The growth of low power, cost effective wireless systems is driving more applications to use ZigBee protocol. While for some of them, the signal needs to propagate over a short range, when covering a large building or operating outdoor over a wide range for example, the system may benefit from transmitting more power. Actually  shows that a ZigBee radio system powered by a 20dBm power amplifier (PA) significantly increases its range compared to a low power 0dBm by 400% from 133 to 543 meters. However, adding a transmit PA may generate higher spurious, in particular -- the harmonic spurs. And additional filtering is required to satisfy radiated emission requirement of the different regulatory standards listed in Table 1.
Radiated Emission Regulatory Requirements
In the United States, radiated emission requirements are provided under Federal Communications Commission regulation FCC CFR47, part 15; in Europe, ETSI EN 300 440-1/EN 300 328; and ARIB STD-T66 in Japan. Annex F of IEEE Std 802.15.4  summarizes the requirements that are also listed in Table 1 for 2.4 GHz operation. Note that spurious requirements in Table 1 are relevant for the harmonic frequencies. Beside the absolute dBm limit, test conditions that include the measurement bandwidth and detector type also differ between the different standards, which may relax the actual requirement.