Sizing Up PMR Devices
Tuesday, July 12, 2011 | Comments
 
By David Lum
 
Why are two-way radios so big? Why can’t two-way radios be as small as cell phones? While two-way radio devices allow for voice communications similar to cell phones, they are different than cell phones on many levels. There is a natural tendency to compare two-way radios to cell phones because of how prevalent cell phones have become to the public globally. The following highlights the differences to help users appreciate the use, utility and design of two-way radios.
 
The Differences
Target Markets. One fundamental difference between cell phones and two-way radios are the users of each. Professionals in industry and government use two-way radios, whereas mostly consumers use cell phones. Two-way radios are considered inputs of production and are required tools of the trade in industry and government to perform missions or produce goods or services. Cell phones are one-to-one communications devices, whereas two-way radios are one-to-many communications devices. Cell phones are used for individual calls, whereas two-way radios are used primarily as communications tools for organizations.
 
Because two-way radio is part of the value chain, most industrial or government users will buy a two-way radio system and use it internally as a tool, similar to how a company procures computers, vehicles and uniforms for its employees. Because a two-way radio system is procured as a capital cost to the organization, the entity attempts to minimize or optimize the cost of the system while still meeting their technical and operational requirements. One way to achieve lower two-way radio system costs is to minimize the number of sites comprising two-way radio infrastructure. Typical infrastructure power output for high-power, two-way radio systems is around 25 to 100 watts. In fact, one of the design philosophies of two-way radio technology is to maximize the coverage area so that as many two-way radios as possible can be used within a given site, minimizing the number of sites.
 
Power Levels. As a consequence of the high-power radio transmitters, a two-way subscriber radio —portable — must operate with a high power level to talk back into the infrastructure (talk-in). Typical power levels are about 1 to 5 watts. In contrast, cell phones are typically 0.6 watts. Because the cell sizes are larger for two-way radio systems, the talk-in range of a portable radio must be compensated with higher power output — up to a point.
 
Cellular networks benefit from smaller cell sizes. A smaller cell size gives cellular systems more capacity. More capacity means more cellular call handling capability and revenue. Contrary to cellular design philosophy, two-way radio systems benefit more from large cell sizes; the larger the two-way radio cell, the better, because a large two-way radio cell size can capture more portable radios to talk to.
 
Is it possible to operate a two-way radio system at lower power levels, thereby shrinking the radio size? Yes, it’s possible, and it has been done. But the tradeoff for a smaller radio size is a larger number of infrastructure sites. This kind of tradeoff impacts the initial capital acquisition costs of the system (higher because of more sites) and the longer-term operating and maintenance costs of the system during the life of the system (higher because of more sites). While the user of the portable radio may see a smaller portable device, the system owner may pay a significantly higher amount of money for the infrastructure. For the system owner, it’s a fine balance between making end users happy with the right size portable device that meets the technical and operational requirements versus the overall capital and ongoing operational and maintenance costs of the system.
 
Direct Mode. An interesting consequence of a higher power output level is that it also increases the reliability of a feature found in two-way radios that is unavailable in cellular — direct mode operation or repeater talk-around. This feature is a last resort form of two-way radio communications in case the portable radios lose connectivity with the infrastructure. This allows users to communicate directly with each other without the use of the infrastructure. For industrial and government users talking in direct mode, while shorter in range and limited in the number of subscribers that can communicate, having limited communications is better than none at all. Higher power output levels increase the reliability of the communications between the radios because the higher power level overcomes the signal path loss better in many environments.
 
Cell phones don’t have a direct mode feature, because there must be a way to charge for airtime usage. The infrastructure tracks the minutes used so that the cell-phone user can be billed properly. Direct mode bypasses the infrastructure, there would be no way to track the cell-phone airtime usage for billing. The low power levels of cell phones further limit the distance of direct-mode operation, making it an impractical feature for low-power devices.
 
Power Amps. High-power output levels have one major design consequence and impact on product design — the need for large, high power amplifiers (PAs). Conversely, low power output devices can use small PAs. The portable radio’s larger PA causes the internal design to be limited in how much it can shrink in overall size.
 
In addition to high PAs, the frequency band also impacts the size of the components inside the radio. A VHF radio has longer wavelengths than an 800 MHz radio. The VHF components and hybrids inside the radio tend to be larger than 800 MHz components, pushing the radio toward larger sizes. Cell phones operate at 850 MHz or higher, allowing for smaller frequency-sensitive components.
 
High PAs also require larger heat sink for dissipating the heat generated by the PA. The larger heat sink drives a larger physical form factor, also adding to the weight of the subscriber device. With cellular, the smaller power output generates less heat, allowing for a smaller heat sink — again, up to a point. Proper heat dissipation is critical for the safe use of a device. Some poor cell-phone designs that don’t have a good heat dissipation design will overheat during a long phone call. In other words, a poorly mechanically designed cell phone will get hot in your hand during a long phone call. The consequence of poor heat dissipation is a shorter life of the PA; the device will fail sooner.
 
Unlike cell phones that operate at 0.6 watts, portable two-way radios operate in the 1- to 5-watt range. Because of the higher power levels, the two-way radios must be designed to higher RF performance specifications than cell phones. The receiver must be able to operate next to a much higher power transmitter inside a compact handheld housing. The two-way radio receiver must still maintain a high level of sensitivity to work effectively in a high-power RF environment. Shielding and receiver designs contribute to better receiver performance and intermodulation (IM) rejection, and this can also impact radio size. Designing to excellent receiver specifications, while keeping the radio as small as possible, is one of the toughest engineering challenges. Once accomplished though, the high-quality RF specifications of portable radios will impact the overall infrastructure design for coverage. A high-performance radio can decrease the number of infrastructure sites.
 
Loudspeakers. In addition to the higher power requirement for a high-power PA, most two-way radio subscribers also have another high-power requirement — the loudspeaker. Two-way radios are found in many industrial environments that are high noise or have a lot of background noise. Unlike cell phones where users put the device directly on their ear, two-way radios use a loudspeaker that must be able to operate in loud environments. Loudspeakers require higher amperage to drive the speaker element to create the appropriate audio sound pressure for the user to hear the receive audio above the noisy environment.
 
The higher amperage requirements for a loudspeaker require a larger battery requirement, impacting radio size. The loudspeakers used in two-way radios are significantly louder than any cell-phone loudspeaker; cell-phone loudspeakers are usually designed for conference call applications inside an office environment and are rarely adequate for use on the street or inside a moving car. The loudspeaker itself may also be larger in diameter to provide the proper sound pressure and sound volume, which requires the size of the radio to be at least as wide as the speaker.
 
Ruggedization. Operating in rugged, highly industrialized and possibly hazardous environments, two-way radios require much thicker housings, stronger metal frames and critical sealing from hazardous environments to work under such extreme conditions. Industrial design and operating requirements constrain the radio design to a slightly larger physical form factor, resulting in increased weight, than commercial-grade products. Consumer products and commercial-grade products, such as cell phones and commercial-grade two-way radios, are not designed with these kinds of considerations, allowing for smaller, thinner, lighter housing designs. The difference is clear: Drop your cell phone a few times. It will break. Ruggedized industrial-grade two-way radios do not break under normal conditions in industrial environments.
 
If the radio is designed to be submersible, then the mechanical design must also be made slightly larger to accommodate special internal sealing in every possible area of water entry. Making a radio that is submersible or workable in a pressurized water environment, such as fire suppression operations, requires more rigorous sealing against water pressure rather than just water entry. This requires special silicon sealing, ultrasonic welding and high-strength bonding materials, all of which need additional space for a tight seal.
 
Encryption. Another factor keeping radios from shrinking are customer demands for unique features that cell phones don’t have. For example, two-way radios can be equipped with government-grade encryption for voice transmissions that require the highest levels of security. Encryption generally requires hardware encryption circuits, adding more circuitry and hardware inside the radio. If cell phones had to meet this kind of sophisticated user requirement, they would be bigger.
 
Market demands for two-way radios to have higher processing power also drive manufacturers to put more electronics into the radio. A modern digital two-way radio has more processing power and more features than a consumer-grade digital cell phone.
 
User Needs. An ironic scenario for portable is that the physical form factor might be too small for some operational environments. For example, firefighters must wear protective gloves to protect themselves from heat, fire and chemicals and can’t remove their gloves inside a hazardous environment to operate a radio. If the radios are too small, user can’t operate the radio properly with a gloved hand, so the radio must have large knobs and buttons. Too small of a form factor makes the radio difficult to handle and easy to drop. Others users who require larger radios include motorcycle police officers, the military, special weapons and tactical (SWAT) teams, oil refinery workers, chemical plant workers, hazmat workers and anyone working outdoors in harsh, cold conditions.
 
More recently, interoperability requirements across frequency bands have driven the creation of new multiband radios, where a single radio can operate in at least three frequency bands. The new multiband radios are larger because they must contain two or three separate transceivers inside the housing. Also driving a slightly larger housing is the additional filtering and shielding between the transceivers to keep the different transceivers from interfering with each other. If proper shielding is not designed, the potential interference will degrade some of the RF performance characteristics and specifications of the radio.
 

 
David Lum is director of product and support operations for Asia/Pacific with Motorola Solutions based in Schaumburg, Illinois, USA. He is an editorial advisor to RadioResource International. Email comments to david.lum@motorola.com.
 
  


 
 
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Comments
On 1/26/17, Pam Lassila said:
There are so many factors to consider with two way radios. But many of these factors are in a sense deal breakers. I want a radio that has good speakers and encryption and will get the job done when I need it to.


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