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Did you know that even if your phone is showing one or two bars of coverage, it might not actually work?

The effective range of your phone might be less than you think.

Maybe you need a repeater/booster?

 
 
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All About Radio Repeaters/Signal Boosters

Extending the range of your cell phone or two-way radio
 

A radio repeater can greatly increase the effective range of a radio or cell phone.

 

 

As amazing as radio devices are, they all have finite range, and sooner or later, we'll find the device we're relying on has lost its signal.

At times like that, and whether it be with Wi-Fi, walkie-talkies, or cell phones, some type of signal booster, extender, or repeater will probably add to the effective range of your device.  In such cases, a repeater/booster/extender might quite literally be a life-saver.

Elsewhere we review specific products for a laptop's Wi-fi signal and for most cell phones; this page here gives you background about how the devices work.  Understanding their operation can help you choose the best devices and then use them to best advantage.

What a Radio Repeater Does

In its simplest form, a radio (or television or any other type of signal) repeater simply takes a weak/distant radio signal, amplifies it, and rebroadcasts it again.

This means that the effective range of a device is increased.

Repeaters can do much more than this, and there are some complexities in what they do and how they do it, so please continue reading through this article.

The Growing Need for Radio Repeaters

There are many reasons why radio repeaters are increasingly necessary.

1.  An evolution to higher frequency services with shorter range and less signal penetration

Way back in the early days of radio, frequencies were very much lower than they are today.

These lower frequency signals had greater range and penetration.  They could penetrate buildings, and they could curve around the earth's surface, and/or be bounced off the atmosphere and back down to ground again.

A good example of such signals are those found on regular AM radio (frequencies in the 0.5 - 1.5 Mhz range) and those used in shortwave broadcasting (2.3 - 25.8 MHz), much amateur radio, and commercial HF (high frequency) radio transmitters (frequencies in the general range of 3 MHz - 30 MHz).

As you may know from your own experiences, AM radio stations can have a range of 100 miles and more, and if you've ever listened to shortwave radio stations, you'll know they can sometimes reach all the way around the world.

And, as you also know, you can generally pick signals up inside a house as well as outside.

By comparison, think now of an FM radio station (88 - 108 MHz, in the VHF frequency band) or a television station (ranging widely from 54 - 806 MHz).  As you know, their range is much shorter, and the quality of the signal is much more sensitive to whether you are trying to receive it inside or out (think rabbit ears vs an outside antenna for your television set).

As radio technology improved, the frequency of radio transmissions steadily increased.  There were (and still are) a number of valid reasons for this.  Higher frequencies have more 'bandwidth' to carry more information in a signal, whether this information be better quality stereo audio (ie in FM radio) or high definition color television (compared to low definition black and white), or data at increasing speeds (eg with a cell phone).

In addition, as lower frequencies became 'used up' - in the sense of all available frequencies being allocated for various different uses, there literally was nowhere to go except 'up', going to higher and higher frequencies, which typically have a range limited to 'line of sight' only and which are increasingly sensitive to all types of obstruction between the transmitter and receiver.

These days frequencies are in use all the way up to about 6.0 GHz, even on such low-cost and seeming low-tech devices as cordless phones (some of which use a 5.8 GHz frequency).

As an aside, there is sort of an upper limit to how high radio frequencies can go.  At a certain point (around about perhaps 300 GHz), they start to evolve from being radio frequencies to infra-red frequencies, and their range and ability to penetrate objects drops down to an impractical level (put a piece of paper in front of your infra-red remote control to see what I mean).

2.  Congested radio waves require stations to share frequencies more ways by becoming less powerful, with shorter ranges

One of the key concepts of making modern cell phone service a viable concept is by limiting the size of each 'cell'.  In the early days of analog cell phone service, a cell might be some miles in diameter, and only capable of handling (say) 50 calls simultaneously.  It was quite common for one's phone to have to switch to a more distant cell (with a poorer quality signal), or not to be able to get service at all, due to congestion in the cell you were in.

Nowadays, think of situations such as a ball game or large convention, where you might have 50,000 - 100,000 people, all with cell phones, and all packed together tightly in a radius of perhaps a quarter mile.  That is an extraordinary density of users, and they all expect reliable service and the ability to make and receive calls.

Part of the solution has been to make each cell very much smaller.  Now you can have very tiny cells with coverage of perhaps only 100 yards (or even less), compared to other cells that might extend 10 miles.

3. A growing demand for 100% coverage

These days cell phones are displacing regular wired phones, and in general are becoming increasingly central and essential to our every-day life.

And - perhaps paradoxically - the better that cell phones get, and the more reliable they become, complete with ever-growing coverage, the more we expect and demand of them.  At the same time, the smaller the phone, the smaller its antenna, and the less effective/efficient it becomes at sending and receiving calls.

When cell phones were still somewhat experimental, we were more forgiving of them, of their poor quality, and of the dropped calls we used to suffer.  Now we expect - we demand - 24/7 perfect coverage, and everywhere.

4.  Other devices need repeaters and boosters too - Wi-fi

Wi-fi service is another part of modern life that is increasingly being taken for granted, but which is very range challenged.  The standard Wi-fi specification anticipates a range of about 100 ft or even slightly less indoors, depending on how many obstructions such as walls are between the router and the device.

Furthermore, as Wi-fi signal strength weakens, its data speed reduces.  That was once not a problem, when the internet speed it was connecting to was perhaps only 500kb/sec or so, but now that internet speeds beyond the Wi-fi router are much higher, a slow Wi-fi speed can materially impact on our internet experience.

5.  Still more devices - GMRS and other amateur radio services

GMRS radio service has become affordable and commonplace, and the FCC seems likely to soon remove the requirement to license GMRS radios - a requirement that 99% of GMRS radio owners have been ignoring for some years now anyway.

Unfortunately, the FCC is also probably going to reduce the power that portable GMRS radios can have down from 5 watts to only 2 watts.

GMRS radio is designed to work with repeaters.  Perhaps the latest FCC changes will accelerate a need for repeaters here too.

Simple Range Boosting Without a Repeater

The simplest way to boost the range of any device is to replace its internal antenna with an external antenna.  The external antenna may be a more efficient antenna, better able to send stronger and receive weaker signals; it might be directional, sending and receiving more strongly in a particular direction; and/or it might be in a better location than the antenna on the device itself.

This is of course not a repeater per se, because it is not rebroadcasting anything.  It is simply an external antenna, or perhaps described as a 'signal booster' or 'range extender'.  But if it gives you the extra reception you need, who cares what it is called!

How a Cell Phone Repeater/Signal Booster Works

In its simplest form, a true repeater/signal booster (as compared to the simple enhanced antenna discussed above) simply receives a weak radio signal and then retransmits it more strongly, enabling the signal to travel further.

There is actually a lot more to the process than this first one sentence description, so let's look at in step by step form.

The repeater needs to have a better aerial/antenna than the devices it will be repeating/boosting the signal for.  If it is no better at receiving a signal than the device it is repeating the signal for, it would not add any extra range or value, because it will fail to receive a usable signal at the same time the unit is is supposed to help does too.

So the first key ingredient of any type of repeater/booster system is a good antenna to receive signals from the far away station.

1.  Receiving a Distant Station's Signal

The first part of providing a repeater or signal booster service is to be able to better receive the distant signal than the device it is supposed to be augmenting.  If you think about it, you'll realize the essential nature of this - if the so-called repeater has no more receiving range than the device that will rely on it, then it too will stop receiving at the same range from the distant transmitter, and therefore it will provide much less practical benefit than if it had a better receiver capability than that belonging to the boosted unit.

The ability of a unit to receive radio signals is a function of two different features.  The first is the antenna, and second is the electronic receiver/tuner/amplifier that 'pulls' a signal off the antenna.

1.1 Antenna issues

A 'better' antenna may be one that is in a better location (especially one that is outdoors and up higher with better line of sight to the distant station).

A 'better' antenna may be one that is more exactly 'tuned' to work best with the frequency of the signal desired to be received and boosted.  This tuning is achieved by making it an optimum 'electrical' length to match the wavelength of the signal being received.  Different antenna styles have different ideal electrical lengths, and the electrical length is not necessarily the same as the physical length - coils somewhere along the antenna can vary its apparent electrical length without requiring its physical length to be the same (but the most efficient antenna design is with straight wire rather than with coils and other devices to 'cheat' the system).

Every antenna has an ideal frequency which it works best at, and a series of multiples of that frequency where it also works somewhat well, but for all other frequencies, it is increasingly less optimized.  It is somewhat of an oversimplification to say 'in general, longer antennas are better than shorter ones' - particularly in cases where the wave length of the radio signal is very short to start with (because an antenna never benefits from being longer in length than the wavelength of the signal it receives).

A CB radio uses frequencies with wave lengths of around 35 ft, a FRS/GMRS radio uses frequencies that have about 2' wave lengths, cell phones use either 12" or 6" radio waves, and Wi-Fi service uses 5" radio waves.  So longer antennas can be a helpful factor with CB and FRS/GMRS radios, but cell phones and Wi-Fi repeaters/boosters don't necessarily need a longer antenna.  But there is another - third - factor in antenna design which might be relevant.

Thirdly, a 'better' antenna may be one that is directional, so that it picks up signals most sensitively in the direction that the distant station is located.  On the face of it, directional antennas would seem only possible with a stationary repeater station, but even mobile repeaters (eg on vehicles) can have a certain type of directional enhancement on their antennas - they can be 'focused' so that they receive and transmit not in an even sphere (or hemisphere) but rather in more of a cylindrical manner so that the antenna 'listens' primarily straight out from the antenna, rather than wasting some of its capability by listening straight up and at other improbable angles - most signals come from transmitters that tend to be more or less horizontally distant from the receiver, rather than at sharp angles up and down.

Directional antennas can readily double or treble the range at which they send or receive signals.

1.2  Receiver issues

The second aspect of receiving a signal relates to the electronics which receive and amplify the signal from the antenna.

A good receiver is very sensitive - it only needs to detect a very weak signal in order to be able to do something with it, and needs to have a low noise amplifier so its own electronic processing of a weak signal doesn't overwhelm the signal with extra background noise.

A good receiver can also be very selective, so when it is tuned to a specific frequency, it only receives that exact frequency and doesn't have any spill-over signal from nearby frequencies interfering.

Good receivers can have 50% or more of increased sensitivity compared to not so good receivers; and can have two or more times less noise and much greater selectivity.

2.  Retransmitting the Distant Signal to the Final Receiver

Most of the factors which apply to receiving a signal now are flipped around and apply again to retransmitting the signal.

In the case of transmitting, the issues apply primarily to the amount of power the transmitter has, although note that, depending on how directional the antenna is for retransmitting the signal, amplifier power is not necessarily the most important factor.  It might be necessary to increase the transmitting power by four or even five times in order to double the range of the signal, and even the most ridiculously powerful signal will still be blocked by major obstructions.

As with the receiver, probably the most important factor is the design and location of the transmitting antenna.

3.  Receiving the reply from the Final Receiver

Of course, the repeater station also needs to be able to clearly receive a reply from the final receiver, and all the same issues apply to this as applied in point (1) above.

4.  Retransmitting the Final Receiver's reply to the Original Distant Station

And now, the last part of the 'round trip' of a conversation or data exchange, the repeater must be able to send the final receiver's response back to the original distant station, with the same issues as in point (2) above applying.

Some Special Situations where Repeaters Can be Used

In addition to the simple/obvious situation where a repeater can be used to extend the range and connectivity between two radios, there are some special situations where perhaps a repeater might not first be thought of but where they can be helpful.

Around obstructions

If you have a line of sight type radio signal/service, there might be a large obstruction that prevents the signal/service from getting where it needs to go.

One example of this would be an 'urban jungle' - the buildings in a downtown area would block or interfere with the transmission/reception of a signal from one side of town to the other, whereas a repeater, located at a high vantage point - possibly even out of town, might have a relatively clear unobstructed view of the downtown area.

In this sort of situation, the total distance traveled by the signal might end up being much longer, through the repeater, than directly by line of sight, but you might get very much better quality as a result.

Into buildings or other 'shielded' areas

Have you ever received a phone call while driving through a tunnel, or on a metro line?  Normal signals can't penetrate into a tunnel, so there have been additional cell sites (repeaters) added to give you coverage in the tunnel or other location.

A similar situation might be in a building.  If you can't get good reception inside your house or office, maybe you need to have a repeater there, too.  You'd have one antenna on the building's roof - it would be high up and outside the structure to get best range/reception.

You'd place the other antenna inside the building, to rebroadcast inside the house/office the signal received from outside.

Split repeaters

The preceding was an example of a split repeater, where one half of it is in one location and the other half is in another location.

Most applications considered until now have implied that the two parts of the repeater are in the same location, but there's no reason why the two parts could not be some distance apart - either a few tens of feet, or even many hundreds of miles, with a wired connection running between the two antennas.  That type of more distant connection can often be found with radio and television stations that are broadcasting into multiple areas.

Multiple Repeaters in a Chain

Sometimes you'll have repeaters that repeat other repeaters.  An example of this is in a fiber optic cable, where the signal needs to be amplified, boosted or repeated every few miles along the cable.

An earlier voice cable across the Atlantic had 22 repeaters, each boosting the signal 1,000,000 times in power.  So the sound coming out one end of the cable had been boosted, in total,

1,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,
000,000,000,000,000,000,000,000,000,000,000,000 times.

That's a lot of boosting.

Multiple Simultaneous Repeaters

Another type of multiple repeater scenario is where a number of repeaters are all linked together (probably by land-line).  When any one of the repeaters (or perhaps certain designated repeaters only) receive an incoming signal to be rebroadcast, they send it to all the other repeaters and all the repeaters rebroadcast it simultaneously.

This can be used to greatly increase the effective coverage area of a wireless service.

Cross-band repeaters

An interesting specialized application for a repeater is when it is used to take a radio signal at one frequency and to then re-broadcast it at a very different frequency.

Of course all repeated signals have to be rebroadcast at a slightly different frequency (otherwise you'd have the repeater end up  receiving and repeating itself in a nasty feedback loop, a bit like you can get with a public address system sometimes), but typically this is just a tiny bit different to the original frequency.  But sometimes you might have a repeater that is designed to patch together two systems with a major difference in frequencies that might be otherwise incompatible - eg a local fire department and police department.

In such a case, one department might operate around the 40 - 45 MHz frequency range and the other around the 150 - 155 MHz range - a huge difference that requires very different electronics and antennas in the radios.  And so a repeater would rebroadcast the frequencies from one system over to the other system and back again, allowing the two systems to be able to communicate.

Another example is where a powerful longer range communication system in a car (eg police car) communicates between the vehicle and a remote base station (eg at the police headquarters or 911 center) and then a separate repeater system rebroadcasts the signal at a different frequency and lower power to a walkie-talkie type radio on the policeman's belt, so when he is out of the car but not far away, his low power portable radio can patch through the car's high power not portable system and keep him in contact with his command center.

Cross-protocol repeaters

Another specialized application is for a repeater that connects between two different types of protocol.  An example of this would be a repeater that converts between cellular radio and Wi-fi.

Sharing repeaters

Yet another specialized application is a repeater that rebroadcasts a secured Wi-fi signal.  You might use this in, for example, a hotel, where you have to pay for each device you wish to connect to the internet.

So rather than paying three or four times to connect your laptop, your phone, your iPad, and who knows what else to the internet, you simply connect your laptop, either through an ethernet cable or via Wi-fi, and rebroadcast the signal for your other devices to connect through.

See our review of the excellent and free Connectify software for more information on how this works.

Can Everything Be Boosted/Repeated

Yes and no.  There are a couple of constraints to the boosting process, especially with analog signals.  Think of each boosting process as being like taking a photocopy - first of the original, then a copy of the copy, then a copy of the copy of the copy, and so on.  You lose some quality each time, and any dirt or dust or imperfections on a copy get passed on to each subsequent copy too.

The same thing happens with boosting radio signals.  If the received signal is of poor quality, with static and low clarity, you'll be boosting the static and distortion right along with the signal.

Sometimes Repeaters Make No Sense

For the sake of completeness, there can be situations where using a repeater doesn't improve the communication at all.  For example, if you have two people in two cars, driving along the same block, but relaying a conversation between themselves through a repeater station that is on a hill ten miles away, they would probably find it easier and better to simply transmit directly to each other rather than going through a repeater.

In addition, if you use a repeater, you are taking up at least twice as much bandwidth (something that is always in short supply) as you would if just communicating directly.  When communicating directly, at the simplest 'simplex' type of communication, you have one frequency that you use for alternately sending or receiving communications (eg a pair of walkie-talkies where only one person can be talking at a time, unlike a phone where both people can be talking simultaneously).

In contrast, the simplest repeater system uses two frequencies, and works so that the originating station transmits on one frequency (F1) and receives on a second frequency (F2).  The repeater receives on F1 and transmits on F2.

And because a repeated signal covers a larger area, one single person using it is making the frequency busy for a greater number of other potential users.

Repeater Delays

Repeaters can be either real-time streaming or 'store and forward' type repeaters.  All voice type repeaters are real-time streaming - they immediately start re-transmitting the signal they receive, with only perhaps 50 msec of delay for processing the signal.

Some types of data repeater are store and forward.

Summary

A repeater can extend the range of a radio signal, and can allow it to reach areas it would not otherwise reach.

Things to look for in choosing a repeater would encompass the antenna design, the receiver sensitivity and noise, and the rebroadcast power of the transmitter.

Related Articles

Please refer to our series on real world radio range for CB/MURS/FRS/GMRS radios.

Here is a review of the Connectify software that allows you to use your Windows laptop as a Wi-fi repeater.

 

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Originally published 22 Sep 2010, last update 31 May 2012

You may freely reproduce or distribute this article for noncommercial purposes as long as you give credit to me (David Rowell - KF7VVM) as original writer.

 
 
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