Radio hobbyists monitor usage of the older equipment with telephone activity in the US AM broadcast band, some 27 MHz frequencies and most older 43-50 MHz frequencies.  1.7 MHz cordless phones were the earliest models available at retailers, and are generally identifiable by their large metal telescoping antennas. Channels just above the AM broadcast band were selected manually by the user. Some of the frequencies used are now part of the expanded AM radio band, and can be heard by anyone with an AM radio. There are reports of people still using these phones, and using them as makeshift AM radio stations that can be heard for a couple of city blocks. These models became obsolete because they are susceptible to eavesdropping, and interference from fluorescent lighting and automobile ignition systems. However, under ideal conditions they could have or more range.  43–50 MHz cordless phones had a large installed base by the early 1990s, and featured shorter flexible antennas and automatic channel selection. Due to their popularity, an overcrowding of the band led to an allocation of additional frequencies; thus manufacturers were able to sell models with 25 channels instead of just 10 channels. Although less susceptible to interference than previous AM units, these models are no longer in production and are considered obsolete because their frequencies are easily heard on practically any radio scanner. Advanced models began to use voice inversion as a basic form of scrambling to help limit unauthorized eavesdropping. These phones share the 49.8 MHz band (49.830 - 49.890) with some wireless baby monitors.  900 MHz cordless phones are rarely sold but have a huge installed base. Features include even shorter antennas, up to 30 auto selecting channels, and higher resistance to interference. Available in several varieties; analog, analog spread spectrum (100 kHz bandwidth), digital, and digital spread spectrum, most being sold today are low-cost analog models, which are still susceptible to eavesdropping. Digital variants can still be scanned, but are received as a digital hiss and therefore are difficult to eavesdrop upon. Digital transmission is immune to static interference but can experience signal fade (brief silence) as the phone goes out of range of the base. Newer Digital Spread Spectrum (DSS) variants spread their signal over a range of frequencies, providing more resistance to signal fade. This technology enabled the digital information to spread in pieces among several frequencies between the receiver and the base, thereby making it almost impossible to eavesdrop on the cordless conversation. The FCC only allows DSS model phones to transmit at the full power of 1 watt, which allows increased range over older analog and digital models.