RESONANCE AND BANDWIDTH
Compliments of Firestik® Antenna Company Technical Support Team
Copyright © 1996 Firestik® Antenna Company
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When an antenna is designed the engineer has a specific band in mind. It could be fairly narrow such as the CB band (440KHz) or relatively broad like the 2-meter band (4000KHz). Knowing full well that an antenna, for the purpose of this discussion, can only be resonant at one particular frequency, the center of the particular bandwidth becomes the target design frequency. For example, the CB band starts at 26.965MHz and is 440KHz wide. So, 1/2 of the bandwidth added to the low frequency places the center frequency at 27.185MHz (which is CB channel 19). For any given antenna length, the two primary starting points for design involves frequency and impedance; frequency as determined by the transmitter/receiver design and impedance as required by the equipment's circuitry. With the antenna resonating at the center frequency, the impedance falling into the acceptable tolerances of the radios circuitry, and the availability of a suitable ground plane it is likely that the SWR will be at or near 1.1:1 at that particular frequency. That reading would indicate that the antenna is absorbing nearly all of the energy that is coming from the transmitter. Nonetheless, even if you achieve a 1.1:1 SWR at the target center frequency that doesn't mean that the antenna will be marketable for there is the problem of bandwidth. The term "bandwidth" primarily refers to the frequency span from the lowest to highest frequency of an allocated service area. The bandwidth of CB is 440MHz, of the 10-meter amateur band it is 1.7MHz, of the 2-meter amateur band it is 4MHz and at the 70-centimeter band it is 30MHz. Ideally, an antenna designed for a particular band will have sufficient bandwidth itself. That is, when the antenna is tuned and resonant at the center of the band that the SWR on the two extreme ends of the band will not exceed the generally acceptable measurement of 2.0:1 or less. When it comes to vertical mobile antennas, frequency and bandwidth go hand and hand. As a general rule, the higher the frequency, the broader the bandwidth will be for a given antenna design. Likewise, the lower the frequency, the more difficult it is to design antennas with sufficient bandwidth. Thus arrives the all too often killer of an otherwise acceptable antenna, namely, bandwidth. As is obvious to those who have spent a little time on our web site, antennas of a specific frequency can be designed in many lengths. Our CB antennas are offered in 2 foot, 3 foot, 4 foot, 5 foot and 7 foot lengths. All can be set to resonate at the CB center frequency of 27.185 and all have impedance characteristics that fall within the acceptable range of the transceivers. However, all have different bandwidths. Specifically, the taller the antenna is, the more bandwidth it will have. For instance, suppose you installed a 2-foot antenna on your vehicle and tuned it to achieve equal SWR on channels 1 and 40. Perhaps the readings would be 1.2:1 on channel 19 and 1.9:1 on channels 1 and 40. Then, you replace the 2-foot antenna with a 5-foot antenna and tune it as you did the 2-foot antenna. Perhaps your readings would now be 1.1:1 on channel 19 and 1.4.1 on channels 1 and 40. Since nothing else changed, it is obvious from the SWR readings that the longer antenna offers more bandwidth Before closing this discussion, the topic of resonance needs to be expanded. Even though we spoke about an antenna as only being resonant on one particular frequency, there is more than what meets the eye. Nearly every antenna will have additional resonant frequencies other than its primary design frequency. In many cases, the harmonic or shadow frequency, as they are often called, could make the antenna technically superior on a frequency other than the primary frequency for which it was designed. An example of this was seen in some field tests using an FS3 CB antenna. On the particular installation, the SWR on CB channel 19 (27.185MHz) was 1.45:1 and the sub 2.0:1 bandwidth was 465KHz. However, at 58.640MHz the SWR was 1.05:1 and the sub 2.0:1 bandwidth was 5140KHz. Furthermore, the antenna had harmonics at 86.160MHz, 119.710MHz and 137.450MHz, all with SWR below 1.7:1. |
Updated 2023.12.07