October 2010

Emergency Call Box: How to Choose The Right One For Your Application

Somewhere in your life you’ve likely seen an emergency call box. The most common place for them is along a busy highway in a big city. These call boxes are linked via wires or wirelessly to a command center to provide assistance to motorists along the highway. However, this article is about the kind of callbox that’s use for emergency purposes in businesses and other buildings and their surrounding areas.

Wired Emergency Call Box

An emergency call box is often used to meet ADA compliance, for safe zones or area of refuge required by local fire code, to notify emergency personnel of an crisis in dangerous work zones, or any application where an urgent situation may occur.

The first question you must answer in choosing an emergency callbox is whether you need a wired or wireless solution. If the building you are installing the callbox in is early in the new construction phase, you should install cable into the areas where you need a callbox. It will NEVER be so inexpensive to install cable again so now is the time to do it.

A wired system has a one primary advantage over a wireless system. That is call reliability. With a new wired system, you know when someone presses a button on the call box, there is nothing to interfere with the signal. With a wireless system there could be interference from neighboring systems or some obstruction that blocks the signal from getting to its destination. There are ways to limit the possibility of both of these issues, which will be discussed later.

But the downside of a wired system is that as the system ages, its reliability could be reduced. Problems usually stem from someone or something accidentally cutting the wire, and then finding the break can be next to impossible. Regardless, if you have a new building going up, put the cable in so you have more options.

If you have an existing building, then the cost of installing cable, or the building construction may prohibit installing a wired system. Then you install a wireless call box which actually does have some advantages over a wired system.

Obviously, the first advantage is the cost of installation. There is no cable running to the system. You may have to run power to the system, but you can get battery powered units where you may only have to change the batteries once or twice a year, and they notify you when they need changing. You can also install solar power in some applications.

One of the best advantages of a wireless system is mobility of your emergency personnel. Since the system communicates through the airwaves, you can give monitoring personnel handheld two-way radios so they can receive distress signals no matter where they are.

As mentioned, you do have to be concerned about interference factors with a wireless system. Building construction may be one concern. If you are putting a callbox in a fire-proof safe zone for instance, that safe zone is likely constructed of solid concrete heavily reinforced with metal rebar. That will seriously reduce the range the wireless call box can communicate. Most callboxes let you attach an external antenna, so you could extend range if you can get an antenna cable through the wall of the safe zone, but this is often not possible. Callboxes do have fairly high radio power so if you don’t need to communicate over a long distance, you may not need to worry about this anyway.

The other kind of interference is from neighboring wireless systems. The airwaves are shared by everyone. In the United States, the airwaves are managed by the Federal Communications Commission (FCC). In most cases you will be required to get a license from the FCC to use the frequencies emergency call boxes use. But there are some callboxes that use license-free frequencies too.

Wireless Emergency Call Box

The advantages of getting a license from the FCC is that if a neighbor installs a wireless system that interferes with yours, the FCC will help you solve the problem (likely by shutting your neighbor down).

If you choose a license-free frequency, you can still share the frequencies with others. but you will implement what are called Interference Eliminator or Quiet Codes. What that means is you can set your system to send these codes with each transmission. If a transmission does not have this code with it, your receiving radios remain quiet.

There are two major formats for wireless emergency call boxes. They are Ultra High Frequency (UHF) radio and Very High Frequency (VHF) radio. Neither frequency band is inherently better than the other. They each have their pluses and minuses. Both formats are effective ways to send a distress call from a callbox so deciding on the right radio for you depends on your application.

Two-way radios and a wireless call box communicate with each other through use of radio waves. Radio waves have different frequencies, and by tuning a radio receiver to a specific frequency you can pick up a specific signal.

Radio waves are transmitted as a series of cycles, one after the other. You will always see the “Hz” abbreviation used to indicate the frequency of a radio. Hertz is equal to one cycle per second.

Radio waves are measured by kilohertz (kHz), which is equal to 1000 cycles per second, or megahertz (MHz), which is equal to 1,000,000 cycles per second–or 1000 kHz. The relationship between these units is like this: 1,000,000 Hertz = 1000 kilohertz = 1 megahertz.

You may also hear the term “wavelength” when you hear about radio waves. This term is from the early days of radio when frequencies were measured in terms of the distance between the peaks of two consecutive cycles of a radio wave instead of the number of cycles per second. Lower frequencies produce a longer wavelength.

While wavelength measures distance between the peaks of cycles, frequency refers to how long the measured time is between the “crest” and “trough” of a wave arriving at the source. So frequency measures time instead of distance, but they are essentially both saying the same thing.

What is significant about wavelength for two-way radios is that it affects transmission range under certain conditions. A longer wavelength as a general rule lets a radio signal travel a greater distance.

Lower frequencies or wavelengths have greater penetrating power. That’s one of the reasons they are used for communicating with submarines. VLF (Very Low Frequency) radio waves (3–30 kHz) can penetrate sea water to a depth of approximately 20 meters. So a submarine at shallow depth can use these frequencies.

So from what you read above you may think VHF is always the better choice for a two-way radio no matter where you’re using it due to its penetrating power and longer distance. That’s not necessarily true. Even though VHF has better penetrating capabilities, that doesn’t necessarily make it the better choice for buildings. Remember the conversation about wavelength above? Wavelength has a big impact on transmission.

To explain this let’s assume we are communicating from one side of a commercial building to the other. In between these two points is a metal wall with a three foot door in it. Metal is an enemy to radio waves and they typically don’t pass through it well.

For our example let’s assume that the UHF wavelength the radio uses is about a foot and a half long and a similar VHF radio is around five feet long. These are in the ballpark of their normal wavelengths.

When the UHF transmits its signal the foot and a half long wave will pass through the door since the door is wider than the wavelength. The VHF signal will be totally reflected since it is wider than the opening of the door.

Your microwave oven is an example of this. The glass front door has a metal mesh with very small holes. Microwaves being a very high frequency have wavelengths that are only several inches long. The mesh keeps the microwaves trapped in the oven but it allows you to see inside because light waves have a microscopic wavelength.

Just imagine walking through the building carrying a five foot wide pole sideways. You will encounter the same challenges a VHF signal encounters. Now imagine walking through the building with a pole that’s only a foot and a half wide like a UHF wave. There are lots fewer doorways you couldn’t get through.

The one difference is that wireless signals will penetrate through drywall, masonry, human bodies, furniture, wall paneling, and other solid objects. All these objects will reduce the signal strength though. The more dense the object, the more it reduces the signal. VHF will penetrate these obstacles better than UHF, but that doesn’t necessarily mean that VHF is better for indoor applications as we will talk about in the UHF section below.

In our example above we assumed you had a metal wall with an opening. If you reverse this and you have a three foot metal object in front of the transmitting radio, then VHF would win. Since the object is three foot wide it will totally block the UHF signal whereas the VHF signal will get around it. Lower frequencies such as VHF diffract around large smooth obstacles more easily, and they also travel more easily through brick and stone.

For most applications, lower radio frequencies are better for longer range. A broadcasting TV station illustrates this. A typical VHF station operates at about 100,000 watts and has a coverage radius range of about 60 miles. A UHF station with a 60-mile coverage radius requires transmitting at 3,000,000 watts.

So there is no clear choice for which is better, VHF or UHF. There is a lot of “black magic” to radio technology so it’s not always easy to tell which will work better for your application. To help you decide on the best technology for you, more detail about each one is included below.

UHF Radio

UHF equipment operates between the frequencies of 300 MHz and 3000 MHz. Until recently, it wasn’t widely used. Now, the UHF radio frequency is used for GPS, Bluetooth, two-way radios, cordless phones, and WiFi.

There are more available channels with UHF so in more populated areas UHF may be less likely to have interference from other systems. If you are in an area where population is thin, VHF should also work fine for you. Plus, recently the FCC opened up a new VHF frequency called MURS that is so far not heavily used in most areas. There’s more about MURS below in the VHF section.

If you are in an area where interference from other radios may be an issue, UHF transmitters and receivers could be your best choice unless you use a MURS VHF radio. UHF is better at finding a way through physical barriers like walls, buildings, and rugged landscape. Anything that obstructs a radio wave, will weaken a radio signal. UHF lessens that effect. Though it may not travel as far, UHF radio waves will traverse through obstacles better than VHF.

To highlight the differences in indoor range, below is an excerpt from a brochure of a leading two-way radio maker on the predicted range of one of their lines of handheld VHF and UHF two-way radios:

“Coverage estimates: At full power, line-of-sight, no obstructions the range is approximately 4+ miles. Indoor coverage at VHF is approximately 270,000 sq ft and 300,000 sq ft at UHF. Expect about 20 floors vertical coverage at VHF and up to 30 floors at UHF. Note: Range and coverage are estimates and are not guaranteed.”

VHF waves are not very good at finding a path through walls, buildings and rugged landscape. Therefore range will be significantly reduced for VHF radios in these environments. That may not necessarily be a problem if the range needed is only a few hundred feet. You can also add an external antenna to an indoor VHF base station that will reduce or eliminate this problem.

The FCC requires you to get a license to operate in UHF frequencies.

One advantage of UHF transmission is the physically short wave that is produced by the high frequency. That means the antenna on the radio can be shorter than an equivalent VHF radio.

VHF Radio

VHF equipment operates between the frequencies of 30 MHz and 300 MHz. FM radio, two-way radios, and television broadcasts operate in this range.

Both UHF and VHF radios are prone to line of sight factors, but VHF a little more so. The waves make it through trees and rugged landscapes, but not as well as UHF frequencies do. However, if a VHF wave and a UHF wave were transmitted over an area without barriers, the VHF wave would travel almost twice as far. This makes VHF easier to broadcast over a long range. If you are working mostly outdoors, a VHF radio is probably the best choice, especially if you are using a base station radio indoors and you add the external antenna.

One disadvantage to VHF equipment can be its size. Since the frequency waves are bigger, its antenna must be bigger.

VHF radios also have a smaller number of available frequencies. Interference with other radios could be more likely to be a problem. However, the FCC recently made this less of a problem when they opened up the MURS frequencies. The 150 MHz frequency is a Citizens Band radio spectrum (not the same CB as truckers use) that is called the MURS service.

MURS stands for Multi-Use Radio Service. This service is for use in the United States and some other countries that use FCC rules. It is a low power, short range service in the VHF 150 MHz Citizens Band radio spectrum. There are 5 channels in the MURS frequencies with 38 privacy codes under each one that enable you to only pick up conversations on your code. The FCC does not require users of products for MURS to be licensed.

With MURS you can add a larger or external antenna to improve range. If you want to put an antenna on top of your house or business, you can do it with MURS. Some antenna manufacturers claim an external antenna can increase the effective radiated power of a transmitter by a factor of 4. These MURS intercoms can transmit up to four miles, and perhaps more with an external antenna depending on the terrain.

One benefit of VHF wireless radios is that battery life is almost always better than for similar UHF units. For handheld radios this is a plus.

In summary, if you are planning on using your two-way radios mainly inside buildings, then UHF is likely the best solution for you. If you are mainly using your two-way radios for communication outside, then VHF would be a good choice. Either radio technology can work for you if you don’t really have a long range to cover. In that case you may want to choose VHF because you can use unlicensed frequencies.

So choosing the right emergency call box for your application may not be as easy as you hoped for, but don’t worry, you can talk to experts to help you figure out what you need for your application. IntercomsOnline.com is one place to do that.

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