by Jim Hanks

Just when you thought you had a high-end cordless phone, the frequencies got higher. But are the new 5.8 GHz phones better than 2.4 GHz and 900 MHz models? Or is this just the latest marketing scheme? After reading this article, which includes some technical background on cordless phones and the frequencies they use, you will have a better idea of whether or not a 5.8 GHz phone is a wise investment for your home or office.

Why higher frequencies can be better
Many people think that just because a phone uses a higher frequency its range will be better. In reality, the physics of wireless technology is a little more complicated. There are 3 main factors that affect the quality and strength of a cordless phone's signal: traffic, how the frequency handles obstructions, and output wattage.

Traffic
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. Unfortunately, many other devices—including cellular phones, ham radios, walkie-talkies, baby monitors, and pagers—are also constantly transmitting wireless signals in the air around you. When any of these devices uses the same frequency or bandwidth as your phone, interference is created.

This is where the FCC plays an important role. In order to prevent interference, the FCC determines which devices are allowed to transmit within specific ranges. When a range reaches its traffic capacity, the FCC will usually open up another range of frequencies for use.

Early generations of cordless phones used the 46-49 MHz bandwidths. Since anything that emits low-band interference in this range, devices such as refrigerators, TVs, and computers diminished the clarity and range of cordless conversations. Additionally, baby monitors and low-band pagers also crowded these same frequencies. So, as more people bought 46-49 MHz cordless phones and interference became even more prevalent, the FCC opened up the 900 MHz range.

With their improved range and clarity, 900 MHz phones became a standard household appliance and subsequent traffic increased. At their peak, 900 MHz phones often allowed users to overhear conversations from their neighbor’s phone. Due to this congestion, the FCC opened up yet another range of frequencies: the 2.4 GHz band. Though this frequency was new for phones, the 2.4 GHz band was also being used for wireless LANs. Additionally, microwave ovens emit transmissions in the 2.4 GHz band. Enter the 5.8 GHz band. As the most recently adopted cordless phone frequency, the 5.8 GHz band offers the least interference.

How the frequency handles obstructions
When dealing with longer distances between a transmitter and a receiver, a lower frequency will usually provide better range because the signal's wavelength is longer and can travel around obstacles.1 This is 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). If such logic stands, 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) 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 could in a less densely occupied area. So what becomes important in these cluttered spaces is a signal's ability to be deflected and then to 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 cover an area. As a result, these signals will often lose their strength before finding the reception antenna. Because signals in the 900 MHz range have wavelengths of only a few feet, it is easier for them to quickly 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 faster 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 a room quicker. Signals in the 5.8 GHz band cover an area even faster.

Output wattage
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 a cordless phone’s range three- to fourfold. Meanwhile, the increased wattage covered up the fact that higher-frequency signals require more power to transmit.

When 5.8 GHz phones were introduced, the allowable wattage was not increased—and here is where the buyer must beware. Because transmitting signals at a higher frequency requires more power, some 5.8 GHz phones use the new frequency only for the base-to-handset transmission. Then, to make sure a handset’s battery has a reasonable life, handset-to-base transmissions are sent on the older 2.4 GHz frequencies.

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 homes use for cordless phones) to 2.4 GHz models. The primary benefit of 5.8 GHz models is the avoidance of interference with 802.11b wLANs and microwaves. If interference is your primary concern, make sure you purchase a fully 5.8 GHz phone.2 Otherwise, a dual transmission phone will give you better battery life.

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
5.8 GHz w/spread spectrum - 300 to 2000 feet

In addition to their other advantages, the 2.4 GHz and 5.8 GHz frequency bands are above the range that most scanners can intercept, making it much more difficult for eavesdroppers to listen to your conversations.

In the end, if you're willing to shell out a few extra dollars for a 2.4 GHz phone, you'll get much better call clarity, better security, and increased range. And if you covet the newest of the new, a 5.8 GHz phone will also avoid interference with your microwave and wLAN.