Are 2.4 GHz Cordless Phones Superior to Others?
by Jim Hanks
Most people have heard about the new entrant to the telephony market: 2.4 GHz
cordless phones. These phones provide greater range and better clarity than
900 MHz models, while offering equal and sometimes improved security from eavesdropping.
But are these phones better because gigahertz beats megahertz in the same way
gazillion beats million? After reading this article (which includes some technical
background on cordless phones and the frequencies they use), you should have
a better idea of whether or not a 2.4 GHz phone is a wise investment for your home or office.
Why higher frequencies can be better
Using a specific range of frequencies assigned by the Federal Communications Commission (FCC),
cordless phones send and receive calls with technology similar to that of an FM radio.
Many people think that just because a phone uses a higher frequency its range will be better.
In actuality, the physics of wireless technology is a little more complicated (though not much more).
There are 3 main factors that affect the quality and strength of a cordless
phone's signal: traffic, how each frequency handles obstructions, and output
Cellular phones, cordless phones, ham radios, radio stations, walkie-talkies,
baby monitors, pagers, and many other devices constantly transmit and receive
wireless signals in the air around you. When a few of these devices simultaneously
utilize the same frequency or bandwidth, interference is created. In order to
prevent interference, the FCC regulates which devices are allowed to transmit
on specific frequencies. When a bandwidth reaches its traffic capacity, the
FCC will usually assign another range of frequencies to be used.
Early generations of cordless phones used the 46-49 MHz bandwidth. Since anything
with an electronic motor emits low-band interference in this range, devices
such as microwaves, refrigerators, TVs, and computers diminished the range and
clarity of cordless conversations. In addition to these household appliances, baby
monitors and low-band pagers also occupied the 46-49 MHz range. As more people
bought cordless phones and interference became even more prevalent, it was quickly
realized that a new frequency was needed. In response, the FCC opened up the
900 MHz range.
With their improved range and clarity, 900 MHz phones became a standard household
appliance. As traffic increased on these bands (exemplified by the neighbors' conversations many of us have heard),
the FCC opened up yet another range of frequencies:
the 2.4 GHz band. As the most recently adopted cordless phone frequency, the
2.4 GHz band offers the least interference.
How each frequency handles obstructions
When dealing with longer distances between a transmitter and a receiver, a lower
frequency usually provides better range because the signal's wavelength is longer
and can travel around obstacles.1 This is the reason
why analog cellular phones (which use the 800 MHz frequency band) often have
greater range than PCS systems (which use the 1900 MHz frequency band). Therefore,
one would assume that a cordless phone utilizing the 900 MHz band would perform
better than one using the 2.4 GHz (or 2400 MHz) frequency band. Not so.
When a system is employed in a confined space such as an office or home, many
obstacles such as office equipment, walls, and doors, obstruct the handset-to-base
path and prevent signals from circumnavigating barriers like they can in less
densely occupied spaces. What becomes important in these cluttered areas is
a signal's ability to be deflected and then find the reception antenna. Low
frequency signals in the 46-49 MHz range have wavelengths of 18 feet, so they
must be repeatedly deflected to completely cover an area. Often these signals
will lose their strength before they find the receiving antenna. Because signals
in the 900 MHz range have wavelengths of only a few feet, it is easier for them
to quickly bounce off obstructions and cover an area more completely.
If the above explanation confuses you, think about how a child would fill in
a square with a crayon: quick side-to-side movements within the square do the
job much quicker than if the child were to wait until he or she hit an edge
before changing directions.
Wavelengths (or crayon strokes) for 2.4 GHz phones are even shorter than 900
MHz wavelengths and consequently cover (or crayon-color) a room even quicker.
To keep neighbors from constantly hearing each other's conversations, the FCC
initially limited the output wattage for cordless phones to just .001 watt.
But when digital and spread spectrum technologies (SSTs) made eavesdropping
a less valid concern by scrambling signals or dividing them across multiple
bandwidths, the allowable wattage for cordless phones was increased to 1 watt.
This action made for clearer calls and increased cordless range 3 to 4 times.
Many 900 MHz phones have SST, as do most 2.4 GHz phones. Unfortunately, this
higher wattage is also the reason SST phones have slightly shorter battery lives.
A downside, yes, but given the higher security and better range SST delivers,
it may not be too important for most people.
What's the bottom line?
Due to the multiple factors involved, the range and clarity of cordless phones
vary. In general, you can expect better clarity as you move from 46-49 MHz models
(which are overcrowded with baby monitors and walkie-talkies) to 900 MHz models
(which most people use for cordless phones) to 2.4 GHz models.
As for range, here are some average maximum distances for the different technologies.
Keep in mind that these numbers are very subjective and depend on factors such
as obstructions, frequency interference, transmission technology, and even weather.
The higher figure listed assumes ideal conditions are present.
46-49 MHz - 40 to 250 feet
900 MHz - 75 to 400 feet
900 MHz w/spread spectrum - 200 to 1500 feet
2.4 GHz w/spread spectrum - 300 to 2000 feet
In addition to its other advantages, the 2.4 GHz frequency band is above the
range that most scanners can intercept, making it much more difficult for eavesdroppers
to listen to your conversations.
So if you're willing to shell out the few extra dollars for a high-end 2.4
GHz phone, you'll get much better call clarity, better security, and increased
1 An interesting note. The size
of a signal's wavelength also determines the required size for the antennae
(both the base station's and the handset's). The wavelengths of 46-49 MHz signals
are 18 feet. In order to "catch" the signals, both the base station and the
handset need antennae that are 2-3 feet in size. When 900 MHz phones were introduced,
antennae were downsized to just 6 inches since the wavelength of their signals
were only a foot long. 2.4 GHz phones are even more portable and user-friendly
as their antennae are only a couple of inches long.