Airport express compatiblity

Another 2nd Gen AirPort Express would make sense if you are happy with the performance of your Express, since Apple designed their "extend" feature to only work with other Apple AirPort devices.

An AirPort Extreme would also work well, if you are looking for better performance, since it be a very close match to the Time Capsule.

That being said, there might be a 3rd party extender that would work with a Time Caspule, but we haven't found one that works well in years of checking and testing other devices.

Although Apple no longer offers the AirPort products, new, refurbished and used AirPorts are available on Ebay.

If you decide to try a 3rd party device that claims to be a "universal" extender, make sure that you understand the store's return policy before you buy.

Both 802.11n AirPort Express base station models would be compatible with extending the wireless range of the 4th generation Time Capsule. However, the 2nd generation Express (A1392) would be able to extent both the 2.4 & 5 GHz bands simultaneously.

Regardless, you do understand that Apple got out of the networking HW business over 2 years ago and no longer provides any of their outdated base stations, regardless of model ... correct?