenna; the trouble with this is that the gain is “flattened” by lowering the resonant frequency performance to that of a level closer to the non-resonant frequency gain. This does help VSWR, however.
In some cases where gain is not too terribly critical and the required bands are not too far apart, adding resistance to the antenna can provide a simple, if inefficient, solution.
Inherently Broadband Antennas
There are antennas that are inherently broadband, such as the spiral antenna.
A noteworthy example is the log-periodic antenna. This antenna type essentially consists of a series of different frequency dipoles all fed in parallel, resulting in broad performance.
This idea opens up another possibility ideally suited for multiband applications: multiple resonances.
An Excellent Solution: Multiple Resonances
A multiple-resonant antenna essentially takes a standard antenna (such as the inverted-F) and adds another resonance, either directly to the feed or as a non-directly connected parasitic antenna placed in close proximity to the main radiating element.
Now, there is one extremely important consideration to recall in multiband antennas, particularly of the multiple-resonant type of antenna. Adding a parasitic element does, in fact, change the performance of the main element. If the main element is barely performing to acceptable criteria, the parasitic element may, even while increasing performance in other bands, wreck the performance of the main element. Thus, we end up with an admittedly complicated situation where improving the performance within one band will affect the performance of another band.
Simulation software (such as Altair FEKO) can be helpful in finding a feasible starting point in complex situations such as this.
Of course, the physical antenna of the desired type will need to be real-world tested to ensure that it actually meets the specified criteria.
ARSI 