Hi Rick,
There is no hard and fast rule for when to use external antennas. The nice thing about the AP1020 is that you have a choice. If you find that the "built in" antenna doesn't give you the coverage you need, due to environmental factors, you can order an appropriate external antenna.I wish I could be more specific but without doing a site survey it's almost impossible to say. Have a look at the following docs that deal with coverage patterns of the Internal vs External antennas.
The AP1020 features both integrated sector antennas and an RP-TNC connector for external antennas. By selecting from a wide range of Cisco antennas, customers can achieve a variety of coverage shapes and sizes. Designed for more challenging RF environments, the AP1020 provides a high degree of installation flexibility.
Designed for enterprise environments and deployments that require coverage flexibility, specific models of Cisco Aironet 1000 Series Lightweight Access Points feature multiple antenna, range, and installation options. A university campus, for example, may place wireless LANs in classrooms using Cisco 1010 lightweight access points. For auditoriums and common areas, Cisco 1020 lightweight access points would be deployed with higher gain external antennas for extended range or specialized coverage patterns
Cisco 1000 Series Lightweight Access Points
http://www.cisco.com/en/US/products/ps6306/products_data_sheet0900aecd8025708a.html
Cisco Aironet Antenna Reference Guide
http://www.cisco.com/en/US/products/hw/wireless/ps469/products_data_sheet09186a008008883b.html
Hope this helps!
Rob
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