MULTI-BAND RADIOS
Compliments of Firestik® Antenna Company Technical Support Team
Copyright © 2002 Firestik® Antenna Company
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Back in 1977 23-channel crystal controlled CB radios were replaced with 40-channel phase-lock-loop (PLL) radios. Soon thereafter, technicians started figuring out how to expand the number of available channels. For the most part, those "conversions" would add 40 channels below CB channel 1 and 40 channels above CB channel 40. These 120 channel hybrids offered a bandwidth of about 1250KHz (26.565 to 27.805MHz). At the time, there were not very many mobile antenna options available with enough bandwidth to maintain safe standing wave ratio (SWR) over the 1.3MHz spectrum. If you were able to tune the antenna to keep SWR below 2.0:1 on the highest and lowest channels, you were fortunate. Even though these "less than legal" radios have been around for many years, during the last 10 years the sale of these so called "export" radios have soared. Some of the new radios offer over 300 AM channels. One of the lesser 240 channel radios, after a simple "from legal to illegal conversion", will transmit from 25.165 to 28.755MHz. In order to satisfy those requirements with a single antenna, the antenna would need to have a minimum bandwidth of 3.6MHz and be tuned to the center resonant frequency of 27.960MHz. If there is an antenna on the market, or one that was ever made for mobile use that would cover 3.6MHz with one setting, we don't know about it. So, where does that leave the owner/operator of the multi-band radio? If you have an understanding of radio and antenna applications then you will understand the dangers of operating a wideband radio with a relatively narrow band antenna. Unfortunately, people who don't understand the pitfall that they are walking into with the equipment are buying most of these radios. They go into a store to buy a CB and are shown a radio with 240 channels. "Wow! That has gotta be a heck of a lot better than 40 channels!" So they buy the radio and some "whatever" antenna and off they go, heading for problems. This discussion is primarily aimed at those of you who need to strengthen your understanding of bandwidth and antennas. There are several components of any two-way radio antenna that you need to understand. First and foremost is resonant frequency. Without getting into harmonics, understand that transmit antennas are single frequency resonant devices. Technically speaking, antennas can only be resonant at one specific frequency. When you move either up or down in frequency (channel) from the resonant point, some efficiency is lost. The further you move from the resonant frequency, the higher the SWR will be. Higher SWR means that the antenna is absorbing less energy, and likewise, will be radiating less energy. As you move further away from the antennas resonant frequency you will eventually hit a point where the SWR gets too high and the final output transistor in the radio will overheat and die. That brings us to the second component, bandwidth. Some antenna designs offer more bandwidth than others do. That is, they allow you to move further from the antennas resonant frequency and absorb enough energy that the SWR doesn't become dangerously high. When we consider bandwidth, we are looking for the distance in kilocycles (KHz) that we can move up and down from the antennas resonant point without exceeding an SWR reading of 2.0:1. Remember that SWR refers to reflected energy, or that which the antenna cannot absorb. If too much is reflected back to the transmitter, it will probably damage the radios output transistors. Generally speaking, taller antennas, antennas that have more diameter and those wound with a heavier gauge of wire will have more bandwidth than shorter, skinnier or those using a lighter gauge wire. The available ground plane and the antennas position over the ground plane can also have an affect on sub-2.0:1 SWR bandwidth. Accordingly, if an antenna has 1MHz of bandwidth on one vehicle, which doesn't mean that it cannot have more or less on another vehicle or in a different location on the same vehicle. Insofar as mobile antennas are concerned, when you are operating below 30MHz it is difficult to make an antenna that can hold SWR below 2.0:1 over a 2MHz bandwidth without exceeding 5 feet (152cm), and that is over a high quality ground plane. So, you can see the problem in trying to get one antenna to safely operate on the radios that are exceeding 3.5MHz of bandwidth. One thing that you need to be careful of when buying antennas is misleading advertising regarding bandwidth coverage. Some companies advertise their antenna as having the ability to cover frequencies from 25 to 30MHz (may be stated as the tunable range). That statement alone is very misleading. The fact that an antenna can be tuned over a 5MHz range does not mean that it has 5MHz of bandwidth. That is, if you think that you can tune the antenna to be resonant at 27.500MHz (center of the 25 to 30MHz claim) and the SWR at 25.000 and 30.000MHz will below 2.0:1 you will most likely be in for a surprise. In most installations you do not have "test-site" quality ground plane and probably will not have the antenna mounted in a location that takes the best advantage of what ground plane is available. So, if you can hold SWR below 2.0:1 over 1MHz that would be considered very good. Therefore, your antenna tuned at 27.500MHz is going to supply you with sub-2.0:1 SWR from 27.000 to 28.000MHz, not the 25 to 30MHz you might have thought you were going to have. To make the antenna work safely from 25 to 30MHz you would need to tune it at five different resonant frequencies to take full advantage of the wideband radios capabilities. That is:
To cover 25 to 26MHz: Center tune at 25.500MHz It isn't an impossible situation but every time you switched bands you would need to pull off to the side of the road and retune the antenna. The bottom-line, if you decide to buy one of these wideband radios you need to be aware of the limitations placed on them by the antenna system. If you ignore it, you will end up spending more money fixing your radio than you did for the radio itself. |
Updated: 2023.12.05