Are PDAs worse clients for WLANs than notebooks?

Q. Our new wireless PDAs don’t seem to pick up signals as easily as the old laptops we’ve been using for several years. Do PDAs have inherently less range than laptops, due to smaller antennae, less power or some other reason?

Our new wireless PDAs don’t seem to pick up signals as easily as the old laptops we’ve been using for several years. Do PDAs have inherently less range than laptops, due to smaller antennae, less power or some other reason?

– Jim, Indiana

Your question touches upon the main differences between wired and wireless technologies – wireless is much more analog and statistical in nature, and its performance varies over a much larger range. User performance in a wireless network is a function of many variables, such as coverage, noise, radio performance and load.

PDAs do have some constraints in terms of radio design that inherently result in lower RF performance. The two biggest constraints are size and power. The size constraint most directly impacts the antennae, and the power/battery life constraint most directly impacts the transmit RF power.

In terms of power, the typical 802.11b card transmits at 17dBm (50mW) to 20dBm (100mW) of RF power and consumes 50 to 60ma of current. Reducing the power in half (i.e., 3dB) to 14-16dBm reduces the current consumption in half. So many of the Compact Flash cards and built-in radios for PDAs will trade transmit RF power for better battery life and give up 2-3dB of RF power.

In terms of size, the PDA designer is typically faced with designing what we in the engineering business call “an electrically small antennae.” While the engineer wants to put a 1.2-inch antenna on a nice big ground plane, the marketing guy says, “All I know is that it has to fit in the PDA that, by the way, has an enclosure made of metal.” This requires a design trade-off, which typically results in another 2-3dB loss of performance.

Does 4-6dB really make a big difference? In the line-of-sight outdoor world, it means that your friend with the laptop will be able to go twice as far as you can with your PDA. In the indoor world, where the typical office wall causes approximately 4-6dB worth of loss, it means that your laptop friend can get an extra office or two further away from an access point than you can with a PDA. More specifically, as the following graph shows, the total difference between the two types of clients should be about 30 feet in range.